@database "Microwave-Repair" @node main "Microwave Repair" NOTES ON MICROWAVE OVEN FAILURE DIAGNOSIS AND REPAIR Contents: @{" Chapter 1 " link "1"} About the Author @{" Chapter 2 " link "2"} Introduction @{" Chapter 3 " link "3"} Installation and Preventive Maintenance @{" Chapter 4 " link "4"} Microwave Oven Troubleshooting @{" Chapter 5 " link "5"} Principles of Operation @{" Chapter 6 " link "6"} Troubleshooting Guide @{" Chapter 7 " link "7"} Testing and Replacement of Components @{" Chapter 8 " link "8"} Items of Interest @{" Chapter 9 " link "9"} Service Information @endnode @node 2 "Chapter 2) Introduction" @{" 2.1 " link "2.1"} Radar Range anyone? @{" 2.2 " link "2.2"} The simplest problems @{" 2.3 " link "2.3"} Repair or replace? @endnode @node 3 "Chapter 3) Installation and Preventive Maintenance" @{" 3.1 " link "3.1"} General microwave oven installation considerations @{" 3.2 " link "3.2"} Microwave oven maintenance @{" 3.3 " link "3.3"} How long do microwaves hang around? @endnode @node 4 "Chapter 4) Microwave Oven Troubleshooting" @{" 4.1 " link "4.1"} Safety @{" 4.2 " link "4.2"} Safety guidelines @{" 4.3 " link "4.3"} Troubleshooting tips @{" 4.4 " link "4.4"} Test equipment @{" 4.5 " link "4.5"} Safe discharging of the high voltage capacitor in a microwave oven @{" 4.6 " link "4.6"} Getting inside a microwave oven @endnode @node 5 "Chapter 5) Principles of Operation" @{" 5.1 " link "5.1"} Instant (2 minutes on HIGH) microwave oven theory @{" 5.2 " link "5.2"} How a microwave oven works @{" 5.3 " link "5.3"} Controller @{" 5.4 " link "5.4"} Sensors @{" 5.5 " link "5.5"} Cooling fans @{" 5.6 " link "5.6"} Microwave generator @{" 5.7 " link "5.7"} Interlock switches @endnode @node 6 "Chapter 6) Troubleshooting Guide" @{" 6.1 " link "6.1"} What can go wrong @{" 6.2 " link "6.2"} General system control problems @{" 6.3 " link "6.3"} Totally dead oven @{" 6.4 " link "6.4"} Oven works but totally dead display @{" 6.5 " link "6.5"} Whacked out controller @{" 6.6 " link "6.6"} Erratic behavior @{" 6.7 " link "6.7"} Incorrect operation for some or all types of cycles @{" 6.8 " link "6.8"} Some of the keys on the touchpad do not function or perform the wrong action @{" 6.9 " link "6.9"} Microwave generator problems @{" 6.10 " link "6.10"} No heat but otherwise normal operation @{" 6.11 " link "6.11"} Loud hum and/or burning smell when attempting to cook @{" 6.12 " link "6.12"} Arcing in or above oven chamber @{" 6.13 " link "6.13"} Fuse blows when pressing start or immediately @{" 6.14 " link "6.14"} Oven heats on high setting regardless of power setting @{" 6.15 " link "6.15"} Oven immediately starts to cook when door is closed @{" 6.16 " link "6.16"} Oven heats but power seems low @{" 6.17 " link "6.17"} Oven heats but shuts off randomly @{" 6.18 " link "6.18"} Fans or turntables that do not work @{" 6.19 " link "6.19"} What to do if the door handle breaks off @{" 6.20 " link "6.20"} Replacing the light bulb @{" 6.21 " link "6.21"} Repairing damage to the oven interior @{" 6.22 " link "6.22"} Microwave/convection oven problems @endnode @node 7 "Chapter 7) Testing and Replacement of Components" @{" 7.1 " link "7.1"} Testing the oven - the water heating test @{" 7.2 " link "7.2"} Testing and replacement of interlock switches @{" 7.3 " link "7.3"} Testing the high voltage components @{" 7.4 " link "7.4"} Testing and replacing the HV diode @{" 7.5 " link "7.5"} Testing and replacing the high voltage capacitor @{" 7.6 " link "7.6"} Testing and replacing the magnetron @{" 7.7 " link "7.7"} Testing and replacing the high voltage transformer @{" 7.8 " link "7.8"} Testing for bad connections (and replacing them with good ones) @{" 7.9 " link "7.9"} Testing and replacing the triac @{" 7.10 " link "7.10"} Testing and replacing the power relay @endnode @node 8 "Chapter 8) Items of Interest" @{" 8.1 " link "8.1"} How safe is a repaired microwave oven? @{" 8.2 " link "8.2"} Problems with running a microwave oven with metal inside or totally empty @{" 8.3 " link "8.3"} Microwave ovens and grounded dedicated circuits @{" 8.4 " link "8.4"} Microwave ovens and GFCIs @{" 8.5 " link "8.5"} Taking a microwave oven oversees (or vice versa) @{" 8.6 " link "8.6"} Using the control panel from defunct microwave oven as an electronic timer @endnode @node 9 "Chapter 9) Service Information" @{" 9.1 " link "9.1"} Advanced troubleshooting @{" 9.2 " link "9.2"} Suggested Reference @{" 9.3 " link "9.3"} Cost of repair parts @{" 9.4 " link "9.4"} Interchangeability of components @{" 9.5 " link "9.5"} Repair parts sources @endnode @node 1 "Chapter 1) About the Author" Author: Samuel M. Goldwasser E-Mail: sam@stdavids.picker.com Corrections/suggestions: [Feedback Form] [mailto] Copyright (c) 1994, 1995, 1996 All Rights Reserved Reproduction of this document in whole or in part is permitted if both of the following conditions are satisfied: 1. This notice is included in its entirety at the beginning. 2. There is no charge except to cover the costs of copying. This document converted to AmigaGuide format by Mike Fuller 27-7-96 ----------------------------------------------------------------------------- @endnode @node 2.1 "Radar Range anyone?" Remember when you actually had to use the real oven to defrost a TV dinner? Think back - way back - before VCRs, before PCs (and yes, before Apple computers as well), almost before dinosaurs, it would seem. There was a time when the term 'nuke' was not used for anything other than bombs and power reactors. For a long time, there was controversy as to whether microwave ovens were safe - in terms of microwave emissions and molecular damage to the food. Whether these issues have been resolved or just brushed aside is not totally clear. Nonetheless, the microwave oven has taken its place in virtually every kitchen on the planet. Connoisseurs of fine dining will turn up their collective noses at the thought of using a microwave oven for much beyond boiling water - if that. However, it is difficult to deny the convenience and cooking speed that is provided by this relatively simple appliance. Microwave ovens are extremely reliable devices. There is a good chance that your oven will operate for 10 years or more without requiring repairs of any kind - and at performance levels indistinguishable from when it was first taken out of the box. Unlike other consumer electronics where a new model is introduced every 20 minutes - some even have useful improvements - the microwave oven has not changed substantially in the last 20 years. Cooking is cooking. Touchpads are now nearly universal because they are cheaper to manufacture than mechanical timers (and also more convenient). However, an old microwave oven will heat foods just as well as a brand new one. This document provides maintenance and repair information applicable to most of the microwave ovens in existence. It will enable you to quickly determine the likely cause and estimate the cost of parts. You will be able to make an informed decision as to whether a new oven is the better alternative. With minor exceptions, specific manufacturers and models will not be covered as there are so many variations that such a treatment would require a huge and very detailed text. Rather, the most common problems will be addressed and enough basic principles of operation will be provided to enable you to narrow the problem down and likely determine a course of action for repair. In many cases, you will be able to do what is required for a fraction of the cost that would be charged by a repair center - or - be able to revive something that would otherwise have gone into the dumpster or continued in its present occupation as a door stop or foot rest. Should you still not be able to find a solution, you will have learned a great deal and be able to ask appropriate questions and supply relevant information if you decide to post to sci.electronics.repair. In any case, you will have the satisfaction of knowing you did as much as you could before taking it in for professional repair. You will be able to decide if it is worth the cost of a repair as well. With your new-found knowledge, you will have the upper hand and will not easily be snowed by a dishonest or incompetent technician. ----------------------------------------------------------------------------- @endnode @node 2.2 "The simplest problems" o Bad interlocks switches or door misalignment causing fuses to blow or no operation when the start button is pressed. Locate and replace defective switches and/or realign door. o Arcing in oven chamber: clean oven chamber and waveguide thoroughly. Replace carbonized or damaged waveguide cover. Smooth rough metal edges. Touch up interior paint. o Blown fuse due to power surge or old age: Replace fuse. o Erratic touchpad operation due to spill - let touchpad dry out for a week. More detailed explanations are provided elsewhere in this document. ----------------------------------------------------------------------------- @endnode @node 2.3 "Repair or replace?" With small to medium size microwave ovens going for $60-100 it hardly makes sense to spend $60 to have one repaired. Even full size microwave ovens with full featured touchpanel can be had for under $200. Thus, replacement should be considered seriously before sinking a large investment into an older oven. However, if you can do the repair yourself, the equation changes dramatically as your parts costs will be 1/2 to 1/4 of what a professional will charge and of course your time is free. The educational aspects may also be appealing. You will learn a lot in the process. Many problems can be solved quickly and inexpensively. Fixing an old microwave for the dorm room may just make sense after all. ----------------------------------------------------------------------------- @endnode @node 3.1 "General microwave oven installation considerations" To assure safety and convenient, follow these recommendations: o Select a stand-alone unit rather than a built-in if possible. It will be cheaper to buy, cheaper and easier to service, and possibly more reliable since ventilation and adjacent heat producing appliances will not be as much of a factor. o Select a convenient location - easy access and not too high or too low. This is particularly important if the door of the oven opens down instead of to the side (only a few models are built this way, however). o Put the microwave oven on its own dedicated 3 wire grounded circuit. Temporary use of a 3 to 2 prong adapter is acceptable only if the outlet box is properly grounded to begin with (BX, Romex, or conduit with ground). Make sure the outlet is in good condition in either case. Check that the plug (or adapter) fits tightly and that there is no appreciable heating of the outlet during use of the microwave oven. If there is, spread the metal pieces of the prongs apart and/or have the outlet replaced. A grounded outlet is essential for safety. Microwave ovens are high power devices and a separate circuit will eliminate nuisance fuse blowing or circuit breaker tripping when multiple appliances are being used at the same time. It will also minimize the possibility of Radio Frequency Interference (RFI) between it and any electronic equipment which might be on the same circuit. A GFCI is not needed as long as the outlet is properly grounded and may result in nuisance tripping with some microwave ovens. o Allow adequate ventilation - do not push it up against the wall or wedge it under a tight fitting wall cabinet (or inside one for that matter!). Leave at least 2 inches on all sides and top if possible. ----------------------------------------------------------------------------- @endnode @node 3.2 "Microwave oven maintenance" Most people do not do anything to maintain a microwave oven. While not much is needed, regular cleaning at least will avoid potentially expensive repairs in the future: o Clean the interior of the oven chamber after use with a damp cloth and a drop of detergent if necessary. Built up food deposits can eventually carbonize resulting in sparks, arcs, heating, and damage to the mica waveguide cover and interior paint - as well as potentially more serious damage to the magnetron. If there is any chance of food deposits having made their way above the waveguide cover in the roof of the chamber, remove the waveguide cover and thoroughly clean inside the waveguide as well. o Periodically check for built up dust and dirt around the ventilation holes or grilles. Clean them up and use a vacuum cleaner to suck up loose dust. Keeping the ventilation free will minimize the chance of overheating. o Inspect the cord and plug for physical damage and to make sure the plug is secure and tight in the outlet - particularly if the unit is installed inside a cabinet (yes, I know it is difficult to get at but I warned you about that!). Heat, especially from a combination microwave/convection oven or from other heat producing appliances can damage the plug and/or cord. ----------------------------------------------------------------------------- @endnode @node 3.3 "How long do microwaves hang around?" You have probably been warned by your mother: "wait a few seconds (or minutes) after the beep for all the microwaves to disappear". My recommendation is that once the beep has sounded (or the door has opened), it is safe. This is because: 1. There is no such thing as residual microwave radiation - it is either being produced or is non-existent. 