Contents:
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. Bad interlock switches probably account for the majority of microwave oven problems. 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). 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. See the section: "Testing and replacing of interlock switches". No adjustments should ever be required for a microwave oven and there are no screws to turn so don't look for any!
The following problems are likely power or controller related and not in
the microwave generator unless due to a blown fuse or bad/intermittent
connections:
* Totally dead oven.
* No response to any buttons on touchpad
* Oven runs when door is still open.
* Oven starts on its own as soon as door is closed.
* Oven works but display is blank.
* Whacked out controller or incorrect operation.
* Erratic behavior.
* Some keys on the touchpad do not function or perform the wrong action.
* Microwave oven does not respond to START button.
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.
If this does not help, there is likely a problem with the controller circuitry
or its power and you will have to get inside the oven.
Some cockroaches (or other lower life forms) may have taken up residence on the controller circuit board. It is warm, cozy, safe, and from their point of view makes an ideal habitat. If you got the microwave oven from a flea market, garage sale, the curb, a relative, or friend, or if your kitchen isn't the cleanest in the world, such visitors are quite possible. Creatures with six or more legs (well, some two legged varieties as well) are not known for their skills in the areas of housekeeping and personal hygiene. Clean the circuit board and connectors thoroughly with water and then isopropyl alcohol. Dry completely. Inspect the circuit traces for corrosion or other damage. If there are any actual breaks, these will have be be jumpered with fine wire and then soldered. Hopefully, no electronic components were affected though there is always a slight possibility of other problems.
First, check power to the outlet using a lamp or radio you know works. The fuse or circuit breaker at your service panel may have blown/tripped due to an overload or fault in the microwave oven or some other appliance. You may just have too many appliances plugged into this circuit - microwave ovens are high current appliances and should be on a dedicated circuit if possible. If you attempt to run a heating appliance like a toaster or fryer at the same time, you *will* blow the fuse or trip the circuit breaker. A refrigerator should never be plugged into the same circuit for this reason as well - you really don't want it to be without power because of your popcorn! If you find the fuse blown or circuit breaker tripped, unplug everything from the circuit to which the microwave is connected (keep in mind that other outlets may be fed from the same circuit). Replace the fuse or reset the circuit breaker. If the same thing happens again, you have a problem with the outlet or other wiring on the same branch circuit. If plugging in the microwave causes the fuse to blow or circuit breaker to trip immediately, there is a short circuit in the power cord or elsewhere. Next, 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". Some models have a thermal fuse as well and this may have failed for no reason or a cooling fan may not be working and the oven overheated (in which case it probably would have died while you were cooking something for an important guest - assuming you would use a microwave oven for such a thing!). 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. Hopefully, only components on the primary side of the power transformer will be affected. In some cases, circuit board traces may have been vaporized (but repair may still be possible by simply jumpering across the crater). Assuming that the main fuse checks out, then check the power supply for the controller next. Also check for bad solder connections.
There can be many causes for this behavior (or lack of behavior): * Door is not closed - on many ovens, there will be no response to any buttons - even setting the clock - unless the door is securely closed. * You waited too long - some models (like Sharp) have a timeout. If you close the door but don't proceed to activate any functions with a couple of minutes, they will require you to open and close the door to reset their pathetic brains. * Controller is confused - a power surge or random non-reproducible action of the universe may have resulted in the controller's program ending up in an infinite loop. Pull the plug for a minute or two to reset it. * Defective interlock switches - this can result in the controller thinking the door is open and ignoring you. * Faulty controller or its power supply - a power surge may have damaged the electronics. Other than checking for bad connections and obviously bad power supply components, diagnosing this will be tough without a schematic (and possibly much more). * Touchpad or controller board contaminated by overenthusiastic cleaning - if you recently power washed the oven (or even if you only use some spray cleaner), some may have gotten inside and shorted out the touchpad or controller. * Defective or damage touchpad - physical abuse is not a recommended technique for getting a microwave oven to cooperate. If there is any visible damage to the touchpad - the outer film is broken - it will probably need to be replaced. Also see the section: "Some of the keys on the touchpad do not function or perform the wrong action".
