Notes on the Troubleshooting and Repair of Microwave Ovens


  8.18) Using the control panel from defunct microwave oven as an electronic timer

It is usually possible to remove just the touchpad and controller board
to use as a stand-alone timer with a switched output.  Be careful when
disconnecting the touchpanel as the printed flex cable is fragile.  With
many models, the touchpanel (membrane touchpad) needs to be peeled off of
the front plastic panel or the entire assembly can be removed intact.

The output will control a 10-15 A AC load using its built in relay or triac
(though these may be mounted separately in the oven).  Note that power on a
microwave oven is regulated by slow pulse width modulation - order of a 30
second cycle if this matters.  If it uses a triac, the triac is NOT phase
angle controlled - just switched on or off.

  8.19) Precise control of microwave oven power

For heating a casserole, the 10 to 30 second cycle time typically used for
microwave oven pulse width heat control is fine.  However, for other purposes,
this results in unsatisfactory results.  This question was posed by someone
who wanted to modify the circuitry to their microwave oven to provide
continuous control and a constant heating rate.

Just cycling faster (without any other modifications is not the answer).  One
problem is that the filament of the magnetron is turned on and off as well.
This would result in a very non-linear relationship between on-time and power
as the cycle became shorter and shorter.

It should be possible to put a Variac (variable autotransformer) on the input
to the high voltage transformer - between the controller and HV primary.  The
power to the filament will still be affected but there will be a range over
which continuous control will be possible.  However, there will be a lag as
the filament heats and cools.  Also, running for an extended period of time at
reduced filament temperature may eventually damage the cathode coating.  I do
not know if this is likely.

Where manual control is all that is needed, this approach may be the adequate.

If the filament were put on its own transformer (with appropriate insulation
ratings), then instantaneous control of power should be possible using a Variac
on the HV transformer primary or a phase control scheme using a triac - a high
power light dimmer or motor speed control might even work.  Alternatively, a
triac or solid state relay can be turned on and off at the peaks of the AC
(to minimize inrush) similar to the pulse width modulation that is normally
used for the oven - but at a much higher frequency.  This could easily be
computer controlled with feedback from a temperature sensor.

In any case, you want everything else - including cooling fans - to be on the
full line voltage not affected by any power control scheme or timer.

  8.20) Has technology gone too far?

Don't you just hate it when your kitchen appliances have the highest IQ in
the household?  What more could you want?  Maybe, a microwave with a robot
arm to retrieve the food from your fridge or freezer!

(From: Dave Marulli (marulli@rdcs.kodak.com)).

We bought a Sharp unit with the Interactive Display feature.

There is a list of common items that you might Defrost, Cook, or Reheat.
You pick one of those tasks, choose a number from the list, enter the
'quantity', hit start and it picks the time and power level. There is
even an 'on-line' help feature.  A typical session goes like this:

    Button Pressed                        Screen Output
   ----------------                 ------------------------------
      CompuCook                      Enter Food Category
          1                          Baked Potato, Enter Quantity
          4                          Press Start

Unit turns on and starts cooking. If the little word HELP lights
up, you press the HELP button and it gives you little hints like, 

For things like CompuDefrost, you tell it what you are defrosting,
how many pounds, and hit start. It will turn on for a while, then
beep at you and tell you to break the pieces apart, cover the edges,
etc. You do as you are told, close the door hit start and it continues
until it's time for you to do some thing else.

Same idea for CompuReHeat: Tell it how many slices of pizza or bowls
of pasta you want to reheat, and it sets itself up and takes off.

It even has the obligatory POPCORN button!

Another neat feature is that you can hold the start button on without
setting any time and it will stay on for as long as you hold the button.
This is great for melting cheese, softening butter or chocolate, etc.

But, does it run Lotus??? :-) --- sam.

(From: Steve Dropkin (sdropkin@isd.net)).

The one we bought has an LCD screen that's maybe three inches square, takes
you step-by-step through anything the oven can do, and includes 600 recipes
(!).  While that sounds like overkill, the attraction for me was that the
menu-driven interface actually seemed simpler and more inviting than the
ovens with timing buttons and 24 others marked "popcorn," "baked potato,"
"hot dog," "frozen dinner," "beverage," "sandwich," "waffles," etc. They
looked just way too busy. (Same argument I have against a lot of mainstream
HiFi equipment these days. I just want to listen to the music, not
reengineer the sound source ...)

(From: Andrew Webber (webbers@magma.ca)).

