Notes on the Troubleshooting and Repair of Microwave Ovens


  6.19) Arcing in or above oven chamber

There is often a simple cause:

* 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 which can lead to more
  expensive damage.  Take extra care to cover all food (which you should do
  anyhow) until the waveguide cover is replaced.

* Any sharp metal edges may also result in arcing or sparking.  However, the
  only way such damage could occur as part of the oven (not added knives or
  forks!) would be through physical abuse.

* If your oven uses a stirrer above the oven chamber (no turntable), it may be
  stuck.  The result will be an uneven distribution of microwave energy and
  localized heating, arcing, and possibly melting plastic or metal.

  6.20) Fuse blows when initiating cook cycle

The fuse may only blow when actually attempting to cook but depending on
design, triacs and/or door switches may always be live and 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):

* Defective interlock switches or misaligned door.  At least one of the
  interlock switches is across the power line and will blow the fuse if not
  activated in the correct sequence.  See the sections: "Fuse blows when closing or opening door" and "Testing and replacing of interlock switches".

* Shorted HV capacitor.  See the section: "Testing the high voltage capacitor".

* Shorted HV diode (see note below).  See the section: "Testing the high voltage diode".

* Shorted magnetron (filament to anode - see note below).  See the section:
  "Testing the magnetron".

* Defective triac (shorted or partially shorted).  See the section: "Testing and replacing the triac".

* Old age or power surge.  Fuses sometimes blow for no apparent reason.

* Defective HV transformer (shorted windings.  See the section: "Testing the high voltage transformer".

* Shorted wiring due to vibration or poor manufacturing quality.  See the
  section: "Testing and repairing the wiring and connections".

Note that a shorted magnetron or shorted HV diode - which you would think
should blow the fuse - probably will not do so because current will be limited
by the impedance of the HV capacitor (assuming it is not shorted as well).
However, there will likely be a loud hum from the HV transformer as it strains
under the excess load.  Such a sound in conjunction with no heat is a likely
symptom of a shorted magnetron or HV diode.  If your oven has a separate
high voltage fuse - somewhat rare in domestic ovens - it may certainly blow
due to a fault in any of the HV components.

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.  A poorly timed power surge (as opposed
to the well timed variety) could also weaken the fuse element resulting in
eventual failure.

The fuses used in microwave ovens are usually ceramic 1-1/4" x 1/4" 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 high voltage
transformer.  One type of failure of a triac is 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 half of the triac is bad - shorted
or open, or doesn't turn on or turn off reliably.  Recall that a triac is in
effect a pair of SCRs in parallel in opposite directions.  If one side is
defective, 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 chapter: "Testing and Replacement of Components" for more information
on this and similar problems.

  6.21) Fuse blows when microwave shuts off (during or at end of cook cycle)

This could be due to a number of faults including shorting wires or defective
relay.  However, a common cause that might not be obvious is that the triac
used to switch power to the high voltage transformer is faulty.  What is
probably happening is that only one half of the triac (recall that a triac
is controlled for both polarities of the line voltage/current) is turning off
completely resulting in DC to the HV transformer, core saturation, and
excessive current which blows the fuse.  Drive to the triac could also be
marginal but the bad triac is more likely.

The following description applies directly to some GE and Hotpoint models.
Modify it accordingly for your oven.  Depending on model, the triac may
be located on the control board or mounted directly on the chassis.

(From: John Gallawa (mtek@pen.net)).

I have seen exactly this problem; and I've seen it baffle many a repair shop.
It is likely that the triac on the 'Power Control Board' is breaking down.
This is a fairly common problem in GE and Hotpoint models that use this board.

You can usually confirm the problem by setting the oven to a lower power level,
say "medium," and heat a cup of water.  You will probably hear a 'thump!' each
time the magnetron cycles on. This is an indication of a weakened triac.

Replace the triac (Q1) with either of the following: ECG 56010, or SK 10265.
Finally, replace the line fuse, install the outer cover, and test the oven
for proper operation.
The only other alternative is to replace the board. The cost used to be pretty
reasonable, but now it's gotten expensive - probably about $80.00.

The triac is probably located beneath a red plastic guard on the power control
board. Its designation is usually Q1.

(From: John Montalbano (jrmont@iquest.net)).

The microwave oven in my General Electric JHP65G002AD cooking center
blew its 15 AMP fuse each time the timing cycle expired. Replacing the
triac GE Part number WB27X5085 ($65.00 from GE) with a new NTE56014
($13.00) solved the problem.

(From: Les Bartel lbartel@veribest.com)).

I had the exact same symptoms on my GE microwave.  I replaced the triac with a
$3 15 amp off-the-shelf triac and it has been working for several years since.

See the chapter: "Testing and Replacement of Components" for more information
on triac testing though replacement is probably the only sure test.

  6.22) 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.

  6.23) 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.

  6.24) Oven heats but power seems low or erratic

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".

* 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%.