WARNING: Needless to say, DO NOT operate the oven with the door open! While extremely unlikely, the microwave be generator could be running! For microwaves to actually be generated with the door still open would require the failure of all 3 interlock switches. The only way this could really happen would be for the 'fingers' from the door that engage the interlocks to break off inside the oven keeping the interlocks engaged. In this case, the controller would think the door was always closed. Where no such damage is evident, a failure of this type is extremely unlikely since power to the microwave generator passes through 2 of the 3 interlock switches. If both of these failed in the closed position, the third switch would have blown the fuse the last time the door was opened. Another more benign possibility is that one or more fans are running as a result of either a defective sensor or normal operation to maintain air flow until all parts have cooled off.
If the oven starts up as soon as the door is closed - regardless of whether a cook cycle has been selected, the cause could be a shorted triac or relay or a problem with the controller or touchpad. First, unplug the oven for a couple of minutes to try to reset the controller. If this doesn't help, put a cup of water into the oven and let it run for a minute to check for heating. (You could also note the normal sound change or slight dimming of lights that accompanies operation of the magnetron.) Much more must be enabled to actually power the magnetron so this might point more to the controller as being faulty but not always. Also see the section: "Whacked out controller or incorrect operation".
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. Try pulling the plug for a minute or two - for some reason the display portion of the controller may have been sent out to lunch by a power surge or alpha particle. It woudn't be the first time. Check for bad connections between the display panel and the power supply and solder joints on the controller board. 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.
The following are some of the possible symptoms: * All the display digits may have come on, EEEE or FFFF, or be displaying in Greek. * The end-of-cooking cycle or 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). * Pressing a button on the touchpad may result in a totally incorrect action such as entering the time resulting in the oven starting to cook. However, for the special case where pressing START results in erratic behavios, see the section: "Erratic behavior". * The oven may start cooking (or at least appear to) as soon as the door is closed. Pressing buttons on the touchpad may or may not have any effect. (This could also be a shorted triac or power relay). First, try unplugging the oven for a couple of minutes - perhaps the controller is just confused due to a power surge, lightning strike or the EMP from a nearby nuclear detonation 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". If the oven seems to have a mind of its own - running a cycle you didn't think you programmed, are you sure a previous cook cycle was not interrupted and forgotten? Try to recreate the problem using a cup of water as a load. 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. 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. 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.
There are three different situation: * Whenever the oven performs unexpectedly both during setup and the cook cycle, suspect the controller power supply or bad connections. * Where problems only occur when entering or during the cook cycle, suspect a power relay or mechanical timer (if used) with dirty or worn contacts, or (less likely) the power surge from energizing the microwave generator or microwave (RF) leakage into the electronics bay affecting the controller. * However, if erratic simply means that it doesn't heat consistently, see the section: "Oven heats but power seems low or erratic". 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. There may be bad connections or loose lugs. The primary relay may have dirty or burnt contacts resulting in erratic operation. If the oven uses a HV relay for power control, this may be defective. If the times and power levels appear on the display reliably but then become scrambled when entering the cook cycle or the oven behaves strangely in some other way when entering the cook cycle, there are several possibilies: * The power surge caused by the cook cycle starting is resulting in changes to the settings or else the microcontroller is not interpreting them properly. This may be due to a faulty part of bad connections in the controller or elsewhere. As with intermittent problems, a thorough search for loose ground and other connections and bad solder joints may locate the source of the difficulty. * Microwave (RF) leakage into the electronics bay due to an faulty joint between the magnetron and the waveguide or structure failure of the magnetron may be interfering with the operation of the microcontroller. Unless the oven was dropped or 'repaired' by an butcher, this sort of failure is unlikely. If you suspect either of these, inspect the integrety of the magnetron-waveguide joint and make sure the RF gasket is in place. Unfortunately, this is sometimes difficult to pinpoint because unless there is obvious mechanical damage, the 'problem' may disappear once the cover is removed for testing. See the section: "Problems with internal microwave leakage". * On rare occasions, the main fuse may become intermittent rather than failing completely. The surge or vibration of starting can jiggle the element open or closed. It is easy to try replacing it!