2. There is little energy storage in the microwave generator compared to the amount being used. The typical high voltage capacitor - the only component that can store energy - has a capacity of less than 15 W-s even for the largest ovens. Power consumption is typically 800 to 1500 W depending on oven size. Therefore, the capacitor will be fully drained in much less than 0.1 second - long before the beep has ended or the door has cleared the front panel. ----------------------------------------------------------------------------- @endnode @node 4.1 "Safety" WARNING! WARNING! WARNING! WARNING! WARNING! WARNING! WARNING! Microwave ovens are probably the most hazardous of consumer appliances to service. Very high voltages (up to 5000 V) at potentially very high currents (AMPs) are present when operating - deadly combination. These dangers do not go away even when unplugged as there is an energy storage device - a high voltage capacitor - that can retain a dangerous charge for a long time. If you have the slightest doubts about your knowledge and abilities to deal with these hazards, replace the oven or have it professionally repaired. Careless troubleshooting of a microwave oven can not only can fry you from high voltages at relatively high currents but can microwave irradiate you as well. When you remove the metal cover of the microwave oven you expose yourself to dangerous - potentially lethal - electrical connections. You may also be exposed to potentially harmful levels of microwave emissions if you run the oven with the cover off and there is damage or misalignment to the waveguide to the oven chamber. To prevent the possibility of extremely dangerous electric shock, do not operate the oven with the cover off if at all possible. If you must probe, remove the connections to the magnetron (see below) to prevent the inadvertent generation of microwaves except when this is absolutely needed during troubleshooting. Use clip leads to make any connections before you plug in the oven and turn it on. There is a high voltage capacitor in the microwave generator. Always ensure that it is totally discharged before even thinking about touching or probing anything in the high voltage power circuits. See the @{"troubleshooting" link "6"} sections later in this document. The microwave oven circuitry is especially hazardous because the return for the high voltage is the chassis - it is not isolated. In addition, the HV may exceed 5000 V peak with a continuous current rating of over .25 AMP at 50/60 Hz - the continuous power rating of the HV transformer may exceed 1500 W with short term availability of greater power. Always observe High Voltage protocol. ----------------------------------------------------------------------------- @endnode @node 4.2 "Safety guidelines" These guidelines are to protect you from potentially deadly electrical shock hazards as well as the equipment from accidental damage. Note that the danger to you is not only in your body providing a conducting path, particularly through your heart. Any involuntary muscle contractions caused by a shock, while perhaps harmless in themselves, may cause collateral damage - there are many sharp edges inside this type of equipment as well as other electrically live parts you may contact accidentally. The purpose of this set of guidelines is not to frighten you but rather to make you aware of the appropriate precautions. Repair of TVs, monitors, microwave ovens, and other consumer and industrial equipment can be both rewarding and economical. Just be sure that it is also safe! o Don't work alone - in the event of an emergency another person's presence may be essential. o Always keep one hand in your pocket when anywhere around a powered line-connected or high voltage system. o Wear rubber bottom shoes or sneakers. o Don't wear any jewelry or other articles that could accidentally contact circuitry and conduct current, or get caught in moving parts. o Set up your work area away from possible grounds that you may accidentally contact. o Know your equipment: TVs and monitors may use parts of the metal chassis as ground return yet the chassis may be electrically live with respect to the earth ground of the AC line. Microwave ovens use the chassis as ground return for the high voltage. In addition, do not assume that the chassis is a suitable ground for your test equipment! o If circuit boards need to be removed from their mountings, put insulating material between the boards and anything they may short to. Hold them in place with string or electrical tape. Prop them up with insulation sticks - plastic or wood. o If you need to probe, solder, or otherwise touch circuits with power off, discharge (across) large power supply filter capacitors with a 25 W or greater resistor of 5 to 50 ohms/V approximate value (e.g., for a 200 V capacitor, use a 1K to 10K ohm resistor). Monitor while discharging and/or verify that there is no residual charge with a suitable voltmeter. In a TV or monitor, if you are removing the high voltage connection to the CRT (to replace the flyback transformer for example) first discharge the CRT contact (under the insulating cup at the end of the fat red wire). Use a 1M to 10M ohm 1W or greater wattage resistor on the end of an insulating stick or the probe of a high voltage meter. Discharge to the metal frame or grounding strap which is connected to the outside of the CRT (NOT THE SIGNAL GROUND). o For TVs and monitors in particular, there is the additional danger of CRT implosion - take care not to bang the CRT envelope with your tools. An implosion will scatter shards of glass at high velocity in every direction. There is several tons of force attempting to crush the typical CRT. Always wear eye protection. o Connect/disconnect any test leads with the equipment unpowered and unplugged. Use clip leads or solder temporary wires to reach cramped locations or difficult to access locations. o If you must probe live, put electrical tape over all but the last 1/16" of the test probes to avoid the possibility of an accidental short which could cause damage to various components. Clip the reference end of the meter or scope to the appropriate ground return so that you need to only probe with one hand. o Perform as many tests as possible with power off and the equipment unplugged. For example, the semiconductors in the power supply section of a TV or monitor can be tested for short circuits with an ohmmeter. o Use an isolation transformer if there is any chance of contacting line connected circuits. A Variac(tm) is not an isolation transformer! The use of a GFCI (Ground Fault Circuit Interrupter) protected outlet is a good idea but will not protect you from shock from many points in a line connected TV or monitor, or the high voltage side of a microwave oven, for example. A circuit breaker is too slow and insensitive to provide any protection for you or in many cases, your equipment. A GFCI may, however prevent your scope probe ground from smoking should you accidentally connect an earth grounded scope to a live chassis. o Don't attempt repair work when you are tired. Not only will you be more careless, but your primary diagnostic tool - deductive reasoning - will not be operating at full capacity. o Finally, never assume anything without checking it out for yourself! Don't take shortcuts! As noted, a GFCI (Ground Fault Circuit Interrupter) will NOT protect you from the high voltage since the secondary of the HV transformer is providing this current and any current drawn off of the secondary to ground will not be detected by the GFCI. However, use of a GFCI is desirable to minimize the risk of a shock from the line portions of the circuitry if you don't have an isolation transformer. An isolation transformer is even limited value as well since the chassis IS the HV return and is a large very tempting place to touch, lean on, or brush up against. And, of course, none of these devices will protect fools from themselves! Take extreme care whenever working with the cover off of a microwave oven. ----------------------------------------------------------------------------- @endnode @node 4.3 "Troubleshooting tips" Many problems have simple solutions. Don't immediately assume that your problem is some combination of esoteric complex convoluted failures. For a microwave oven, there may be a defective door interlock switch or just a tired fuse. If you get stuck, sleep on it. Sometimes, just letting the problem bounce around in your head will lead to a different more successful approach or solution. Don't work when you are really tired - it is both dangerous (particularly with microwave ovens) and mostly non-productive (or possibly destructive - very destructive). If you need to remove the cover or other disassembly, make notes of which screw went where - they may not all be identical. More notes is better than less. Pill bottles, film canisters, and plastic ice cube trays come in handy for sorting and storing screws and other small parts after disassembly. Select a work area which is well lighted and where dropped parts can be located - not on a deep pile shag rug. Something like a large plastic tray with a slight lip may come in handy as it prevents small parts from rolling off of the work table. The best location will also be relatively dust free and allow you to suspend your troubleshooting to eat or sleep or think without having to pile everything into a cardboard box for storage. A basic set of high quality hand tools will be all you need to work on a microwave oven. These do not need to be really expensive but poor quality tools are worse than useless and can cause damage. Stanley or Craftsman are fine. Needed tools include a selection of Philips and straight blade screwdrivers, needlenose pliers, wire cutters and wire strippers. A medium power soldering iron and rosin core solder (never never use acid core solder or the stuff for sweating copper pipes on electronic equipment) will be needed if you should need to disconnect any soldered wires (on purpose or by accident) or replace soldered components. However, most of the power components in microwave ovens use solderless connectors (lugs) and replacements usually come with these as well. An assortment of solderless connectors (lugs and wirenuts) is handy when repairing the internal wiring. A crimping tool will be needed as well but the $4 variety is fine for occasional use. For info on useful chemicals, adhesives, and lubricants, see "Repair Briefs, an Introduction" as well as other documents available at this site. Old dead microwaves can often be valuable source of hardware and sometimes even components like interlock switches and magnetrons as these components are often interchangeable. While not advocating being a pack rat, this does have its advantages at times. ----------------------------------------------------------------------------- @endnode @node 4.4 "Test equipment" Don't start with the electronic test equipment, start with some analytical thinking. Many problems associated with consumer electronic equipment do not require a schematic (though one may be useful). The majority of microwave oven problems are easily solved with at most a multimeter (DMM or VOM). You do not need an oscilloscope for microwave oven repair unless you end up trying to fix the logic in the controller - extremely unlikely. A DMM or VOM is necessary for checking of power supply voltages (NOT the high voltage, however) and testing of interlock switches, fuses, wiring, and most of the components of the microwave generator. This does not need to be expensive but since you will be depending on its readings, reliability is important. Even a relatively inexpensive DMM from Radio Shack will be fine for most repair work. You will wonder how you ever lived without one! Cost: $25-50. Other useful pieces of 'test equipment': o A microwave leakage detector. Inexpensive types are readily available at home centers or by mail order. These are not super accurate or sensitive but are better than nothing. o A microwave power detector. These can be purchased or you can make one from a small neon (NE2) or incandescent bulb with its lead wires twisted together. Sometimes these homemade solutions do not survive for long but will definitely confirm that microwave power is present inside the oven chamber. NOTE: always have a load inside the