Notes on the Troubleshooting and Repair of Microwave Ovens


  7.19) Testing thermal protectors and thermal fuses

There may be two types of devices present in your oven:

* Thermal protectors are thermostats that open a set of high current contacts
  at a preset temperature.  They should reset when they cool off.  However,
  like a relay or switch, the contacts sometimes deteriorate.

* Thermal fuses will open at a preset temperature but do not reset.  They blow
  and need to be replaced.

At room temperature, both types should read as a dead short with an ohmmeter
(disconnect one terminal as there may be low resistance components or wiring
which may confuse your readings).  If the resistance is more than a small
fraction of an ohm, the device is bad.  Replacements are somewhat readily
available.  You must match both the temperature and current ratings.

If you suspect a bad thermal protector in the HV transformer primary, clip a
100 W light bulb or AC voltmeter across it and operate the oven.  If the
thermal protector is functioning properly, there should never be any voltage
across it unless there is actual overheating.  If the bulb lights up or
the meter indicates approximately line voltage - and there is no sign of
overheating - the thermal protector is defective and will need to be replaced.

An overheating condition would generally be obvious as the mounting surface
on which the thermal protector is located would be scorching hot when it
tripped - too hot to touch (but discharge the HV capacitor first - a burn from
the heat will be nothing compared to the potential shock!).

Replacement of a thermal protector is very straightforward as it is almost
always screwed in place with push-on lug terminals.  The new thermal fuse will
probably come with lugs attached.

  7.20) Testing and replacing the triac

A triac may fail in a variety of ways:

* A shorted triac would result in the oven coming on as soon as the door is
  closed or the power being stuck on high no matter what the touchpad setting.

* An open triac or one that didn't respond to the gate would result in no heat
  and possibly other things like the fan and turntable not working as well.

* A triac that didn't turn off would result in the parts of the oven continuing
  to run even after the timer counted to zero.

* A triac where one half was shorted would result in a blown fuse due to it
  acting as a rectifier pumping DC through the HV transformer.

* A triac where one half doesn't properly turn off would result in the main
  fuse blowing when the cook cycle completed.

Nearly all triac failures will be shorts.  Thus, measuring across the
MT1 and MT2 terminals of the triac (the power connections) should read
as a high resistance with a multimeter.  A few ohms means a bad triac.

As noted above, triacs can fail in other - possibly peculiar ways - so
substitution or bypassing may be necessary to rule out all possibilities.

Replacement is very straightforward - just don't get the wires mixed up.

  7.21) Testing and replacing the power relay

A defective relay can result in a variety of symptoms:

* A relay with its contacts welded (stuck) closed would result in the oven
  coming on as soon as the door is closed or the power being stuck on high
  no matter what the touchpad setting.

* A relay that doesn't close (due to defective contacts or a bad coil) would
  result in no heat and possibly other things like the fan and turntable not
  working as well.

If the relay is totally inoperative, test for voltage to the coil.  If the
voltage is correct, the relay may have an open coil.  If the voltage is low
or zero, the coil may be shorted or the driving circuit may be defective.
If the relay makes a normal switching sound but does not correctly control
its output connections, the contacts may be corroded, dirty, worn, welded
closed, binding, or there may be other mechanical problems.

Remove the relay from the circuit (if possible) and measure the coil
resistance.  Compare your reading with the marked or specified value
and/or compare with a known working relay of the same type.  An open
coil is obviously defective but sometimes the break is right at the
terminal connections and can be repaired easily.  If you can gain access
by removing the cover, a visual examination will confirm this.  If the
resistance is too low, some of the windings are probably shorted.  This
will result in overheating as well as no or erratic operation.  Replacement
will be required.

The resistance of closed contacts on a relay that is in good condition
should be very low - probably below the measurable limits on a typical
multimeter - a few milliohms.  If you measure significant or erratic
resistance for the closed contacts as the relay is switched or if very
gentle tapping results in erratic resistance changes, the contacts are
probably dirty, corroded, or worn.  If you can get at the contacts, the
use of contact cleaner first and a piece of paper pulled back and forth
through the closed contacts may help. Superfine sandpaper may be used as
a last resort but this is only a short term fix.  The relay will most likely
need to be replaced if as in this case the contacts are switching any
substantial power.