Our microwave has a button for popcorn.  As far as I can tell, all it does is
automatically set 5 minutes.  The manual says to monitor the popcorn anyway
since it varies based on bag size, etc.  So on principal I choose 5 minutes on
high and stop it at 1:45 (why not set for 3:15? because the one time I tried it
the popcorn was burnt!).  I can choose 5 minutes with two presses (QUICK, 5)
and popcorn with two presses (POPCORN, START).

But that popcorn button sure is a good selling point! :)

  8.21) Microwave ovens for non-standard applications

Occasionally, people ask questions about the use of a microwave oven to
do things other than heating food.  In general, these have to be taken on
a case-by-case basis.  Obviously, softening sticks of Dynamite is probably
not to be recommended!  (There actually is a reason for this - a microwave
can develop hot spots - heating is not as uniform as with normal ovens.  Do
your dynamite softening in a normal oven).

Special kilns that will fit inside a microwave oven are apparently available
to achieve really high temperatures.  They consist of a ceramic (expanded
alumina or something similar) insulating cylinder lined with a microwave
susceptor - possibly a ferrite material.  Temperatures exceeding 1000 degrees
C (yellow-white heat) are possible after a few minutes on high.

If any modifications are made to the oven that would compromise the integrity
of the door seals or provide other places where microwave radiation could 
escape, then special tests MUST be done to assure the safety of the users
of the equipment.  The following is one such case in point:

"My Dad and I are using a microwave oven to heat oak strips by passing them
 through the microwave field of a 1000W oven. We cut out squares (4"x 4") in
 the glass front and metal back of the oven to allow these strips to pass
 through the field. I am concerned about potential microwave leakage of a
 harmful nature."

Geez!!!  You guys are out of your collective mind.  Sorry, having said that
I feel much better :-(.

My first recommendation (though this is too weak a term) would to not do this.

My second (and up to N where N is a very large number) recommendation would
be not to do this.

However, if you insist, use a good conductive sheet metal such as copper or
aluminum  to reduce the size of the opening as close to the material as
possible.  The wood stock will tend to reduce leakage while it is in place
but the opening will leak like crazy when there is nothing in the hole.  The
sheet metal must be in electrical contact with the mesh in the door and the
metal back.  The smaller the opening, the less will be the leakage.  Also,
make sure there is always a load in the oven (a cup of water, for example) to
keep the magnetron happy.

Next, borrow an accurate microwave leakage detector.  A large appliance repair
shop or electronics store may rent you one if you are persistent enough.  Use
this to identify the safe limits front and back.  Label these and don't go
closer while the oven is in operation.  The operators may have to remain
further away or some additional shields may needed if these distances are not
satisfactory.  The leakage detector or microwave field strength meter should
come with information on acceptable power limits.  It is something like 2 mW
per square cm a foot or so from the oven - check it out.  However, there is
no assurance that even this limit is safe.

CAUTION (In addition to the loony nature of this entire project!): Since the
leakage you encounter may be orders of magnitude greater than what is typical
of even a misaligned microwave oven, start with the probe at a distance of a
few feet and slowly move it closer while watching the meter or readout.  Don't
set it next the opening as you hit START!  This will prevent the possibility
of damage to the expensive leakage tester (which could be costly) and exposure
risk to you as well.

The only known confirmed danger from microwave radiation is from internal
heating effects.  The eye is particularly sensitive to this and it doesn't
take much of an increase in temperature to denature the tissue of the central
nervous system (i.e., scramble your brain).  The human body does not have an
adequate warning system since nerve endings sensitive to heat are somewhat
sparse.  Thus, while the dangers may be overstated, it doesn't make sense to
take chances.

What is wrong with radiant heat???

(From Barry Collins (bcollins@mindspring.com)).

You did the right thing to discourage people from breaching the integrity of a
microwave oven, because there are so many factors involved that one has to
assume personal (or property) injury (or damage) may result from such actions.

I personally don't feel uncomfortable with what the person was doing, provided
they had taken reasonable precautions (too numerous to list).  Power does fall
off with the square of the distance and microwaves, barring any reflective
surface, are very directional by nature.  Just don't stand in front of the
source.  (I met one of the Japanese engineers who had unintentionally placed
his head in a test oven that was working.  He reported warmth, but no lasting
damage, aside from the resulting joke.)  Field density and exposure time is a
large factor.  One tends to remove one's hand when one senses heat.  I think
the story goes that this was how the heating affect was originally discovered.

The number one precaution I've always held near and dear to me is to protect
one's eyes.  The Narda manual has multiple warning in it about this.  The
aqueous membranes of the eyes are perfect absorption material for stray
microwaves.  This can happen much faster than with fleshy parts of the body
and don't heal anywhere near the way a flesh injury does.  It is this that you
might want to point out in your FAQ's.

