Notes on the Troubleshooting and Repair of Microwave Ovens


  6.2) 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.  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!

  6.3) General system problems

The following problems are likely power or controller related and not in
the microwave generator unless due to a blown fuse or bad/intermittent

    * 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.

  6.4) Uninvited guests

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.

  6.5) Totally dead oven

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.

  6.6) No response to any buttons on touchpad

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

* 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

Also see the section: "Some of the keys on the touchpad do not function or perform the wrong action".

  6.7) Oven runs when door is still open

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.

  6.8) Oven starts on its own as soon as door is closed

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".

  6.9) 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.

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.

  6.10) Whacked out controller or incorrect operation

The following are some of the possible symptoms:

* All the display digits may have come on, EEEE or FFFF, or be displaying in

* 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.

  6.11) Erratic behavior

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

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!

  6.12) Problems with internal microwave leakage

(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

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.)

  6.13) Some of the keys on the touchpad do not function or perform the wrong action

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.

  6.14) Microwave oven does not respond to START button

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.

  6.15) Microwave generator problems

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.

  6.16) 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.

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.

  6.17) Fuse blows when closing or opening door

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

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

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.

  6.18) Loud hum and/or burning smell when attempting to cook

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

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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Written by Samuel M. Goldwasser. | [mailto]. The most recent version is available on the WWW server http://www.repairfaq.org/ [Copyright] [Disclaimer]