Chapter 8) Items of Interest

  8.1) Microwave leakage meters

A routine test for radiation leakage should be done before returning an oven
you have worked on especially if the door or magnetron/waveguide were disturbed
during the repair process.  Use it around the door seem and ventilation holes
in the cabinet.  An inexpensive meter is better than nothing but will not be
as sensitive and will not allow you to quantify the amount of any leakage.

If you work on microwave ovens, such a meter is a *must* for personal safety
reasons as well as minimizing the risk of liability after returning them to
your customers.

These should be available wherever you buy quality test instruments.  They
are usually made by the same companies that manufacture other service
equipment.  Prices and capabilities vary widely.  MCM Electronics sells an
inexpensive unit suitable for quick checks on a go/no-go basis for $6.99
and an FDA approved unit (including calibration), for $388.

Note: you should also perform an electrical leakage test to assure that all
case parts are securely connected to the Ground of the AC plug.

  8.2) Comments on microwave leakage meters

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

I found an old manual for a Narda 8100B Electromagnetic Leakage Monitor.  (I
used to work for a manufacturer of Microwave ovens.)  While I don't personally
recall ever having damaged a probe while checking for leakage, I do know that
it is possible to do so and did happen on rare occasions.

The Narda manual states that their probes use an antenna/thermocouples design.
Holaday (sp?) makes another line of detectors and those may use a thermistor

I have confirmed that by removing the styrofoam cone from the end of a Holaday
uW leakage detector's probe and then bringing its tip near a heat source (40W
bulb) caused the meter to have a significant deflection.  Thus, the cones are
not only used as spacers.  They prevent radiant heat sources from affecting
the meter reading, as well.

The Holaday probes that I used had 8 diodes in the tip that formed an array.

Newer designs (Holaday) claim to be more or less immune to damage resulting
from placing them into high energy fields.  I do know that the older Narda
equipment was prone to such damage.

There is a section in the Narda manual that details how to select the proper
probe to measure "unknown" leakage levels.  In a nutshell, one should start
with the highest power rated probe and work toward the lowest power rated
probe (three listed in all).  The goal is to have a meter deflection of more
than 10% of it's scale while not going off scale for sake of accuracy.  While
it didn't specifically mention damage to the probes, there were overtones
throughout the text that implied such (watch needle, listen for alarm, stop
and replace probe, etc...).

The three probes were listed as (high/low range for each):

     Probe          Range
     8120A    0.2 mW to   2.0 mW/square cm
     8121A    2.0 mW to  20.0 mW/square cm
     8122A   20.0 mW to 200.0 mW/square cm

This is from memory, but I believe that the maximum leakages we were allowed
by the governmental agency were:

    * Less than 2.0 mw/square cm off of our assembly line
    * Less than 3.0 mw/square cm leaving the warehouse
    * Less than 5.0 mw/square cm in consumers home

As you no doubt know, with a hole cut in the oven (in reference to those who
want to modify one - see the section: "Microwave ovens for non-standard applications" --- sam), the density can easily reach several times these
numbers, especially on the newer 1,000 watt plus models.  Damage would occur
where one intentionally held the lower power rated probe in the strong field
until the thermocouple (or thermistor?) overheated.

  8.3) Simple microwave leak detectors

Since these do not really provide an absolute measurement, their utility is
somewhat limited.  All microwave ovens leak to some extent.  Determining by
how much is why you pay the big bucks for a real leakage meter!

WARNING: These are no substitute for a properly calibrated commercial unit!

(From: Leon Heller (leon@lfheller.demon.co.uk)).

A very simple design I saw somewhere (Electronics World, probably) consisted
of a half-wave dipole with a Shottky diode detector between the two elements.
I think one measured the voltage across the diode via a resistor and capacitor
smoothing arrangement using a 50 uA meter. You can buy these detectors quite

(From: Ren Tescher (ren@rap.ucar.edu)).

I saw an article about it in Modern Electronics in the early eighties.  It is
simply a Schottky Barrier Diode (SBD) and an LED wired together.  The leads of
the SBD are left intact and straight and act as a 1/4 wavelength dipole.  

Here's the circuit:

                             |     |

The LED is soldered close to SBD using as short of leads as possible (being
careful not to ruin either part with too much heat).  (Note that the diodes
are connected anode to cathode, not cathode to cathode.)

I then taped/glued it 1 1/2 and perpendicular from the end of a popsicle stick
(this gives it a 'standoff' distance).