(From: Charles Godard (cgodard@iamerica.net)). I only service Amana's, but have serviced lot's of them over the years. I've only found a few that leaked with my expensive leak detector. The most memorable was the one with the leak that was due to the copper gasket that's between the magnetron tube and the cavity. I just reformed the gasket and reseated the magnetron and that fixed the leak. The symptom was that the Touch Pad timer lights and indicators would change while the unit was cooking. I thought I had a timer problem. I took it apart and checked for loose solder joints and even cleaned the glass touch pad contacts. For some reason that I don't remember now, I checked for radiation with the cover off the unit and found it extremely high. It turned out that the radiation was affecting the controller. From the outside, with the cover on, the unit didn't leak. Long ago, I tried one of the cheapie detectors because one of my parts supply houses suggested it, and it detected leaks on everything. After that I shelled out the bucks and bought a real detector. (From: Matthew Sekulic (goatboy@telusplanet.net)). I have had a similar experience with a Sanyo, similar symptoms, but with the leakage from the spot welded waveguide inside the unit. Our calibration meter showed a two watt leakage, with none escaping the outer case when attached. (My worst case of actual external leakage was from a misaligned door at .75 watts with the probe's styrofoam spacer placed against the door, of course dropping off to near zero a few inches away. My clue in was a spark between the waveguide and the case, when I was messing with the Controller PCB.)
Touchpads are normally quite reliable in the grand scheme of things but can fail as a result of physical damage (your spouse threw the roast at the oven), liquid contamination (from overzealous cleaning, for example), or for no reason at all. 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. Some people have reported at least temporary improvement by simple peeling the touch pad off of the front panel and flexing it back and forth a few times. Presumably, this dislodges some bit of contamination. I am skeptical as this could just be a side effect of a bad connection elsewhere. 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! If the problem was the result of a spill into the touchpad, replacement will probably be needed. However, if you have nothing to lose, and would dump it otherwise, remove the touchpad entirely and wash it in clean water in an effort to clear out any contamination, then do the same using high purity alcohol to drive out the water, and then dry it out thoroughly. This is a long shot but might work.
While all other functions operate normally including clock, cook time, and power setting, pressing START does nothing, including no relay action and the timer digits do not count down. It is as though the START button is being totally ignored. (However, if there is a momentary response but then the oven shuts off, see the section: "Erratic behavior". If there is an alternate way of activating the cook cycle, try it. For example, Sharp Carousel IIs have a 'Minute Plus' button which will cook for one minute on HIGH. Use this to confirm the basic controller logic and interlock circuitry. If it works, then the problem may indeed be a faulty START button. If it is also ignored, then there may be a bad interlock or some other problem with the controller. Check for bad interlocks or interlocks that are not being properly activated. Next confirm if possible that the START touch pad button is not itself faulty. If you can locate the matrix connections for this button, the resistance should go down dramatically (similar to the other buttons). See the section: "Some of the keys on the touchpad do not function or perform the wrong action" The START button does, after all, sees quite a lot of action! Assuming it is not the touch pad, it sounds like the controller is either not sensing the start command or refusing to cooperate for some reason - perhaps it thinks an interlock is open. Otherwise, the timer would start counting. Testing the relay or triac control signal will likely show that it is not there. Check that there are no missing power supply voltages for the controller and bad connection.
Failures in the microwave generator can cause various symptoms including: * No heat but otherwise normal operations. * Fuse blows when closing or opening door. * Loud hum and/or burning smell when attempting to cook. * Arcing in or above oven chamber. * Fuse blows when initiating cook cycle. * Fuse blows when microwave shuts off (during or at end of cook cycle). * Oven heats on high setting regardless of power setting. * Oven immediately starts to cook when door is closed. * Oven heats but power seems low or erratic. * Oven heats but shuts off randomly. Most of these are easy to diagnose and the required parts are readily available at reasonable prices.
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. Some models may have a separate high voltage fuse. If this is blown, there will be no heating but no other symptoms. However, high voltage fuses are somewhat rare on domestic ovens. A number of failures can result in the fuse NOT blowing but still no heat: * Bad connections - these may be almost anywhere in the microwave generator or the primary circuit of the HV transformer. A common location is at the crimp connections to the magnetron filament as they are high current and can overheat and result in no or intermittent contact. See the section: See the section: "Testing the magnetron". * Open thermal protector - usually located on magnetron case. Test for continuity. It should read as a dead short - near zero ohms. See the section: "Testing thermal protectors and thermal fuses". * Open thermal fuse - some ovens have one of these in the primary circuit. It may be in either connection to the HV transformer or elsewhere. Test for continuity. It should read as a dead short - near zero ohms. * Open HV capacitor - see the section: "Testing the high voltage capacitor". A shorted HV capacitor would likely immediately blow the fuse. * Open HV diode - see the section: "Testing the high voltage diode". * Open magnetron filament - This failure may also be due to loose, burnt, or deteriorated press (Fast-on) lugs for the filament connections and not an actual magnetron problem. See the section: "Testing the magnetron". * Open winding in HV transformer. See the section: "Testing the high voltage transformer". * Defective HV relay. A few models use a relay in the actual high voltage circuitry (rather than the primary) to regulate cooking power. This may have dirty or burnt contacts, a defective coil, or bad connections * Shorted HV diode - see the section: "Testing the high voltage diode". * Short or other fault in the magnetron - see the section: "Testing the magnetron". * Short in certain portions of the HV wiring. See the section: "Testing and repairing the wiring and connections". A shorted HV diode, magnetron, or certain parts of the HV wiring would probably result in a loud hum from the HV transformer but will likely not blow the main fuse. (However, the HV fuse - not present on most domestic ovens - might blow.) Depending on design, a number of other component failures could result in no heat as well including a defective relay or triac, interlock switch(s), and controller.