oven when testing - a cup of water is adequate. o A thermometer (glass not metal) to monitor water temperature during power tests. o High voltage probe (professional, not homemade!). However, this is only rarely actually required. Low voltage, resistance, or continuity checks will identify most problems. WARNING: the high voltage in a microwave oven is NEGATIVE (-) with respect to the chassis. Should you accidentally use the wrong test probe polarity with your meter, don't just interchange the probes = it may be last thing you ever do. Unplug the oven, discharge the HV capacitor, and only then change the connections. There are special magnetron and microwave test instruments but unless you are in the business, these are unnecessary extravagances. ----------------------------------------------------------------------------- @endnode @node 4.5 "Safe discharging of the high voltage capacitor in a microwave oven" It is essential - for your safety and to prevent damage to the device under test as well as your test equipment - that the large high voltage capacitor in the microwave generator be fully discharged before touching anything or making measurements. While these are supposed to include internal bleeder resistors, these can fail. In any case, several minutes may be required for the voltage to drop to negligible levels. The technique I recommend is to use a high wattage resistor of about 5-50 ohms/V of the working voltage of the capacitor. This will prevent the arc-welding associated with screwdriver discharge but will have a short enough time constant so that the capacitor will drop to a low voltage in at most a few seconds (dependent of course on the RC time constant and its original voltage). o For the high voltage capacitor in a microwave oven, use a 25 W or larger 100 K ohm resistor for your discharge widget with a clip lead to the chassis. The reason to use a large (high wattage) resistor is again not so much power dissipation as voltage holdoff. You don't want the HV zapping across the terminals terminals of the resistor. o Clip the ground wire to an unpainted spot on the chassis. Use the discharge probe on each side of the capacitor in turn for a second or two. Since the time constant RC is about 0.1 second, this should drain the charge quickly and safely. o Then, confirm with a WELL INSULATED screwdriver across the capacitor terminals. If there is a big spark, you will know that somehow, your original attempt was less than entirely successful. At least there will be no danger. DO NOT use a DMM for this unless you have a proper high voltage probe. If your discharging did not work, you may blow everything - including yourself. A suitable discharge tool can be made as follows: o Solder one end of the appropriate size resistor (100K ohms, 25W in this case) to a well insulated clip lead about 2-3 feet long. Don't just wrap it around - this connection must be secure for safety reasons. o Solder the other end of the resistor to a well insulated contact point such as a 2 inch length of bare #14 copper wire mounted on the end of a 2 foot piece of PVC or Plexiglas rod which will act as an extension handle. o Secure the resistor to the insulating rod with some plastic electrical tape. This discharge tool will keep you safely clear of the danger area. The capacitor discharge indicator circuit described in the document: "Testing Capacitors with a Multimeter and Safe Discharging" can be built into the discharge tool if desired. Again, always double check with a reliable high voltage meter or by shorting with an insulated screwdriver! Reasons to use a resistor and not a screwdriver to discharge capacitors: 1. It will not destroy screwdrivers and capacitor terminals. 2. It will not damage the capacitor (due to the current pulse). 3. It will reduce your spouse's stress level in not having to hear those scary snaps and crackles. ----------------------------------------------------------------------------- @endnode @node 4.6 "Getting inside a microwave oven" You will void the warranty - at least in principle. There are usually no warranty seals on a microwave so unless you cause visible damage or mangle the screws or plastic, it is unlikely that this would be detected. You need to decide. A microwave still under warranty should probably be returned for warranty service for any covered problems except those with the most obvious and easy solutions. Unplug the unit! Usually, the sheet metal cover over the top and sides is easily removed after unscrewing 8-16 philips head sheet metal screws. Most of these are on the back but a few may screw into the sides. They are not usually all the same! At least one of these includes a lockwasher to securely ground the cover to the case. Make note of any differences in screw types so they can be put back in the same place. The cover will then lift up and off. Note how fingers on the cover interlock with the main cabinet - these are critical to ensure prevention of microwave leakage after reassembly. Discharge the high voltage capacitor as described in the section: @{"Safe discharging of the high voltage capacitor in a microwave oven" link "4.5"} before even thinking about touching anything. A schematic showing all of the power generation components is usually glued to the inside of the cover. How much of the controller is included varies but is usually minimal. Fortunately, all the parts in a microwave can be easily replaced and most of the parts for the microwave generator are readily available from places like MCM Electronics, Dalbani, and Premium Parts. Reassemble in reverse order. Take particular care to avoid pinching any wires when reinstalling the cover. Fortunately, the inside of a microwave is wide open and this is not difficult. Make sure ALL of the metal fingers around the front edge engage properly with the front panel lip. This is critical to avoid microwave emissions should the waveguide or magnetron become physically damaged in any way. Confirm that the screws you removed go back in the proper locations, particularly the one that grounds the cover to the chassis. ----------------------------------------------------------------------------- @endnode @node 5.1 "Instant (2 minutes on HIGH) microwave oven theory" A typical microwave oven uses between 500 and 1000 W of microwave energy at 2.45 GHz to heat the food. This heating is caused mainly by the vibration of the water molecules. Thus plastic, glass, or even paper containers will heat only through conduction from the hot food. There is little transfer of energy directly to these materials. This also means that the food does not need to be a conductor of electricity (try heating a cup of distilled water) and that electromagnetic induction (used elsewhere for high frequency non-contact heating) is not involved. Since the oven chamber cavity is a good reflector of microwaves, nearly all the energy generated by the oven is available to heat the food and heating speed is thus only dependent on the available power and how much food is being cooked. Ignoring losses through convection, the time to heat food is roughly proportional to its weight. Thus two cups of water will take around twice as long to bring to a boil as one. Heating is not (as popularly assumed) from the inside out. The penetration depth of the microwave energy is a few cm so that the outside is cooked faster than the inside. However, unlike a conventional oven, the microwave energy does penetrate these few cm rather than being totally applied to the exterior of the food. The misconception may arise when sampling something like a pie filling just out of the microwave (or conventional oven for that matter). Since the pie can only cool from the outside, the interior filling will appear to be much hotter than the crust and will remain that way for a long time. One very real effect that may occur with liquids is superheating. It is possible to heat a pure liquid like water to above its boiling point if there are no centers for bubbles to form such as dust specks or container imperfections. Such a superheated liquid may boil suddenly and violently upon removal from the oven with dangerous consequences. This can take place in a microwave since the heating is relatively uniform throughout the liquid. With a stovetop, heating is via conduction from the burner or coil and there will be ample opportunity for small bubbles to form on the bottom long before the entire volume has reached the boiling point. Most metal objects should be excluded from a microwave oven as any sharp edges (areas of high electric field gradient) may create sparking or arcing which at the very least is a fire hazard. Microwave-safe metal shelves will have nicely rounded corners. A microwave oven should never be operated without anything inside as the microwave generator then has no load - all the energy bounces around inside an a great deal is reflected back to the source. This may cause expensive damage to the magnetron and other components. ----------------------------------------------------------------------------- @endnode @node 5.2 "How a microwave oven works" The operation of a microwave oven is really very simple. It consists of two parts: the controller and the microwave generator. A schematic diagram of the microwave generating circuitry and portions of the controller is usually glued to the inside of the cover. The controller is what times the cooking by turning the microwave energy on and off. Power level is determined by the ratio of on time to off time in a 10-30 second cycle. The microwave generator takes AC line power. steps it up to a high voltage, and applies this to a special type of vacuum tube called a magnetron - little changed from its invention during World War II (for Radar). ----------------------------------------------------------------------------- @endnode @node 5.3 "Controller" The controller usually includes a microcomputer, though very inexpensive units may simply have a mechanical timer (which ironically, is probably more expensive to manufacture!). The controller runs the digital clock and cook timer; sets microwave power levels; runs the display; and in high performance ovens, monitors the moisture or temperature sensors. Power level is set by pulse width control of the microwave generator usually with a cycle that lasts 10-30 seconds. For example, HIGH will be continuous on, MEDIUM may be 10 seconds on, 10 seconds off, and LOW may be 5 seconds on, 15 seconds off. The power ratios are not quite linear as there is a 1-3 second warmup period after microwave power is switched on. The operating voltages for the controller usually are derived from a stepdown transformer. The controller activates the microwave generating circuitry using either a relay or triac. ----------------------------------------------------------------------------- @endnode @node 5.4 "Sensors" More sophisticated ovens may include various sensors. Most common are probes for temperature and moisture. A convection oven will include a temperature sensor above the oven chamber. Since these sensors are exposed to the food or its vapors, failures of the sensor probes themselves are common. ----------------------------------------------------------------------------- @endnode @node 5.5 "Cooling fans" Since 30-50 percent of the power into a microwave oven is dissipated as heat in the Magnetron, cooling is extremely important. Always inspect the cooling fan/motor for dust and dirt and lubricate if necessary. A couple of drops of electric motor oil or 3-in-One will go a long way. If there are any belts, inspect for deterioration and replace if necessary. An oven that shuts off after a few minutes of operation could have a cooling problem, a defective overtemperature thermostat, a bad magnetron, or is being operated from very high AC line voltage increasing power to the oven. One interesting note: Since 30-50 percent of the power goes out the vents in the back as heat, a microwave oven is really only more efficient than conventional means such as a stovetop or gas or electric oven for heating small quantities of anything. With a normal oven or stovetop, wasted energy goes into heating the pot or oven, the air, and so on. However, this is relatively independent of the quantity of food and may be considered to be a fixed overhead. Therefore, there is a crossover point beyond which it is more efficient to use conventional heat than high tech microwaves. ----------------------------------------------------------------------------- @endnode @node 5.6 "Microwave generator" This is the subsystem that converts AC line power into microwave energy. It consists of 5 parts: High Voltage Transformer, Rectifier Diode, Capacitor, Magnetron, Waveguide to oven chamber. o High Voltage Transformer. Typically has a secondary of around 2000 VRMS at .25 amp - more or less depending on the power rating of the oven. There will also be a low voltage