  8.22) Short course on Amana

(From: Charles Godard (cgodard@iamerica.net)).

Everything depends on "Air Flow".  If the stirrer does not turn, you will
always get a "Hot! spot" on the left bottom of the door.  In addition the
stirrer bearing will sometimes arc and may melt at the spots where it arcs.

If your blower is running up to speed, remove the cover and replace the foam
gasket material.  This forces air over the stirrer when the cover is replaced.
If stirrer still does not turn, remove the grease shield and check the stirrer
for burns that are causing it to stick.  If this is ok or you correct it and
stirrer still does not turn, then replace the grease shield with a later model
that looks almost the same as the original, but has one small modification
which you will see when you compare the two.

Never let one go out of the shop unless the stirrer is turning.  It will soon
be back unless all they do is heat coffee.  Next time it may be a cavity or
magnetron overload that has opened due to the stirrer not turning.

It's good work on a quality product.  I wish I had a hundred restaurant
customers using them.  The older Amana's power stays near 1500 watts forever.
Retail customers are junking them because of $100 - to $125 repair bills.
What a waste!

Chapter 9) Service Information

  9.1) Advanced troubleshooting

If the solutions to your problems have not been covered in this document,
you still have some options other than surrendering your microwave to the
local service center or the dumpster.

Unlike most other types of consumer electronic equipment, a service manual
is rarely required.  A sufficiently detailed schematic is nearly always
pasted to the inside of the cover and includes all power components,
interlocks, fuses, protectors, and wiring.  This is entirely sufficient
to deal with any problems in the microwave generator.  No adjustments or
alignment should even be required so detailed procedures for these are not

However, when tackling electronic faults in the controller, a service manual
with schematics will prove essential.  Whether these are available depends
on the manufacturer.  For legal reasons, some manufacturers are reluctant
to sell service information or replacement parts for microwave ovens.  They
are concerned with litigation should an unqualified person be injured or

  9.2) Suggested Reference

I know of at least one book dealing specifically with microwave oven repair.
It is very complete and includes many actual repair case histories.  There
is a good chance that your specific problem is covered.

1. Microwave Oven Repair, 2nd Edition
   Homer L. Davidson
   TAB Books, a division of McGraw Hill, Inc., 1991
   Blue Ridge Summit, PA 17294-0850
   ISBN 0-8306-6457-2 (hard), ISBN 0-8306-3457-6 (pbk.)

This may be available at your public library (621.83 or 683.83 if your
library is numbered that way) or from a technical bookstore. 

  9.3) Cost of repair parts

Assuming you have located one or more bad components, the question is
whether an oven that is a few years old is worth fixing.  Typical parts cost
for generic replacements:

    * HV diode: $2-5 (except for the bolt-on variety which can range up
      to $50.  It should be possible to replace these with the $2 variety
      with wire leads);

    * Power fuse: $.40.

    * HV Capacitor: $10-20.

    * Magnetron: $30-100.  Common generic replacements are $30-40.

    * Overtemperature thermostat (thermal protector): $4.50.

    * Interlock Switch: $2.50.

    * Triac: $12.00 (unless original replacement in which case you will
      need to take out a mortgage - try the generic variety).

Parts suppliers like MCM Electronics can provide these components to fit
the vast majority of microwave ovens.

Touchpads and controller parts like the microprocessor chip are usually only
available from the manufacturer of the oven.  Prices are high - a touchpad
may cost $30 or more.

Sensors and other manufacturer specific parts will be expensive.

While the HV transformers are fairly standard, they are not readily available
from the common replacement parts sources.  However, they do not fail that
often, either.

Here is one place that seems to stock some: AMI Parts, Eagle Grove, IA.  Voice
phone: 1-800-522-1264.  However, they won't be cheap - expect to pay $50 or
more!!!  In addition, MCM Electronics now lists at least one Goldstar model

With the prices of microwave ovens dropping almost as fast as PCs, a few year
old oven may not be worth fixing if the problem is a bad magnetron or touchpad.
However, except for a slight decrease in power output as the oven is used over
the years and the magnetron ages, there is little to go bad or deteriorate.
Therefore, you can expect a repaired oven to behave just about like new.

  9.4) Interchangeability of components

The question may arise: If I cannot obtain an exact replacement or if I
have another microwave oven carcass gathering dust, can I substitute a
part that is not a precise match?  Sometimes, this is simply desired to
confirm a diagnosis and avoid the risk of ordering an expensive replacement
and/or having to wait until it arrives.