This means that the main fuse in the microwave (or less commonly, the fuse or circuit breaker for the power outlet) pops when the microwave oven door is closed or opened. This may be erratic, occurring only 1 out of 10 times, for example. The cause is almost certainly related to either the door interlock switches or the door itself. Marginal door alignment, broken 'fingers' which operate the switches, dislocated parts in the interlock mechanism, or a defective interlock switch may result in either consistent or erratic behavior of this type. On some ovens, this can happen at any time regardless of the control panel settings or whether the oven is in the cook cycle or not. On others, it can only happen when interrupting the cook cycle by opening the door or when initiating the cook cycle from the front panel (if the switches are in the wrong state). The rational for this basic design - some form of which is used in virtually all microwave ovens - is that a defect in the interlock switches or door alignment, which might result in dangerous microwave radiation leakage, will produce a hard permanent failure. This will prevent the oven from being used until it is inspected and repaired. * As noted, one of the interlock switches is actually across the power line. If the switches are activated in the wrong sequence due to a misaligned door, that switch will not turn off before the other switches turn on shorting the power line. Similarly, if its contacts are welded closed, the power line will be shorted when the other switches close. See the section: "Testing and replacing of interlock switches". * Inspect the door, its mounting, and the plastic 'fingers' which operate the interlock switches as well. Again, if the sequence is not correct, the power line will be shorted blowing the fuse. If the oven was dropped, then such damage is quite likely. Look for broken or dislocated parts, warpage, and other indications of problems with the door and interlock mechanism Of course, if the oven was dropped, there could be much more extensive internal damage as well.
A loud abnormal hum 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. There may be a burnt odor associated with this behavior:
* Shorted HV diode - see the section: "Testing the high voltage diode".
* Shorted magnetron (filament to anode) or other internal fault in the
magnetron - see the section: "Testing the magnetron". Arcing within
the Magnetron case (visible through ventilation holes in the bottom
section) is usually an indication of a bad magnetron.
* Other short resulting from frayed insulation or wires touching in the
microwave generator.
* Shorted HV transformer - see the section: "Testing the high voltage transformer".
* Short resulting from burnt on food (usually) in or around the waveguide.
If the odor is coming from the oven chamber, see the section: "Arcing in or above oven chamber".
The following procedure will quickly identify the most likely component if
the problem is not food/spills/carbon related:
(Usually a loud hum is caused by a short in the HV transformer, HV diode, or
magnetron. The other items listed below would likely blow the main fuse but
possibly not always.)
(Portions from: Tony (tonyb@ramhb.co.nz)).
1. Discharge HV capacitor! (If there is a short it is doubtful if it has any
charge but never hurts to be safe).
2. Remove one end of the lead from the HV capacitor to the transformer.
3. Start the oven.
* Hum gone? If so, it is the HV circuitry, go to step 4.
* If it still hums you probably have a faulty HV Transformer. (Not
uncommon.)
4. Discharge the HV capacitor again, reconnect wire and disconnect the 2 wires
to the magnetron.
5. Restart oven.
* Hum Gone? If so, magnetron is shorted. Replace or get a new oven.
* Hum still there? If so, go to step 6.
6. You have either
* Shorted HV capacitor,
* Shorted HV Diode,
* Shorted clamp diode across the HV Cap terminals (if one is present, about
30% of microwave ovens use these). (The oven will run 100% without this
protection for the HV capacitor but it should be replaced if possible.)
* Some older Panasonic ovens have a HV reed switch and these can also short
but these are rare now because of the age.
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