winding for the Magnetron filament. You cannot miss this as it is the largest and heaviest component visible once the cover is removed. There will be a pair of quick-connect terminals for the AC input, a pair of leads for the Magnetron filament. and a single connection for the HV output. The HV return will be fastened directly to the transformer frame and thus the chassis. o Rectifier - usually rated 12,000-15,000 PRV at around .5 amp. Most commonly, this will be rectangular or cylindrical, about .5 inch long with wire leads. Sometimes, it is a box bolted to the chassis. One end will be electrically connected to the chassis. o Capacitor - .65-1.2 uF at a working voltage of around 2,000. Note that this use of 'working voltage' may be deceiving as the actual voltage on the capacitor may exceed this value during operation. The capacitor is metal cased with quick-connect terminals on top (one end). Always discharge the capacitor as described below before touching anything inside once the cover is removed. o Magnetron - the microwave producing tube includes a heated filament cathode, multiple resonant cavities with a pair of permanent ceramic ring magnets to force the electron beams into helical orbits, and output antenna. The magnetron is most often box shaped with cooling fins in its midsection, the filament/HV connections on the bottom section, and the antenna (hidden by the waveguide) on top. Sometimes, it is cylindrical in shape but this is less common. The frequency of the microwaves is usually 2.45 GHz. The typical circuit is as follows: || +------------------------+ ||( 3 VAC, 4 A, typical | Magne- Relay or || +------------+------+ | tron fuse I TP Triac || | | | +-|----|-+ ___-\ ______/ __--_____/ ____ || +-----| |----+ | |_ _| | \- | )||( Cap | | | | \/ | AC I \ I=Interlock )||( __|__ | ___ | Line | TP=Thermal Prot. )||( 2000 VAC _\_/_ +----|:--+ _________|____________________)||( .25 A | Diode |'--> ||( typical | | Microwaves (Controller not shown) || +------------+---------+ Note the unusual circuit configuration - the magnetron is across the diode, not the capacitor as in a 'normal' power supply. What this means is that the peak voltage across the magnetron is the transformer secondary + the voltage across the capacitor, so the peaks will approach the peak-peak value of the transformer or nearly 5000 V in the example above. This is a half wave voltage doubler. The output waveform looks like a sinusoid with a p-p voltage equal to the p-p voltage of the transformer secondary with its positive peaks at chassis ground (no load). The peaks are negative with respect to the chassis. The negative peaks will get squashed somewhat under load. Take extreme care - up to 5000 V at AMPs available! There is also usually a bleeder resistor as part of the capacitor, not shown. HOWEVER: DO NOT ASSUME THAT THIS IS SUFFICIENT TO DISCHARGE THE CAPACITOR - ALWAYS DO IT WHENEVER THE OVEN HAS BEEN POWERED. The bleeder may be defective and open as this does not effect operation of oven and the time constant may be long - minutes. In addition, there will likely be an overtemperature thermostat somewhere in the primary circuit, often bolted to the Magnetron case. Other parts include the oven interlock switches, cooling fan, turntable motor (if any), oven light, etc. ----------------------------------------------------------------------------- @endnode @node 5.7 "Interlock switches" Various door interlock switches prevent inadvertent generation of microwaves unless the door is closed completely. At least one of these will be directly in series with the transformer primary so that a short in the relay or triac cannot accidentally turn on the microwaves with the door open. The interlocks must be activated in the correct sequence when the door is closed or opened. Interestingly, another interlock is set up to directly short the power line if it is activated in an incorrect sequence. The interlocks are designed so that if the door is correctly aligned, they will sequence correctly. Otherwise, a short will be put across the power line causing the fuse to blow forcing the oven to be serviced. At least that is the most likely rational for putting a switch across the power line. Failed door interlocks account for the majority of microwave oven problems - perhaps as high as 75 percent. ----------------------------------------------------------------------------- @endnode @node 6.1 "What can go wrong" The most common problems occur in the microwave generating portion of the system, though the controller can be blown by a lightning strike or other power surge. Also, since the touchpad is exposed, there is a chance that it can get wet or damaged. If wet, a week or so of non-use may cure keys that don't work. If damaged, it will probably need to be replaced - this is straightforward if the part can be obtained, usually direct from the manufacturer. Unfortunately, it is an expensive part ($20-50 typical). No adjustments should ever be required for a microwave oven and there are no screws to turn so don't look for any. The interlock switches, being electromechanical can fail to complete the primary circuit on an oven which appears to operate normally with no blown fuses but no heat as well. Faulty interlocks or a misaligned door may result in the fuse blowing as described above due to the incorrect sequencing of the door interlock switches. Failed interlocks are considered to be the most common problems with microwave ovens, perhaps as high as 75% of all failures. Tracing the circuitry of the interlocks is straightforward, if a bit tedious to keep track of where the wires connect. It is a simply matter to check each of the switches with an ohmmeter. Contact resistance of more than a tiny fraction of an ohm when in the 'on' position is a failure as is anything less than infinity ohms when open. You may need to temporarily remove the quick connects to assure that the measurements are not being confused by other circuitry. Measure across the fuse and neutral while opening and closing the door with various amounts of jiggling thrown in. Any indication of continuity means that the fuse will blow if the oven is powered up and the door is opened or closed. If a switch needs to be replaced due to it having blown the fuse, replace any other switches upstream of it as well since it is likely that the momentary current surge has damaged them also. Generic replacement interlock switches are readily available. ----------------------------------------------------------------------------- @endnode @node 6.2 "General system control problems" The following problems are likely caused by the controller and not the microwave generator unless due to a blown fuse. o Totally dead oven. o Oven works but display is blank. o Whacked out controller. o Erratic behavior. o Incorrect operation for some or all types of cycles. o Some keys on the touchpad do not function or perform the wrong action. First, unplug the microwave oven for a couple of minutes. Sometimes, the microcontroller will get into a whacko mode for some unknown reason - perhaps a power surge - and simply needs to be reset. The problem may never reoccur. NOTE: when working on controller related problems, unplug the connection to the microwave generator (HV transformer primary) from the power relay or triac - it is often a separate connector. This will prevent any possible accidental generation of microwave energy as well as eliminating the high voltage (but not the AC line) shock hazard during servicing. ----------------------------------------------------------------------------- @endnode @node 6.3 "Totally dead oven" First, try to set the clock. With some ovens the screen will be totally blank following a power outage - there may be nothing wrong with it. Furthermore, some ovens will not allow you perform any cooking related actions until the clock is set to a valid time. Assuming these are not your problems, a fuse has probably blown although a dead controller is a possibility. If the main fuse is upstream of the controller, then any short circuit in the microwave generator will also disable the controller and display. If this is the case, then putting in a new fuse will enable the touchpad/display to function but may blow again as soon as a cook cycle is initiated if there is an actual fault in the microwave circuits. Therefore, try a new fuse. If this blows immediately, there may be a short very near the line cord, in the controller, or a defective triac (if your oven uses a triac). If it does not blow, initiate a cook cycle (with a cup of water inside). If the oven now works, the fuse may simply have been tired of living. This is common. If the fuse still blows immediately, confirm that the controller is operational by unplugging the microwave generator, power relay, and/or triac from the controller. If a new fuse does not now blow when a cook cycle is initiated - and it appears to operate normally - then one of the components in the microwave generator is defective (shorted). See the section: @{"Microwave generator problems" link "6.9"} Other possible causes: bad controller power supply or bad controller chip. The most common way that the controller circuitry can be harmed is by a power surge such as from a lightning strike. Assuming that the main fuse checks out, then check the power supply for the controller next. Also check for bad solder connections. ----------------------------------------------------------------------------- @endnode @node 6.4 "Oven works but totally dead display" If all functions work normally including heating but the display is blank (assuming you can issue them without being able to see the display), the problem is almost certainly in the controller or its power supply. Check for bad connections between the display panel and the power supply. With everything else operational, a bad microcontroller chip is not that likely but is still a possibility. If the oven was physically abused, the display panel may have fractured though it would take quite a bit of violence. In this case, more serious damage to the door seals may have resulted as well which would be a definite hazard. ----------------------------------------------------------------------------- @endnode @node 6.5 "Whacked out controller" All the display digits may have come on or the end-of-cooking cycle or the keypress tone may be wailing away continuously. By tone I mean from the controller (not a low buzzing or humming when attempting to cook which would indicate a microwave generator power problem like a shorted magnetron). First, try unplugging it for a couple of minutes - perhaps the controller is just confused due to a power surge or because it wanted attention. If you recently cleaned the oven, some liquid may have accidentally gotten inside the touchpad or even the controller circuitry (though this is less likely). See the section: @{"Some of the keys on the touchpad do not function or perform the wrong action" link "6.8"} Assuming this does not apply, it sounds like a controller problem - possibly just a power supply but could also be the controller chip. My guess is that unless you were to find some simple bad connections or an obvious problem with the controller's power supply, the cost to repair would be very high as the custom parts are likely only available from the manufacturer. A controller failure does little to predict the reliability of the rest of the oven. The microwave generator circuits could last a long time or fail tomorrow. The output of the magnetron tube may decrease slightly with use but there is no particular reason to expect it to fail any time soon. This and the other parts are easily replaceable. However, unless this oven has a lot of fancy features, you can buy a replacement (depending on size) for $100-200 so it is probably not worth fixing unless it is something relatively simple and inexpensive. ----------------------------------------------------------------------------- @endnode @node 6.6 "Erratic behavior" Whenever the oven performs unexpectedly, suspect the controller power supply, bad connections, or a power relay with dirty or worn contacts. The filter capacitor(s) in the controller's power supply may be dried up or faulty. Check with a capacitor meter or substitute known good ones. Prod the logic board to see if the problem comes and goes. Reseat the flex cable connector to the touchpad. For mechanical timers, the timing motor could be defective or require lubrication. The contacts could be dirty or worn. Bad connections in the microwave generator could also result in erratic behavior either by preventing the microwave power from being produced (e.g., corroded filament connections to the magnetron) or by resulting in electrical noise which is erroneously resetting the microcontroller's logic. ----------------------------------------------------------------------------- @endnode @node 6.7 "Incorrect operation for some or all types of cycles" Check touchpad as above and interlocks. The controller's program may be corrupted (unlikely) but we have no real way of diagnosing this except by exclusion of all other possibilities. Depending on the model, some or all operations - even setting the clock - may be conditional on the door interlocks being closed, so these should be checked. Some ovens will not allow any actions to be performed if the door has been closed for more than a few minutes - open and close the door to reset. ----------------------------------------------------------------------------- @endnode @node 6.8 "Some of the keys on the touchpad do not function or perform the wrong action" Look carefully for any visible signs of damage or spills. The touchpads often use pressure sensitive resistive elements which are supposed to be sealed. However, any damage or just old age may permit spilled liquid to enter and short the sensors. A week or so of drying may cure these problems. If there is actual visible damage, it may be necessary to replace the touchpad unit, usually only available from the original manufacturer. Also, check the snap type connector where the touchpad flex-cable plugs into the controller board. Reseating this cable may cur a some keys dead problem. With a little bit of effort (or perhaps a lot of effort), the internal circuitry of the touchpad can be determined. This may require peeling it off of the front panel). Then, use resistors to jumper the proper contacts on the flex cable connector to simulate key presses. This should permit the functions to be verified before a new touchpad is ordered. CAUTION: unplug the microwave generator from the controller when doing this sort of experiment! ----------------------------------------------------------------------------- @endnode @node 6.9 "Microwave generator problems" Failures in the microwave generator can cause various symptoms including: o No heat but otherwise normal operations, o Loud hum and/or burning smell when attempting to cook. o Arcing in or above oven chamber. o Dead oven due to blown fuse in AC line input, o Oven heats but power seems low. Most of these are easy to diagnose and the required parts are readily available at reasonable prices. ----------------------------------------------------------------------------- @endnode @node 6.10 "No heat but otherwise normal operation" If the main power fuse is located in the primary of the high voltage transformer rather then at the line input, the clock and touchpad will work but the fuse will blow upon initiating a cook cycle. Or, if the fuse has already blown there will simply be no heating action once the cook cycle is started. There are other variations depending on whether the cooling fan, oven light, and so forth are located down stream of the fuse. A defective thermal protector (usually mounted on the magnetron) can also prevent heating action. ----------------------------------------------------------------------------- @endnode @node 6.11 "Loud hum and/or burning smell when attempting to cook" A loud abnormal hum which may be followed by the fuse blowing or not, is an indication of a short somewhere. The sound may originate from the HV transformer vibrating and/or from within the Magnetron depending on cause. Arcing within the Magnetron case (visible through ventilation holes in the bottom section) is usually an indication of a bad Magnetron. There may be a burnt odor associated with this behavior. If the odor is coming from the oven chamber, see the section: @{"Arcing in or above oven chamber" link "6.12"} ----------------------------------------------------------------------------- @endnode @node 6.12 "Arcing in or above oven chamber" Arcing in the oven chamber with a normal load (a cup of water, for example), often just indicates that a thorough cleaning of the oven chamber is needed, particularly around and inside/above the waveguide cover. Any food that gets trapped here will eventually burn and carbonize resulting in a focal point for further arcing. Usually, the waveguide cover is designed to be removable without taking the cover off of the oven. However, burnt food and carbon often make this difficult so that some disassembly will be required. Clean the waveguide cover and clean inside the waveguide as well. If the waveguide cover is broken or damaged seriously, replacement material is available. The oven will work fine without it but replacement will prevent contamination of the waveguide with food vapors or splatters. ----------------------------------------------------------------------------- @endnode @node 6.13 "Fuse blows when pressing start or immediately" Most often, the fuse would only blow when actually attempting to cook but some designs using triacs and/or door switches that are always live may result in a blown fuse at any time when plugged in or when the door is opened or closed. The following can cause the fuse to blow (in approximate order of likelihood): 1. Defective interlock switches or misaligned door. 2. Shorted HV capacitor. 3. Shorted HV diode. 4. Shorted magnetron (filament to anode (case)). 5. Defective triac (shorted or partially shorted). 6. Defective HV transformer (shorted windings. 7. Shorted wiring due to vibration or poor manufacturing quality. However, fuses also die of old age. The types of fuses used in microwave ovens are subjected to a heavy load and you may find that all that is needed is to replace the fuse with one with equivalent ratings. (but check for shorts first). There could be an intermittent problem as well which will only show up at some random time in the future. The fuses used in microwave ovens are usually ceramic 1-1/4"x1/4" 15 or 20 A 250 V fast blow type. Replace with exactly the same type and rating. Another possible cause of a blown fuse is a partially bad triac. Some ovens use a triac rather than a relay to control the main power to the HV transformer. The most common failures of a triac are for it to be totally shorted - causing the oven to come on whenever the door is closed. Alternatively, the gate may be defective preventing the triac from ever turning on. A third, and most interesting possibility, is that one side of the triac is bad - shorted or open. While the exact mechanism is unclear, if either of these situations occurs, the main fuse will blow due to transformer core saturation since the triac will act as a rectifier and transformers really do not like DC. See the section: @{"Testing and Replacement of Components" link "7"} for more information on this and similar problems. ----------------------------------------------------------------------------- @endnode @node 6.14 "Oven heats on high setting regardless of power setting" Power levels in a microwave oven are controlled by cycling the microwave generator on and off with a variable duty cycle - kind of like slow pulse width modulation. For 'HIGH', it runs continuously; for low, it may run 10% on and 90% off; other settings are in between. When the oven always seems to be stuck at high power, it is likely to be due to one of two possible causes - a faulty relay or Triac, or controller. The relay or Triac may have failed in the on state. This will probably show up with ohmmeter tests (with the oven unplugged!) but not always. Replacements should be readily available. If the problem is is the controller, it will be more difficult to diagnose as schematics for the controller are usually not readily available. However, it could be something simple like a bad connection or dirty connector. ----------------------------------------------------------------------------- @endnode @node 6.15 "Oven immediately starts to cook when door is closed" This means that the relay or triac which controls power to the microwave generator, or the controller or timer has failed in the on state. If the problem is the relay or triac, it is simple to diagnose and repair since the component can be easily identified and tested. If the problem is is the controller, it will be more difficult as schematics are usually not readily available. Check the relay or triac with an ohmmeter. Disconnect the output of the timer or controller and see if it still come on immediately. With a mechanical timer, repair may be possible. ----------------------------------------------------------------------------- @endnode @node 6.16 "Oven heats but power seems low" Some considerations are how old the oven is and did the problem happen suddenly or did it just gradually weaken over the years. First, are you sure the problem is real? Perhaps you are just a little less patient than you used to be. Perform a water heating test or try to pop a bag of popcorn using you usual time setting. See the section: @{"Testing the oven - the water heating test" link "7.1"} If you are subject to brownouts or are running on your own generator, the line voltage may be low. Power output is quite sensitive to the AC input - there is no regulation. A 10% drop in line voltage is likely to reduce microwave power output by more than 20%. The magnetron - the main microwave generating tube - can conceivably weaken with a lot of use like any other vacuum tube but other problems could be the causes as well. If you run the oven on HIGH, can you tell if it is actually heating continuously or rather it thinks you want LOW? Many microwave ovens make a clicking sound as they use a relay to switch microwave power on and off - see if you can hear this. You should not on HIGH except when you push START. Check to see if your model uses a 'mode mixer blade' or fan above the oven chamber (likely if it does not have a turntable). If this gets stuck due to gummed up lubrication or deterioration, then certain parts of the oven chamber will not be heated uniformly (to the extent that any microwave oven is uniform!) and apparent reduced heating may result. Clean and adjust as needed. There could be intermittent connections to the magnetron filament or possibly others. But, these would likely show up as erratic operation - no heat at all sometimes - not just a weak oven. Clean and tighten all connections at the magnetron, HV Diode and HV capacitor. Be sure to unplug the unit first and discharge the HV capacitor before touching anything. ----------------------------------------------------------------------------- @endnode @node 6.17 "Oven heats but shuts off randomly" Everything operates normally, but the oven shuts off after varying amounts of time. This could be a faulty magnetron, bad cooling fan (or just built up dust and grime block ventilation grilles) or thermal cutout, faulty controller, some other intermittent component, or bad connections. If resetting it allows cooking to resume immediately, if even for a few seconds, I would not suspect the magnetron or thermal problem as no cool down time is required. It could be a marginal door interlock switch or a controller problem. If the magnetron were shorting, a fuse would blow. If it were overheating, you would not be able to resume cooking until it cooled. If it just stopped working, everything would appear normal but there would be no heating. If it is not possible to resume cooking for a few minutes indicating that something needs time to cool off, then the magnetron could be faulty but check for the obvious cooling problems first: blocked or dirty ventilation grille. Determine if the magnetron cooling fan is operating by listening for its sound or looking through the ventilation opening in the back of the oven. If it is not, there could be a broken or weak belt, gummed up or lack of lubrication, other mechanical problems, a bad motor, or bad connections. ----------------------------------------------------------------------------- @endnode @node 6.18 "Fans or turntables that do not work" There are up to 4 motors in a microwave oven: 1. Magnetron cooling fan - always present 2. Mechanical timer (on inexpensive non-touchpanel or older units) 3. Turntable 4. Convection air circulation (combo units only) When any of these do not operate properly, the most likely causes are: o Bad connections - trace wiring and check continuity (unplugged, capacitor discharge) to motor terminals. o Gummed up lubrication/dry bearings. Check for free rotation of the affected part(s). Clean and lubrication as needed. Also confirm that there are no other mechanical problems (e.g., turntable improperly installed). o Loose or broken belt. Confirm that belt is properly installed. Test to determine if it is worn and flabby - stretch it by about 25%. It should return to its relaxed length instantly. Clean and/or replace if needed. o Bad motor. Disconnect one wire and check for continuity with an ohmmeter. If open, winding is bad but check for break at terminal which you can resolder. o Bad thermostat. Where a fan only runs when the oven is hot as in a microwave/convection oven, the thermostat or controller could also be at