Put a large container of water (>=2 cups) in the microwave and run it on HIGH
for 2 minutes.  While it is running, slowly sweep the tester around the door
seal, hinges and door latch.  You may have to dim the lights to see if the LED
lights up.

Any leaking uwaves will be picked up by the dipole 'antenna', the SBD will
rectify the waves, and when sufficient rectified voltage has built up, the LED
will light up.

I built 10 of these at home and then compared them to the commercial tester we
had at work.  The commercial tester had three ranges and the most sensitive
range was divided into 3 color bands, red, yellow, green.  The home-built
testers all 'fired' at some point in the 'yellow' range.  I attribute the
variances within the yellow (caution) range to individual characteristics of
the diodes - they all came from the bargain bin at Radio shacks....

A solid glow would indicate excessive leakage, especially if the tester still
glows if it is pulled beyond the 1-1/2 inch standoff distance to 3 inches.
Typically the LED just flickers, around the hinge/latch areas.  (US law allows
increased leakage as the oven ages).

You may notice that no radiation leaks through the viewing window, contrary to
the old wives tale of not looking through the window while it's cooking.  (The
screen really is a very good microwave shield --- sam).

Small leaks may be remedied by adjusting or cleaning the door and hinges
and/or by distance (square law= doubling the distance quarters the power).
Large leaks - trash the oven.

(From: James P. Meyer (jimbob@acpub.duke.edu)).

Get a small neon bulb.  The NE-2 size is a good one.  Use some resistors to
make a voltage divider for 115 VAC to feed the bulb.  Adjust the voltage
across the bulb so that it's just barely glowing.  Make the divider network
resistance large enough to limit the current through the bulb to just a couple
of mA.  Put the bulb on the end of a line cord and plug.  INSULATE everything

Adding this onto a neon circuit tester is one option and will provide an
insulated housing as well.

Plug the whole thing into an AC outlet.  Wave the bulb around the door gaskets 
and if it gets brighter when the oven is turned on, then you have located a
leak.  The bulb detector can be very sensitive.  You may even be able to use
it to find wires behind drywall in your house.

  8.4) How safe is a repaired microwave oven?

So you fixed up Aunt Minnie's Radarange or picked up a microwave at a yard
sale or scavenged one off the curb.  The only problem you could find was a
blown fuse, truly horrible mess of decayed burnt-on food, or a thriving
community of cockroaches inside.  How safe is it to use (assuming you evicted
the cockroaches)?

As long as there is no serious damage to the door (a 6 inch hole would quality
as serious damage) and the door fits square, it should be properly sealed.  As
long as the waveguide is tightly mounted and undamaged, there should be no
leakage from there.  Make sure the metal cover has all its fingers engaged
around the front (though with a properly installed magnetron, there should be
minimal microwave leakage into the electronics bay).

An inexpensive leakage tester - around $8 - will not be as sensitive or
accurate as the $500 variety by may provide some peace of mind.  However, as
noted below, they may indicate dangerous leakage even when your oven is within
acceptable limits.

The most important considerations are the door and door seal.

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

Those inexpensive hand held meters (from Radio Shack, etc..) can give very
inaccurate readings. While they definitely serve a purpose, they have caused a
more than a few people to unnecessarily fear microwave ovens over the years.
Also, I just changed jobs from working for a company that made gas ranges.  CO
detectors caused similar panic among users of the appliances.  I'd highly
recommend anyone with gas heat or appliances to purchase a quality CO
detector, but not one of those inexpensive type that go off whenever there is
a thermal inversion of smog a city.

  8.5) Efficiency of microwave ovens

The efficiency of an electric heating element is 100% - period.  However, using
an electric stove to heat 1 cup of tea may result in much wasted energy as the
element and pot must be heated as well and there are losses due to convection
and conduction to the surrounding environment.  Furthermore, you won't heat
just *1 cup* of tea but more likely 2 or 3 just to be sure you have enough!

A microwave oven is not likely to be more than 60% efficient - possibly as low
as 50 percent or even less.  While the magnetron tube itself may have an
efficiency rating of 75%, there are losses in the high voltage transformer,
cooling fans, and turntable motor (if used).  The light bulb and controller
also use small amounts of power.   These all add up to a significant overhead.
In addition, the waveform applied to the magnetron by the half wave doubler
circuit is not ideal for maximum efficiency.