For safety related items, the answer is generally NO - an exact replacement
part is needed to maintain the specifications within acceptable limits with
respect to line isolation, radiation emission, and to minimize fire hazards.
For microwave ovens such parts include the power fuses, interlock switches,
and anything else that could potentially lead to microwave radiation leakage -
like a magnetron which did not fit the waveguide properly.

Fortunately, while an exact match may be required, it doesn't have to
be from the original manufacturer - most parts are interchangeable.
Thus the organs from that carcass may be able to provide renewed vitality
to your ailing microwave.

Here are some guidelines:

1.  Fuses - exact same current rating and at least equal voltage rating.
    This will probably be a ceramic 1-1/4" x 1/4" 15 or 20 A 250 V fast
    blow type.  For the repair, use an exact replacement.  For testing
    only, a similar type may be used.

2.  Thermal protectors - same temperature and maximum current rating.  You
    must be able to mount it securely and flush against the same surface as
    the old one.

3.  Interlock switches - must have the same terminal configuration and
    at least equal current rating.  Of course, a secure fit is very
    important as well for it to perform its safety function.  Many of
    these are interchangeable.

4.  HV capacitor - similar (within 5%) and at least equal working voltage.
    Note that the working voltage rating of these capacitors is not consistent
    with the way capacitors in other electronic equipment are specified and
    is usually the RMS voltage of the AC input from the HV transformer.  There-
    fore, it is not possible to substitute something from your junkbox unless
    it is from a microwave oven.  In addition, this is one situation where
    higher capacity (uF) is not better.  The power output is related to
    capacitance.  Thus, the value should be matched fairly closely or else
    other parts may be overloaded.  However, a smaller one can be used for

5.  HV diode - most of these have similar electrical ratings so a substitution
    is possible if you can make it fit physically.  This would be particularly
    desirable where your oven has one of those chassis mount $50 dollar
    varieties - it may be acceptable to use a $2.75 generic replacement.

6.  Relays and triacs - substitutes will generally work as long as their
    specifications meet or exceed those of the original.  Creative mounting
    may be required.  

7.  Magnetrons - a large number of microwave ovens use the same basic
    type but the mounting arrangement - holes vs. studs, orientation of
    the cooling fins, etc., differ.  You can safely substitute a not
    exact match for testing purposes IF you can make it fit the waveguide
    securely without gaps.  However, if the cooling fins end up being on
    the wrong side, it will heat up very quickly - 50% of the input power
    goes to heat - and will not be suitable as a permanent replacement.

8.  HV transformer - same (within 5%) voltage and at least equal current
    rating.  Mounting should not be a problem but don't just leave it
    loose - you could end up with a disaster.

9.  Fans and motors - speed/power and direction must match and mounting must be
    possible.  Speed isn't so critical for a turntable but for a magnetron
    cooling fan, inadequate air flow will result in overheating and shutdown
    or failure.  Common shaded pole type motors may be interchangeable with
    other appliances or if a mounting arrangement can be cobbled together.

10. Mica waveguide cover - cut to match.

11. Turntable and mode mixer components - if they fit, use them.

12. Light bulb - similar ratings and base.

13, Temperature sensors, thermistors, etc. - depends on the particular

14. Mechanical timers - compatible switching and mounting arrangement.

15. Cordsets - must be 3 wire heavy duty grounded type.  Make sure the
    replacement has at least as high a current rating as the original.
    Observe the color code!

16. Controller and touchpad - small parts like resistors, diodes, capacitors,
    and so forth can often be substituted.  Forget about the controller
    ICs or display.  The touchpad is likely to be custom both electrically
    and physically as well unless you have a similar model microwave to

  9.5) Can I substitute a slightly different HV capacitor for a blown one?

It is not always possible or convenient to obtain an exact replacement
high voltage capacitor.  What will the effects be of using one that is
a slightly different value?

First, the voltage rating must be at least equat to that of the original.
It can be higher but never never lower or you will probably be replacing
it again in the very near future.

Now for the uF rating:

Unlike a conventional power supply filter capacitor, the capacitor in a
microwave is in a voltage doubler and effectively in series with the
load (magnetron).  Therefore, its value **does** have an impact on output
power.  A larger capacitor will slightly increase the output power - as
well as heat dissipation in the magnetron.  Too small a capacitor and
the doubler will not produce full output.