fault. Locate the thermostat and jumper across its terminals with power off. Plug the oven in and see if the fan now runs all the time or at least when the appropriate mode(s) are entered. ----------------------------------------------------------------------------- @endnode @node 6.19 "What to do if the door handle breaks off" Usually this happens at the places where the handle is screwed to the door. I would NOT recommend making the repair in any manner that compromises the shielding properties of the door. (I have visions of someone using 1/2" stove bolts through the door and handle which would definitely be a bad idea). Anything that penetrates the door seal is a potential hazard - likely a very small one but it is not worth the risk. Therefore, I would recommend staying with repairs that can be made totally externally unless there is no possibility of a change to the integrity of the door. For example, replacing the screws with similar sized screws that gripped better or using filler to reconstruct or strengthen the threaded holes would be acceptable. Plastic is generally tough to glue where a strong bond is needed and where the joint is subject to abuse. However, depending on the type of plastic, one or more of the following may work: semiflexible adhesive like windshield sealer, plastic cement (the kind that fuses the plastic, not model cement), Duco cement, PVC (pipe) cement, or even superglue (though it seems not all brands are equally effective). Make sure the surfaces to be glued are perfectly clean (remove any residual library paste if you tried that!) and provide a means of clamping the pieces until the bond sets up (adhesive tape and/or rubber bands may be all you need). Consider providing some reinforcements around the joint (i.e., plastic splints or sisters depending on your profession) for added durability. Replacement door handles and/or entire doors may be available from the manufacturer of the oven. Replacements for a few Panasonic models are even stocked by MCM Electronics (and no doubt other places as well). ----------------------------------------------------------------------------- @endnode @node 6.20 "Replacing the light bulb" If the oven light no longer works, believe it or not, a burned out light bulb is likely. You would think that something like replacing a light bulb would be trivial and self evident. Unfortunately, not always so with microwave ovens. Light bulbs may be typically located in any of 3 places: 1. Oven chamber - it may be behind a mesh grill requiring a screw or snap to be removed. This is the easiest. 2. Rear - the bulb may be in a recessed compartment accessible by removing a screw or two on the back of the oven. 3. Inside - it may be behind a non-removable grille requiring the removal of the cover. These are typically not your usual vanilla flavored appliance bulbs either. ----------------------------------------------------------------------------- @endnode @node 6.21 "Repairing damage to the oven interior" If spilled food - solid or liquid - is not cleaned up soon after the oven is used, it will tend to harden and carbonize. Not only will this be much more difficult to remove, but hot spots may develop and result in possible sparking, arcing, and damage to the interior paint. If this happens in the vicinity of the mica waveguide cover, it may be damaged as well. In addition, sometimes splatters may find their way above the waveguide cover and cause problems above the roof of the oven chamber in the waveguide. Needless to say, clean up spills and food explosions as soon as possible. Not only will it be easier, the chance of future expensive problems will be minimized. Once damage occurs - paint blisters and peels, or totally hardened impossible to remove carbon deposits - more drastic action is called for. To prevent arcing and sparking, the interior needs to be smooth. Sharp edges and hard carbon in particular creates places where electric field gradients can become great enough to cause problems. Thus the warning not to use any metal utensils in a microwave. Assuming cleaning does not work on the carbon - even after repeated attempts - carefully scrape it off with a blunt knife or other suitable tool. This will probably damage the paint. Use fine sandpaper to completely smooth out the metal and feather the edges of the paint in the immediate area. Special microwave oven cavity paint is available but I imagine that any non-flammable high temperature enamel will work just as well. Until you can obtain the paint, the oven will work fine but since the chamber is made of sheet steel, rust will set in eventually. So, do paint it. If the waveguide cover is damaged seriously - such that it no longer will prevent splatters from entering the waveguide, obtain replacement material, cut to fit. Leaving it larger than necessary is fine as well. Use a suitable bit in a hand drill to make holes in the mica for the mounting screws or plastic snaps. If the interior of the door is damaged seriously such that either it will not longer seal around the edge properly or that the mesh screening is breeched, a replacement will be required to assure continued safety with respect to minimizing microwave emissions. Microwave oven cavity paint, waveguide cover mica sheets, and even some replacement doors are available from the parts suppliers listed at the end of this document. For most ovens, parts like doors will need to be obtained direct from the manufacturer, however. ----------------------------------------------------------------------------- @endnode @node 6.22 "Microwave/convection oven problems" In addition to the microwave components, these ovens also include an air circulating fan and an electric heating element as well as a temperature sensing themister. Any of these can fail. A convection oven which shuts down after a couple of minutes during the preheat cycle with the temperature display (if any) stuck at LOW (even though the oven is hot when opened) may have a bad thermister temperature sensor. The overtemperature protection sensor is shutting the oven down. The termister will usually be accessible after removing the oven cover. It will be located centrally just above the oven ceiling duct. It is a two terminal device that looks like a tiny resistor or diode and may be mounted on a metal header fastened with a couple of screws. Remove and test with an ohmmeter. An infinite reading means it is bad. As a test, jumper a 50 K ohm potentiometer in place of the thermister. During preheat, as you lower the resistance of the pot you should see the temperature readout climb. The oven will then indicate READY when the simulated temperature exceeds the setpoint. Replacement thermisters are available from the oven manufacturer - about $20. If the convection preheat cycle never completes and the oven is cool when opened, then either the heating element is bad (test with an ohmmeter) or the relay controlling the heating element or the controller itself is bad. If the circulating fan runs off of the same relay and it is operating, then the problem must be the heating element. The heating element will be either a Calrod type (GE trade name?) which is a steel tube enclosing a Nichrome wire coil embedded in ceramic filler or a coiled Nichrome element strung between ceramic insulators. The former is probably only available from the oven manufacture, though it is worth trying an appliance parts distributor or a place like MCM electronics first. It may be possible to find a replacement Nichrome coil and form it to fit. Make sure the wire gauge and length are identical. The circulating fan is probably driven by a belt, which may break or deteriorate. Inspect the belt. If it is loose, cracked, or does not return to its normal length instantly after being stretched by 25% replace it. Check the fan motor and fan itself for adequate lubrication. Check the fan blades for corrosion and damage. ----------------------------------------------------------------------------- @endnode @node 7.1 "Testing the oven - the water heating test" The precise number of degrees a known quantity of water increases in temperature for a known time and power level is a very accurate test of the actual useful microwave power. A couple of minutes with a cup of water and a thermometer will conclusively determine if your microwave oven is weak or you are just less patient (or the manufacturer of your frozen dinners has increased their weight - sure, fat chance of that!) You can skip the heavy math below and jump right to the final result if you like. However, for those who are interested: o 1 Calorie (C) will raise the temperature of 1 gram (g) of liquid water exactly 1 degree Centigrade (DegC) or 9/5 degree Fahrenheit (DegF) o 1 Calorie is equal to 4.184 Joules (J) or 1 J = .239 C o 1 Watt (W) of power is 1 J/s or 1 KW is 1000 J/s o 1 cup is 8 ounces (oz) which is 8 x 28.35 g/oz = 226.8 g o 1 minute equals 60 s (but you know this!) Therefore, in one minute, a 1 KW microwave oven will raise the temperature of 1 cup of water by: T(rise) = (60 s * 1000 J/s * .239C/J * (g * DegC)/C)/(226.8 g) = 63 DegC. Or, if your prefer Fahrenheit: 114 DegF. To account for estimated losses due to conduction, convection, and imperfect power transfer, I suggest using temperature rises of 60 DegC and 109 DegF. Therefore, a very simple test is to place a measured cup of water in the microwave from the tap and measure its temperature before and after heating for exactly 1 minute on HIGH. Scale the expected temperature rise by the ratio of the microwave (not AC line) power of your oven compared to a 1 KW unit. Or, from a Litton microwave handbook: Heat one Liter (L) of water on HIGH for 1 minute. Oven power = temperature rise in DegC multiplied by 70. Use a plastic container rather than a glass one to minimize the needed energy loss to raise its temperature by conduction from the hot water. There will be some losses due to convection but this should not be that significant for these short tests. (NOTE: if the water is boiling when it comes out - at 100 DegC or 212 DegF, then the test is invalid - use colder water or a shorter time.) The intermediate power levels can be tested as well. The heating effect of a microwave oven is nearly linear. Thus, a cup of water should take nearly roughly twice as long to heat a specific number of degrees on 50% power or 3.3 times as long on 30% power as on full power. However, for low power tests, increasing the time to 2 minutes with 2 cups of water will result in more accurate measurements due to the long period pulse width power control use by microwave ovens which may have a cycle of up to 30 seconds. Any significant discrepancy between your measurements and the specified microwave power levels - say more than 10 % on HIGH - may indicate a problem. (Due to conduction and convection losses as well as the time required to heat the filament of the magnetron for each on-cycle, the accuracies of the intermediate power level measurements may be slightly lower). See the section: @{"Oven heats but power seems low" link "6.16"} ----------------------------------------------------------------------------- @endnode @node 7.2 "Testing and replacement of interlock switches" With power disconnected and after waiting ample time (a couple of minutes, but check with a meter to be sure, or discharge it with a resistor) put an ohmmeter across the AC input just before the interlocks but beyond the power relay or triac. Open and close the door several times - there should be no significant change in resistance and it should be more than a few ohms. If it approaches zero while opening or closing the door, the interlock switches and door alignment should be checked. (You may need to disconnect one side of the transformer primary since its resistance is a fraction of an ohm. Refer to the schematic pasted inside the cover.) Replace with switches having a precisely identical fit and equal or better electrical specifications (terminal configuration, current rating). When removing the old switch make a note as to where each wire goes. Check the embossed marking on the old switch - don't depend on location as your replacement might just have a different arrangement. Make sure the new switch aligns correctly with the actuating mechanism and then check for correct electrical operation with an ohmmeter before applying power. ----------------------------------------------------------------------------- @endnode @node 7.3 "Testing the high voltage components" Assuming it passes the above test for interlocks and door alignment, the triac (if used) may be defective. There could also be a wire shorting to the chassis. However, the most likely problems are in the microwave generator. First, with power disconnected, discharge the high voltage capacitor (see the section @{"Discharging the high voltage capacitor in microwave oven" link "4.5"}) An ohmmeter can be safely used