However, you are not heating the surrounding countryside as the microwaves only
affects what you are cooking and not the container or oven cavity itself and
you are more likely to only load the amount of food you expect to be eating.
For a single cup of tea, the microwave oven may use 1/10th the energy of a
typical electric cooktop element to bring it to a boil!

Therefore, it makes sense to use a microwave oven for small short tasks where
the losses of an electric or gas oven or cooktop would dominate.  However,
gastronomic preferences aside, a conventional oven is better suited for that
20 pound turkey - even if you could distort its anatomy enough to fit the
typical mid-size microwave!

  8.6) Microwave oven design and cost reduction

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

Microwave oven design is a black art.  What one hopes for is to deliver all
the power from the magnetron into the food and not have a high SWR reflect
back into the magnetron and burn it out.  Size, shape, placement of food items
affect the SWR.  The microwaves are designed for the most part to work
optimally with an average load.  Models equipped with turn-table models
compensate for this by breaking up the SWR as the food revolves.  My oven has
a stirrer fan design and has been working for going on 18 years now without
the first hint of a problem (maybe a little less power).  I personally know
that it had one of the lowest SWRs available at the time.  Not to mention it
has an older design, non-cost reduced, cooler running, more efficient
magnetron (that cost $13.00 instead of $9.45).  The thing that I found
disturbing about microwave oven design was the trends to go with hotter an
hotter insulation classes on the components used in them.  The original
transformers were class H while the newer ones are now class N.  This was all
done in the name of cost reduction to remain competitive.  The windings AWG
got smaller and the temperature rise went up accordingly.  The magnetrons were
cost reduced in a similar fashion.  Size was reduced and the number of fins
were reduced.  Their temperature went up while their efficiency went down.
But then the cost went from $300 to $149 while life went from 10 years-plus to
5 years or less and they became disposable items.  That's one area, I'd
almost hesitate to hope the Government would have mandated an efficiency.

  8.7) Problems with running a microwave oven with metal inside or totally empty

Metal in microwave ovens may or may not be a problem depending on the specific
situation.  Sharp edges and points create strong field gradients which tend to
spark, arc, or create other fireworks.  With some food in the oven to absorb
the power, this is probably not likely to damage the oven.  You will note that
some ovens come with metal fixtures in addition to the oven walls themselves
(e.g., Sharp convection/microwave combo).

Having absolutely nothing in the oven chamber or just metal is the potentially
more likely damaging situation for the magnetron as you are dumping several
hundred W to over a KW of power into a reflective cavity with no load.  In the
worst case, you could end up with a meltdown inside the waveguide requiring
replacement of various expensive components including the magnetron.

  8.8) More on metal in the microwave

(From: Don Klipstein (don@Misty.com)).

Mainly, you need exposed water or food to absorb the microwaves.  Otherwise,
they just reflect around the oven and get back to the magnetron tube.  This
may be bad for the tube, and in an unpredictable manner.

It is even not too good to run a microwave empty.  The walls of the main
cooking chamber are metal.

In the event the microwave runs empty OK, adding metal objects change the
microwave reflection pattern and might possibly unfavorably change things.

If you have exposed food or water, the tube should not mind some stray metal
too much.  If the added metal does not interfere with microwaves mainly
getting from the tube to the target food or water and being absorbed, the
magnetron should be OK.

Even if the tube does not mind, there is another concern.  Metal objects close
to other metal objects or to the walls of the cooking chamber may arc to these.
Any arcing is generally not a good thing.  If you add metal objects in a manner
safe for the tube, try to keep these at lease a half inch (a bit over a cm.)
from the walls to avoid arcing.  Safe distances are uncertain and are usually
less if the metal objects are small and a large amount of food or water is

If any metal object has major contact with a microwave absorbing food target
and such target is still heavily exposed, you should be OK.  Examples would
be wrapping foil around the wingtips of a whole chicken or whole turkey, or
a bottle of liquid (on its side) with a metal lid with liquid contacting much
of the lid.  This is usually OK.  Just avoid unrelated problems due to major
temperature change of anything in contact with a non-heat-rated glass

A plain glass bottle if ice-cold stuff might possibly break from thermal shock
when heated, but any metal lid on a bottle largely full of microwave-absorbing
stuff should not present a problem especially if the bottle is on its side so
that stuff is contacting or very nearly contacting much of the lid.