* Magnetrons, like other vacuum tubes, can weaken with age and use.  An oven
  that sees daily use may indeed weaken over the course of several years.  It
  is unlikely that any other electronic components could change value in such a
  way as to significantly affect power output.  However, a failure of the
  controller or sensor (if you have one) could result in short cycling.

  Testing on HIGH will eliminate this possibility.  Make sure the magnetron is
  powered continuously and it is not cycling.  You can often tell by listening
  for the relay clicks and/or by observing the oven light/other lights dimming
  as the magnetron kicks in.  50% power should result in approximately equal
  on and off times.

* 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 -
  check if you can hear this.  Alternatively, lights on the same circuit or
  the oven light may dim slightly when the magnetron kicks in.  There should
  not be any cycling on HIGH - the microwave power should stay on continuously
  while it is cooking.  If it is cycling, there may be a problem with the
  controller or you may unknowingly be in a low power mode - check it.

* Mechanical problems are also possible.  Where a spinning paddle wheel is
  used to 'stir' the microwave energy (often where there is no turntable), its
  failure to rotate can result in hot and cold spots.  Thus, you may see an
  unexplained variation in cooking times.  The paddle is often accessible by
  unclipping a plastic cover above the oven cavity.  Check for bearing failure,
  binding, broken or lose belt if direct driven, etc.  Note that some are
  rotated by air flow from the cooling fan and require that cover to be in
  place to rotate.  Therefore, it is not really possible to inspect for correct
  operation with the cover removed.  However, you can put a microwave power
  indicator (NE2 neon light bulb with its leads twisted together) in the oven
  (with a cup of water for a load) and observe it through the window.  You
  should see a periodic variation in intensity as the paddles do their job.

* There could be intermittent connections to the magnetron filament, thermal
  protector, or elsewhere.  But, these would likely show up as erratic
  operation - no heat at all sometimes - not just a weak oven.

  Inspect and clean and tighten (if necessary) all connections in the microwave
  generator including the magnetron filament, HV transformer, HV Diode, HV
  capacitor, and thermal protector.  Be sure to unplug the unit first and
  discharge the HV capacitor before touching anything!

* The thermal protector may be intermittent.  Test by clipping a light bulb
  across it or monitoring with a multimeter on AC voltage.  See the section:
  "Testing thermal protectors and thermal fuses".

  6.25) 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), bad thermal protector, 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 bad connections in the controller or
  elasewhere, a marginal door interlock switch, or a controller problem.
  Jiggle the door to see if this will cause it to shut off.

* If the magnetron was overheating, you would not be able to resume cooking
  until it cooled and the thermal protector reset.  If it just stopped working
  (i.e., the filament opened), everything would appear normal but there would
  be no heating.  If the magnetron were shorting, there would likely be a loud
  hum associated with the periods where there was no heat.

* 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
  grill.  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.

* Extremely high power line voltage may also result in overheating on a poorly
  designed or oven where the components are marginal.

  6.26) Oven makes (possibly erratic) buzzing noise when heating

Assuming operation is normal otherwise, this is most likely either a fan or
other motor vibrating on its mounts, fan blades hitting something, or some
sheet metal or the high voltage power transformer laminations vibrating.
There may be something stuck under the turntable or above the waveguide cover
interfering with the stirrer.

Something may have loosened up with age and use.

If the noise is caused be simple vibrations, no damage is likely to result.
However, if the main cooling fan is on its way out and it stops or gets stuck,
parts will overheat quite quickly at which point the oven will shut down
(hopefully) and there could be damage to the magnetron or other components.
Therefore, at least identifying the cause is probably a good idea.

The solution may be as simple as tightening a screw or weging a shim
between two pieces of vibrating sheet metal.

  6.27) Oven light does not work

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.

Bad connections are also possible but not that likely.

  6.28) Fans or turntables that do not work

There are up to 4 motors in a microwave oven:

* Magnetron cooling fan - always present.

* Mechanical timer (on inexpensive non-touchpanel or older units).

* Turntable

* Convection air circulation (combo units only).

When any of these do not operate properly, the most likely causes are:

* 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

* 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.

* 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

* 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.

* Bad connections - trace wiring and check continuity (unplugged, capacitor
  discharge) to motor terminals.

  6.29) 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).

(From: John Gallawa (mtek@pen.net)).

Here are the door disassembly instructions from the Amana service manual.
Many others are similar:

1. Pry out the inner door trim with a small screwdriver on the latch side of
   the door.

2. Remove two screws securing the latch assembly and door handle to the outer
   panel (this may be all that's needed to replace the handle).

3. Remove six screws and release 4 spring fingers that secure the choke to the
   outer panel.

Warning: A microwave leakage test must be performed any time a door is
removed, replaced, disassembled, or adjusted for any reason.

  6.30) Crack in door

"My microwave oven has a crack in the glass of its door. Is this safe to
 continue using or should I get it fixed? Will there be any radiation leakage?"

So you were throwing roasts at the oven again, huh? :-)

If the metal screen/mesh is behind and separate from the glass, there is no
danger.  In this case, the function of the glass is mostly cosmetic and a
small crack should not be a problem.