to quickly determine if the capacitor, HV diode, or magnetron are a dead short (as well as for an open magnetron filament). Use an ohmmeter to test the diode and capacitor. While connected in circuit, the resistance in at least one direction should be several M ohms. (try in both directions, use the higher reading). Test the magnetron from the filament to chassis - it should be high in at least one direction. Test the filament for continuity - the resistance of a good filament is nearly 0. These may be considered to fail/no conclusion tests - they can definitively identify parts that are bad but will not guarantee that they are good. They may test ok with no voltage applied but then fail once operated in-circuit. Connections may open up when they heat up. More complete information is provided in the next few sections including replacement procedures. ----------------------------------------------------------------------------- @endnode @node 7.4 "Testing and replacing the HV diode" The resistance measured across the leads of the HV diode should be greater than 10 M ohm in at least one direction when disconnected from the circuit. However, since it is composed of multiple silicon diodes in series to get the voltage rating, testing with a DMM may not produce a definitive answer as to whether it actually works as a rectifier. The HV diode can be tested with an adjustable AC source (such as a Variac), resistor load, and AC voltmeter for proper behavior, at least at low voltages, or assume for now that the diode is good if it is not shorted - which is likely. If your DMM or VOM has a resistance scale operated off a 6 volt or higher battery, you may get a reading in one direction (but only one direction). Most HV diodes have press fit (Fast-On) or ring lugs so replacement is very straightforward. Discharge the high voltage capacitor. Make sure you get the polarity correct if your replacement can be installed either way. Putting the diode in backwards will result in positive instead of negative high voltage and, needless to say, no heat. ----------------------------------------------------------------------------- @endnode @node 7.5 "Testing and replacing the high voltage capacitor" (The following assumes no internal rectifier or other circuitry except of a bleeder resistor. Adjust procedures accordingly if your oven is different.) The resistance measure across the terminals of the high voltage capacitor should be very high - several M ohms for bleeder resistor. If it is less than 1 M ohms, the capacitor is definitely shorted. Yes, if you measure 0.00 ohms across the terminals (and they are not bussed together on the case), then the capacitor is positively, without a shadow of a doubt, bad! A high resistance does not prove that the capacitor is actually functional, just not shorted with no voltage across it. If you have a capacitance meter, check it for proper value (should be printed on the case). Even this does not prove that it will not short when full voltage is applied. Substitution is the only sure test beyond this. Before removing the old capacitor, double check that it is fully discharged (even a bad one may hold a partial charge). Make a diagram of the precise wiring as multiple connections are often made to the capacitor terminals. The capacitor is usually mounted with a clamp which is easily loosened. Sometimes, the capacitor is jammed into a location that requires moving some other components to extract it. Replace in reverse order. Tighten the clamp securely but not so much as to distort the case. ----------------------------------------------------------------------------- @endnode @node 7.6 "Testing and replacing the magnetron" The resistance should be infinite from filament connections to chassis and a fraction of an ohm between filament connections with the wiring disconnected from the magnetron. Evidence of arcing (visible blackening around ventilation holes in base or burnt odor) usually indicates a bad Magnetron. While measuring resistance from filament chassis, gently tap the magnetron to determine if there is an intermittent short. However, such problems may only show up once the filament heats up and parts expand. Possible failure modes: o Filament could be open - check with ohmmeter. o Filament could be shorted to case - check with ohmmeter. Anything less than infinity means the tube is bad though it could be charring due to arcing outside the vacuum in the box with the filament connections. o Filament could be shorted to itself - tough to test since it is such a low resistance to start. Compare with good magnetron. o Any of these could take place once the tube is powered. If there is HV and the filament is lit, there will be microwave power unless the magnetron has failed in a very strange manner (like one of the magnets fell off!). Magnetron replacement is generally straightforward but other assemblies like the cooling fan may need to be removed to gain access. Make careful notes of both the wiring and mechanical relationships. Usually, the magnetron is fastened to the waveguide with 4 nuts on studs. When removing it from its mounting, do not lose the RF gasket - a metal mesh ring which seals the connection against microwave leakage. Reuse it unless your replacement magnetron comes with a new one. Be careful - the magnetron is a vacuum tube and damage is possible from careless handling. Transfer any thermal protector to the new unit. Replace other components in reverse order and then reattach the filament/HV wires. NOTE: the old magnetron is fairly harmless but contains powerful magnets which are really nice toys but will suck all the bits right off your tapes and diskettes. There is no residual radiation! ----------------------------------------------------------------------------- @endnode @node 7.7 "Testing and replacing the high voltage transformer" Disconnect terminals as required to make the following tests. The resistances of the primary should be .1 to .5 ohms (.2 ohms typical) The resistance of the filament windings will likely be so low as to be detectable with your multimeter. The only measurement easily made would be that there is no short to the chassis. Typical resistance readings for the transformer secondary are in the 25 to 150 ohms range (depending on the power rating of the oven) from HV connection to chassis. A typical midsize might be 65 ohms. An open would be an obvious failure. However, based on the way these are wound, a winding-winding short would not cause enough of a resistance change to be detected with an ohmmeter unless you could compare with an identical model transformer from the same lot number. Testing the high voltage transformer more fully is difficult without fancy equipment. Only major short circuits can be identified in the transformer with an ohmmeter since the nominal resistance of the windings is unknown. However, open windings (not very likely) can be located and other faults can be located by the process of elimination: If the other components - HV diode, Hv capacitor, magnetron - test out, remove the high voltage connections to the transformer, power up the oven, and initiate a cook cycle. If the fuse now does not blow, there are still problems in the high voltage components - possibly something is failing only when full voltage is applied. If the fuse still blows, then the problem is likely with the triac (if used), a shorted wire, or shorted transformer. Unplug the oven, discharge the HV capacitor. Check for damaged wires that may be shorting to the chassis. Repair or replace as necessary. If your oven uses a triac, remove and bypass it. Now, if the fuse blows when the oven is plugged in, the problem is quite possibly with the transformer. Replacement is straightforward but other assemblies may be using the transformer bolts for their mounting and/or may block your way. Confirm that the high voltage capacitor is discharged. Make notes as to wiring and try not to drop the transformer on your foot! ----------------------------------------------------------------------------- @endnode @node 7.8 "Testing for bad connections (and replacing them with good ones)" Inspect the wire crimps and terminal lugs for corrosion or being loose - you should have to work at pulling them off. This is mostly a problem at the magnetron filament terminals as these are high current (many amps to heat the filament). Sometimes, the old lugs can be carefully squished - just a little - with a pair of pliers so that the connection is tight. However, a badly corroded terminal should be cleaned with fine sandpaper and the lug should be replaced on the wire. Alternatively, carefully drill a hole for a screw in the male terminal (e.g., filament connection on the magnetron) and use a nut and bolt to reattach the wire. ----------------------------------------------------------------------------- @endnode @node 7.9 "Testing and replacing the triac" Nearly all triac failures will be shorts. Thus, measuring across the MT1 and MT2 terminals of the triac (the power connections) should read as a high resistance with a multimeter. A few ohms means a bad triac. Triacs can fail in other - possibly peculiar ways - so substitution or bypassing may be necessary to rule out all possibilities. Replacement is very straightforward - just don't get the wires mixed up. ----------------------------------------------------------------------------- @endnode @node 7.10 "Testing and replacing the power relay" If the relay is totally inoperative, test for voltage to the coil. If the voltage is correct, the relay may have an open coil. If the voltage is low or zero, the coil may be shorted or the driving circuit may be defective. If the relay makes a normal switching sound but does not correctly control its output connections, the contacts may be corroded, dirty, worn, welded closed, binding, or there may be other mechanical problems. Remove the relay from the circuit (if possible) and measure the coil resistance. Compare your reading with the marked or specified value and/or compare with a known working relay of the same type. An open coil is obviously defective but sometimes the break is right at the terminal connections and can be repaired easily. If you can gain access by removing the cover, a visual examination will confirm this. If the resistance is too low, some of the windings are probably shorted. This will result in overheating as well as no or erratic operation. Replacement will be required. The resistance of closed contacts on a relay that is in good condition should be very low - probably below the measurable limits on a typical multimeter - a few milliohms. If you measure significant or erratic resistance for the closed contacts as the relay is switched or if very gentle tapping results in erratic resistance changes, the contacts are probably dirty, corroded, or worn. If you can get at the contacts, the use of contact cleaner first and a piece of paper pulled back and forth through the closed contacts may help. Superfine sandpaper may be used as a last resort but this is only a short term fix. The relay will most likely need to be replaced if as in this case the contacts are switching any substantial power. ----------------------------------------------------------------------------- @endnode @node 8.1 "How safe is a repaired microwave oven?" So you fixed up Aunt Minnie's Radar Range or picked up a microwave at a yard sale or scavenged one off the curb. The only problem you could find was a blown fuse or truly horrible mess of decayed burnt on food. How safe is it to use? As long as there is no serious damage to the door (a 6 inch hole would quality as serious damage) and the door fits square, it should be properly sealed. As long as the waveguide is tightly mounted and undamaged, there should be no leakage from there. Make sure the metal cover has all its fingers engaged around the front (though with a properly installed magnetron, there should be minimal microwave leakage into the electronics bay). An inexpensive leakage tester - around $8 - will not be as sensitive or accurate as the $500 variety by may provide some peace of mind. The main thing is the door seal. ----------------------------------------------------------------------------- @endnode @node 8.2 "Problems with running a microwave oven with metal inside or totally empty" Metal in microwave ovens may or may not be a problem depending on the specific situation. Sharp edges and points create strong field gradients which tend to spark, arc, or create other fireworks. With some food in the oven to absorb the power, this is probably not likely to damage the oven. You will note that some ovens come with metal fixtures in addition to the oven walls themselves (e.g., Sharp convection/microwave combo). Having absolutely nothing in the oven chamber or just metal is the potentially more likely damaging situation for the magnetron as you are dumping several