  8.9) Burnt smell from oven - after incident

"My daughter tried to heat up one of those 'soup in a box' containers and it
 burned - actually charred. I wasn't home at the time, so I don't know if it
 was neglect or inappropriate use, but the lasting effect is that there is a
 strong odor, similar to that which you smell after a fire that I cannot seem
 to get rid of.  What do you recommend.  I have a Sharp Convection/Microwave,
 that even after the incident described still performs well."

Start by cleaning the interior of the oven thoroughly with mild detergent and
water.  You may have to do this several times to get all of the sticky film
left behind.  However,  the odor may persist since the smoke can penetrate
to places you cannot access for cleaning.  With a combination convection and
microwave oven especially, there are many passages where the air would normally
circulate in convection mode which will be coated even if the oven was used in
microwave mode.  However, I would expect that the smell will decrease and
eventually go away.  Most likely, nothing in the oven has actually sustained
any damage.

  8.10) Microwave ovens and grounded dedicated circuits

A microwave oven should be used only on a properly wired 3 wire grounded
circuit.  Check with a circuit tester to make sure your 3 prong outlet is
correctly wired.  Many are not.  Install one if it is not grounded.  There
is a very important safety reason for this requirement: the return for the
high voltage is through the chassis.  While unlikely, it is theoretically
possible for the entire high voltage to appear on the metal case should
certain internal connections come loose.  With a properly grounded outlet,
this will at most blow a fuse.  However, with the case floating, a shocking
(or worse) situation could develop - especially considering that microwave
ovens are usually situated near grounded appliances like ranges and normal
ovens and wet areas like kitchen sinks.

A dedicated circuit is desirable since microwave ovens are significant users
of power.  Only about 50 to 60% of the electricity used by a microwave oven
actually gets turned into microwaves.  The rest is wasted as heat. Therefore,
a 700 W oven will actually use up to 1400 W of power - nearly an entire 15 Amp
circuit.  Convection ovens have heating elements which are similar energy hogs.
At least, do not put your refrigerator on the same circuit!

  8.11) Microwave ovens and GFCIs

A Ground Fault Circuit Interrupter (GFCI) protects people from shocks should a
situation develop where an accessible part of an appliance should short to a
live wire.  Touching this may result in a shock or worse.  A GFCI detects any
difference between the currents in the Hot and Neutral wires and shuts off the
power should this difference exceed a few mA.

A GFCI is not needed with a properly grounded microwave oven as any such fault
will blow a fuse or trip a circuit breaker.  In most cases, it will not hurt
to have a GFCI as well.  However, with some combinations of oven design and
your particular wiring, due to the highly inductive nature of the high voltage
transformer, nuisance tripping of the GFCI may occur when you attempt to cook
anything - or at random times.  However, this usually does not indicate any
problem.  Plug the oven into a properly grounded circuit not on a GFCI.

  8.12) Can a microwave oven be built into (or hung under) a cabinet?

Assuming it is a regular microwave and not a convection/microwave combo, the
major issues are:

* Providing adequate air flow through its ventilation grill which is usually
  located in the rear.

  (A convection/microwave can get quite hot and have ventilation in other
  places.  In this case I would suggest contacting the manufacturer of the
  oven for specific requirements.)

* Providing adequate structural support so the microwave doesn't end up in the
  soup :-(.  These are HEAVY appliances - cabinetry and/or drywall may not
  be up to the task.  Models designed as over-the-range or combined microwave
  and exhaust fan units mount via a massive plate fastened securely into the
  wall structure (screwed directly to the studs, not just the sheetrock!).

* Local building codes may specify when and if this approach can be used.  So,
  before doing any demolition, check with your friendly township inspector!

There are special (likely highly overpriced) models available for this type of

To use a normal microwave, my recommendation would be to build a shelf rather
than a totally sealed, enclosed, conformal cabinet.  It can have sides and a
top as long as you leave a couple of inches all around.  This will result in
a microwave oven that is much more easily serviced should the need arise and
replaced in the future with a model that is not quite identical.

Just make sure it is securely supported - the microwave weighs quite a bit and
must endure a fair amount of abuse from heavy casseroles and the inevitable
door yanking/slamming!

  8.13) Taking a microwave oven oversees (or vice versa)

Microwave ovens are high power appliances.  Low cost transformers or
international voltage adapters will not work.  You will need a heavy and
expensive step down or step up transformer which will likely cost as much
as a new microwave oven.  Sell the oven before you leave and buy a new one
at your destination.