However, if the screen is inside the glass and now broken as well, there
could be microwave leakage.  Even if it is not actually broken at this
time, future failure is possible.  Therefore, the glass panel or entire
door should be replaced.

  6.31) 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.

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.

Once damage occurs - paint blisters and peels, or totally hardened impossible
to remove carbon deposits - more drastic action is called for:

* 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 any common gloss enamel
  will work just as well (and costs about 1/10th as much).  Use touch-up paint
  (with a small brush) or spray paint.  The typical color is beige, almond, or
  some other form of off-white - just match it to your oven (if you care).

  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.

  Alternatives to mica which can stand the elevated temperatures in a microwave
  oven may also be acceptable.  Possible choices include plastic or fiberglass
  laminate but not all materials will allow microwaves to pass without some
  heating - check it out.  Heat a cup of water and the candidate material on
  high for a couple of minutes.  If the material doesn't heat up, it should be
  fine.  Of course, it must also not have any metal coating (don't use a piece
  of one of those 'browning disks' :-).  Mica is also non-flammable which is
  may not be the case with other materials.

* 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.

  6.32) 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 thermistor temperature sensor.

* The overtemperature protection sensor (rather than the normal temperature
  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 or elsewhere in the convection air flow.  It is a two
  terminal device that may look 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 thermistor.  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 thermistors are available from the oven
  manufacturer - about $20.  Cheaper alternatives may be possible but you
  would need to know the exact specifications and it is probably impossible to
  obtain this information.

  Also see the section: "Sensor problems", below.

* 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.

  6.33) Sensor problems

Fancier microwave or microwave/convection ovens include various probes that
can be used to shut off the oven when the food is supposedly done or maintain
it at a preset temperature.

A problem with a sensor, controller, or wiring, may result in incorrect
operation (never getting past 'preheat' or not terminating a cook cycle) or
in a display of 'EEEE', 'FFFF', ERROR, or something similar:

  (From: Wilton Itamoto (witam40231@aol.com)).

  "FFFF" display is a common problem in older Panasonic convection ovens.  The
  problem is the temperature sensor thermostat located on the top rear of the
  oven.  This is the convection temp. sensor for the correct oven temperature.
  Replacing this open sensor will correct the problem.

When problems develop with these automatic features, the sensor and the probe
cable are the primary suspects.  However, it is possible that the electronic
circuitry could also be affected by a damaged or defective probe unit.

* Check for bad connections where the probe plugs in as well as broken wires
  inside the cable particularly near the ends where it gets flexed.

* Temperature probes may use a thermistor similar to one that controls the
  convection portion of a microwave/convection oven.  Steam/humidity probes
  may also behave similarly.

* If you have never tried the probe before, check your users manual.  It may
  only be active in certain modes, etc.

The best test of the probe unit is to substitute a known good one.  Of course,
this is generally not convenient.

* There should be some resistance when measuring between the signal conductors
  of the probe cable.  It may be high (hundreds of K ohms) but probably should
  not be open.  A very low value (a few ohms or less) might indicate a short
  in the cable or sensor.

* See the section: "Microwave/convection oven problems" for a discussion of
  thermistors.  Testing to determine if the controller is responding to the
  input from the sensor can be done in a similar manner except that access
  must be from inside the electronics bay while the oven is running (the probe
  normally plugs in inside the oven chamber).  Substitute a fixed or variable
  resistor and see if you can get the oven to shut off (or stay on) as a
  function of resistance.  Caution: Don't forget to put a cup of water in as
  a load if you are testing microwave operation.

If the resistor test determines that the controller is responding, than a
bad probe unit is likely.

If the probe checks out or substituting a known good one makes no difference
in behavior, look for corrosion or other deterioration of the socket in the
oven chamber as well as bad connections.  Faulty circuitry in the controller
is also possible.

Chapter 7) Testing and Replacement of Components

  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:

  * 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).

  * 1 Calorie is equal to 4.184 Joules (J) or 1 J = .239 C.

  * 1 Watt (W) of power is 1 J/s or 1 KW is 1000 J/s.

  * 1 cup is 8 ounces (oz) which is 8 x 28.35 g/oz = 226.8 g.

  * 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

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 or erratic".

  7.2) Testing the main fuse

Where the oven is dead or mostly dead, the main fuse is the place to start.

Locate and remove the main fuse.  It will usually be a 1" x 1-1/4" ABC
ceramic type directly in-line with the Hot (black wire) of the power cord.

Test it with an ohmmeter - the reading should be zero ohms.

If it is blown, suspect problems with the interlock switches, high voltage
capacitor, or high voltage wiring.

If it is good but the oven makes a loud humming sound when you attempt to
cook, suspect the magnetron or high voltage diode.

  7.3) Testing and replacing of interlock switches

With the oven unplugged, put an ohmmeter across the AC input just before the
interlocks (but beyond the power relay or triac if it precedes these).  Open
and close the door slowly 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.

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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]