hundred W to over a KW of power into a reflective cavity with no load. In the worst case, you could end up with a meltdown inside the waveguide requiring replacement of various expensive components including the magnetron. ----------------------------------------------------------------------------- @endnode @node 8.3 "Microwave ovens and grounded dedicated circuits" A microwave oven should be used only on a properly wired 3 wire grounded circuit. Check with a circuit tester to make sure your 3 prong outlet is correctly wired. Many are not. Install one if it is not grounded. There is a very important safety reason for this requirement: the return for the high voltage is through the chassis. While unlikely, it is theoretically possible for the entire high voltage to appear on the metal case should certain internal connections come loose. With a properly grounded outlet, this will at most blow a fuse. However, with the case floating, a shocking (or worse) situation could develop - especially considering that microwave ovens are usually situated near grounded appliances like ranges and normal ovens and wet areas like kitchen sinks. A dedicated circuit is desirable since microwave ovens are significant users of power. Only about 50% of the electricity used by a microwave oven actually gets turned into microwaves. The rest is wasted as heat. Therefore, a 700 W oven will actually use up to 1400 W of power - nearly an entire 15 Amp circuit. Convection ovens have heating elements which are similar energy hogs. A least, do not put your refrigerator on the same circuit! ----------------------------------------------------------------------------- @endnode @node 8.4 "Microwave ovens and GFCIs" A Ground Fault Circuit Interrupter (GFCI) protects people from shocks should a situation develop where an accessible part of an appliance should short to a live wire. Touching this may result in a shock or worse. A GFCI detects any difference between the currents in the Hot and Neutral wires and shuts off the power should this difference exceed a few mA. A GFCI is not needed with a properly grounded microwave oven as any such fault will blow a fuse or trip a circuit breaker. In most cases, it will not hurt tp jave a GFCI as well. However, with some combinations of oven design and your particular wiring, due to the highly inductive nature of the high voltage transformer, nuisance tripping of the GFCI may occur when you attempt to cook anything - or at random times. However, this usually does not indicate any problem. Plug the oven into a properly grounded circuit not on a GFCI. ----------------------------------------------------------------------------- @endnode @node 8.5 "Taking a microwave oven oversees (or vice versa)" Microwave ovens are high power appliances. Low cost transformers or international voltage adapters will not work. You will need a heavy and expensive step down or step up transformer which will likely cost as much as a new microwave oven. Sell the oven before you leave and buy a new one at your destination. ----------------------------------------------------------------------------- @endnode @node 8.6 "Using the control panel from defunct microwave oven as an electronic timer" It is usually possible to remove just the touchpad and controller board to use as a stand-alone timer with a switched output. Be careful when disconnecting the touchpanel as the printed flex cable is fragile. With many models, the touchpanel (membrane touchpad) needs to be peeled off of the front plastic panel or the entire assembly can be removed intact. The output will control a 10-15 A AC load using its built in relay or triac (though these may be mounted separately in the oven). Note that power on a microwave oven is regulated by slow pulse width modulation - order of a 30 second cycle if this matters. If it uses a triac, the triac is NOT phase angle controlled - just switched on or off. ----------------------------------------------------------------------------- @endnode @node 9.1 "Advanced troubleshooting" If the solutions to your problems have not been covered in this document, you still have some options other than surrendering your microwave to the local service center or the dumpster. Unlike most other types of consumer electronic equipment, a service manual is rarely required. A sufficiently detailed schematic is nearly always pasted to the inside of the cover and includes all power components, interlocks, fuses, protectors, and wiring. This is entirely sufficient to deal with any problems in the microwave generator. No adjustments or alignment should even be required so detailed procedures for these are not needed. However, when tackling electronic faults in the controller, a service manual with schematics will prove essential. Whether these are available depends on the manufacturer. For legal reasons, some manufacturers are reluctant to sell service information or replacement parts for microwave ovens. They are concerned with litigation should an unqualified person be injured or killed. ----------------------------------------------------------------------------- @endnode @node 9.2 "Suggested Reference" I know of at least one book dealing specifically with microwave oven repair. It is very complete and includes many actual repair case histories. There is a good chance that your specific problem is covered. Microwave Oven Repair, 2nd Edition Homer L. Davidson TAB Books, a division of McGraw Hill, Inc. Blue Ridge Summit, PA 17294-0850 This may be available at your public library (621.83 if your library is numbered that way) or from a technical bookstore. ----------------------------------------------------------------------------- @endnode @node 9.3 "Cost of repair parts" Assuming you have located one or more bad components, the question is whether an oven that is a few years old is worth fixing. Typical parts cost for generic replacements: o HV diode: $2-5 (except for the bolt-on variety which can range up to $50. It should be possible to replace these with the $2 variety with wire leads) o Power fuse: $.40 o HV Capacitor: $10-20 o Magnetron: $30-100. Common generic replacements are $30-40 o Overtemperature thermostat (thermal protector): $4.50 o Interlock Switch: $2.50 o Triac: $12.00 Parts suppliers like MCM Electronics can provide these components to fit the vast majority of microwave ovens. Touchpads and controller parts like the microprocessor chip are usually only available from the manufacturer of the oven. Prices are high - a touchpad may cost $20-30 or more. Sensors and other manufacturer specific parts will be expensive. While the HV transformers are fairly standard, they are not readily available from the common replacement parts sources. However, they do not fail that often, either. With the prices of microwave ovens dropping almost as fast as PCs, a few year old oven may not be worth fixing if the problem is a bad magnetron or touchpad. However, except for a slight decrease in power output as the oven is used over the years and the magnetron ages, there is little to go bad or deteriorate. Therefore, you can expect a repaired oven to behave just about like new. ----------------------------------------------------------------------------- @endnode @node 9.4 "Interchangeability of components" Q: If I cannot obtain an exact replacement or if I have another microwave oven carcass gathering dust, can I substitute a part that is not a precise match? Sometimes, this is simply desired to confirm a diagnosis and avoid the risk of ordering an expensive replacement and/or having to wait until it arrives. For safety related items, the answer is generally NO - an exact replacement part is needed to maintain the specifications within acceptable limits with respect to line isolation, radiation emission, and to minimize fire hazards. For microwave ovens such parts include the power fuses, interlock switches, and anything else that could potentially lead to microwave radiation leakage - like a magnetron which did not fit the waveguide properly. Fortunately, while an exact match may be required, it doesn't have to be from the original manufacturer - most parts are interchangeable. Thus the organs from that carcass may be able to provide renewed vitality to your ailing microwave. Here are some guidelines: 1. Fuses - exact same current rating and at least equal voltage rating. This will probably be a ceramic 1-1/4" x 1/4" 15 or 20 A 250 V fast blow type. For the repair, use an exact replacement. For testing only, a similar type may be used. 2. Thermal protectors - same temperature and maximum current rating. You must be able to mount it securely and flush against the same surface as the old one. 3. Interlock switches - must have the same terminal configuration and at least equal current rating. Of course, a secure fit is very important as well for it to perform its safety function. Many of these are interchangeable. 4. HV capacitor - similar (within 5%) and at least equal working voltage. Note that the working voltage rating of these capacitors is not consistent with the way capacitors in other electronic equipment are specified and is usually the RMS voltage of the AC input from the HV transformer. There- fore, it is not possible to substitute something from your junkbox unless it is from a microwave oven. In addition, this is one situation where higher capacity(uF) is not better. The power output is related to capacitance. Thus, the value should be matched fairly closely or else other parts may be overloaded. A smaller one can be used for testing, however. 5. HV diode - most of these have similar electrical ratings so a substitution is possible if you can make it fit physically. This would be particularly desirable where your oven has one of those chassis mount $50 dollar varieties - it may be ok to use a $2.75 generic replacement. 6. Relays and triacs - substitutes will generally work as long as their specifications meet or exceed those of the original. Creative mounting may be required. 7. Magnetrons - a large number of microwave ovens use the same basic type but the mounting arrangement - holes vs. studs, orientation of the cooling fins, etc., differ. If you can safely substitute a not exact match for testing purposes IF you can make it fit the waveguide securely without gaps. However, if the cooling fins end up being on the wrong side, it will heat up very quickly - 50% of the input power goes to heat. 8. HV transformer - same (within 5%) voltage and at least equal current rating. Mounting should not be a problem but don't just leave it loose - you could end up with a disaster. 9. Fans and motors - speed/power and direction must match and mounting must be possible. Speed isn't so critical for a turntable but for a magnetron cooling fan, inadequate air flow will result in overheating and shutdown or failure. 10. Mica waveguide cover - cut to match. 11. Turntable and mode mixer components - if they fit, use them. 12. Light bulb - similar ratings and base. 13. Temperature sensors, thermistors, etc. - depends on the particular model. 14. Mechanical timers - compatible switching and mounting arrangement. 15. Cordsets - must be 3 wire heavy duty grounded type. Make sure the replacement has at least as high a current rating as the original. Observe the color code! 16. Controller and touchpad - small parts like resistors, diodes, capacitors, and so forth can often be substituted. Forget about the controller ICs or display. The touchpad is likely to be custom both electrically and physically as well unless you have a similar model microwave to cannibalize. ----------------------------------------------------------------------------- @endnode @node 9.5 "Repair parts sources" For general electronic components like resistors and capacitors, most electronics distributors will have a sufficient variety at reasonable cost. Even Radio Shack can be considered in a pinch. However, places like Digikey, Allied, and Newark do not have the specialized parts like magnetrons, HV capacitors and diodes, interlock switches, thermal protectors, etc., needed for microwave oven repair. The following are good sources for consumer electronics replacement parts including common microwave oven parts: MCM Electronics (VCR parts, Japanese semiconductors, U.S. Voice: 1-800-543-4330. tools, test equipment, audio, consumer U.S. Fax: 1-513-434-6959. electronics including microwave oven parts and electric range elements, etc.) Dalbani (Excellent Japanese semiconductor source, U.S. Voice: 1-800-325-2264. VCR parts, other consumer electronics, U.S. Fax: 1-305-594-6588. Xenon flash tubes, car stereo, CATV). Int. Voice: 1-305-716-0947. Int. Fax: 1-305-716-9719. Premium Parts (Very complete VCR parts, some tools, adapter U.S. Voice: 1-800-558-9572. cables, other replacement parts.) U.S. Fax: 1-800-887-2727. ----------------------------------------------------------------------------- @endnode