As an example, the impedance of a 1 uF capacitor at 60 Hz is about 2.5 K ohms.
The cap is in effect in series with the magnetron.  A 1 KW magnetron running
on just over 3 KV RMS is about 10 K ohms.  These are really really rough

Thus the power difference is not a straight percent for percent change - I
estimate that it is about a 1:4 change - increase the capacitor's uF rating
by 10 percent and the power will go up by 2.5% (assuming the relationship
is linear right around the nominal value).  I have not confirmed this, however.

Therefore, I would say that using a capacitor with up to a 10-15% difference
(either way) in uF rating is probably acceptable but a closer match is better.

  9.6) Obtaining replacement parts for microwave ovens

For general electronic components like resistors and capacitors, most
electronics distributors will have a sufficient variety at reasonable
cost.  Even Radio Shack can be considered in a pinch.

However, places like Digikey, Allied, and Newark do not have the specialized
parts like magnetrons, HV capacitors and diodes, interlock switches, thermal
protectors, etc., needed for microwave oven repair.

Your local appliance distributor or repair parts outlet may be able to obtain
an exact replacement or something that is an ecceptable substitute.  However,
the cost will be higher than for generic parts from the places listed below
if they carry what you need.

Going direct to the manufacturer is a possibility but expect to pay more than
might be charged for generic replacement parts by an independent company.
Also, some places like Sears, may refuse to sell you anything microwave oven
related due to safety concerns - unless they are convinced you are a certified
repair technician, whatever that might mean.  Their prices are inflated as

Another alternative is to determine who actually made your oven.  This is
obvious with name brands like Panasonic and Sharp.  However, Sears doesn't
manufacture their own appliances, but an inspection inside may reveal the
actual manufacturer.  Then, go direct to the horse's mouth.  Many companies
will be happy to sell service parts but availability may be a problem on
older ovens.  I had to give up on a Sharp microwave/convection oven that
was 15 years old because specialized replacement parts were no longer
available from Sharp.

  9.7) Sources for replacement microwave oven parts

The following are good sources for consumer electronics replacement parts
including common microwave oven parts:

* MCM Electronics                  (VCR parts, Japanese semiconductors,
  U.S. Voice: 1-800-543-4330.       tools, test equipment, audio, consumer
  U.S. Fax: 1-513-434-6959.         electronics including microwave oven parts
                                    and electric range elements, etc.)
  Web: http://www.mcmelectronics.com/

* Dalbani                          (Excellent Japanese semiconductor source,
  U.S. Voice: 1-800-325-2264.       VCR parts, other consumer electronics,
  U.S. Fax: 1-305-594-6588.         car stereo, CATV).
  Int. Voice: 1-305-716-0947.
  Int. Fax: 1-305-716-9719.
  Web: http://www.dalbani.com/

* Premium Parts                    (Very complete VCR parts, some tools,
  U.S. Voice: 1-800-558-9572.       adapter cables, other replacement parts.)
  U.S. Fax: 1-800-887-2727.

The following suppliers have web sites with on-line catalogs and list a very
extensive selection of microwave oven parts.  There is a chance that they may
not want to sell to the general public.  I suppose this may be due to several
factors including the potential liability issues, complaints/attempts to return
parts when a repair doesn't work, and the small quantities involved.  However,
it is definitely worth checking as the public web sites implie a desire to deal
with the entire Internet community.

* Global/MPI/All Appliance Parts   (Their web site includes a very extensive
  Phone: 1-800-325-8488             selection of microwave oven parts.  For
  http://www.allapplianceparts.com  example, nearly 50 different magnetrons are
                                    listed along with little photos of each!)

* AMI (Appliance Maintenance)      (Distributor of major appliance replacement
  International                     parts.  Extensive on-line catalog of
  U.S. Phone: 1-800-522-1264        microwave oven parts with web pages for
  U.S. Fax: 1-800-442-3601          other major appliance parts under
  Int. Phone: 1-515-448-5311        construction.  On-line parts lookup and
  Int. Fax: 1-515-448-3601          ordering.)
  email: microwav@netins.net

Here is another one:

* Electronix, Corporation          (Magnetrons, interlock switches, lamps, http://www.electronix.com/        glass trays, diodes, thermal fuses,
  (also: Techweb, $6/month)         couplers, latches, rivets, stirrers, fans,
                                    waveguides, more...)

The following company will definitely not sell you anything but should be able
to provide the name of a local appliance parts distributor.

* QB Products                      (Master distributor, they sell only to
  Phone: 1-800-323-6856             appliance and electronics parts
                                    distributors like Marcone, Tritronics,
                                    Johnstone, etc.  You can call them to
                                    find the nearest distributor.)

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