Furthermore, for microwave ovens in particular, line frequency may make a
difference.  Due to the way the high voltage power supply works in a microwave
oven, the HV capacitor is in series with the magnetron and thus its impedance,
which depends on line frequency, affects output power.

High voltage transformer core saturation may also be a problem.  Even with no
load, these may run hot even at the correct line frequency of 60 Hz.  So going
to 50 Hz would make it worse - perhaps terminally - though this is not likely.

* Going from 50 Hz to 60 Hz at the same line voltage may slightly increase
  output cooking power (and heating of the magnetron).  The line voltage
  could be reduced by a small amount to compensate.

* Going from 60 Hz to 50 Hz may slightly decrease output power and possibly
  increase heating of the HV transformer due to core losses.  Using a slightly
  lower line voltage will reduce the heating but will further decrease the
  cooking power.

The digital clock and timer will likely run slow or fast if the line frequency
changes as they usually use the power line for reference.  Of course, this may
partially make up for your change in output power! :-)

  8.14) Microwave oven test-mode

(From Mark Paladino (paladino@frontiernet.net)).

Some microwave ovens have a self-test feature. This self-test is usually 
accessed by pressing a couple of keys on the touch pad. You can usually test 
things like keys, switches controller etc. Check the manual for any 
self-test info. Some microwaves have this information tucked in a pocket 
or hidden somewhere behind panels.

  8.15) High frequency inverter type HV power supplies

While the vast majority of microwave ovens - perhaps every single one you will
ever see - use minor variations on the tried and trusted half wave doubler
circuit, a few models have been designed using solid state high frequency
inverters - in many ways similar to the deflection/HV flyback power supply of
a TV or monitor.

A typical circuit (from a Sharp microwave oven) uses full wave rectified
but mostly unfiltered pulsating DC as the power to a large ferrite inverter
transformer which sort of looks like a flyback on steroids.  This means that
the microwave output is pulsing at both 60 Hz and the frequency of the

        Bridge Rectifier         Inverter Transformer            Magnetron
  H o----+---|>|------+--------+-------+ |:| +--------------------------+
        ~|            |+      _|_ Drive )|:|( Filament 1T #18           |
         +---|<|---+  |       ---   25T )|:| +--------------+------+    |
 115 VAC           |  |        |    #12 )|:|   HV Cap       |    +-|----|-+
         +---|>|---|--+        +-------+ |:| +-------||-----+    | |_  _| |
         |         |           |         |:|(    .018 uF    |    |   \/   |
  N o----+---|<|---+   Drive |/ C        |:|(    2,400 V  __|__  |   ___  |
        ~          |-    o---|   Chopper |:|( HV          _\_/_  +----|:--+
 (Interlocks and   |         |\ E        |:|( 250T          | HV      |'-->
  fuses/protectors |           |         |:|( #26   Sense   | diode   | uWaves
  not shown)       +-----------+         |:| +--+---/\/\----+---------+
                                             o  |    1.2   _|_
 (Except for filament, # turns estimated)       o H1        -  Chassis Ground

The chopper transistor is marked: Mitsubishi, QM50HJ-H, 01AA2.  It is a LARGE
NPN type on a LARGE heatsink :-).

Note the similarity between the normal half wave doubler circuit and this
output configuration!  Base drive to the chopper transistor is provided
by some relatively complex control circuitry using two additional sets of
windings on the inverter transformer (not shown) for feedback and other
functions in addition to current monitoring via the 'Sense' resistor in the
transformer return.

It is not known whether power levels in this over were set by the normal
long cycle pulse width modulation or by control over a much shorter time
scale.  However, since the filament of the magnetron is powered from the same
transformer as the HV - just as in a 'normal' microwave oven, this may not be
very effective.

Compared to the simplicity of the common half wave doubler, it isn't at all
surprising why these never caught on (what is diagramed above includes perhaps
1/10th the actual number of components in a typical inverter module).  Except
for obvious problems like a tired fuse, component level troubleshooting and
repair would be too time consuming.  Furthermore, as with a switchmode power
supply (which is what these really are) there could be multiple faults which
would result in immediate failure or long term reliability problems if all
bad parts were not located.  Schematics are not likely available either.  And,
a replacement module would likely cost as much as a new oven!

This is simply a situation where a high tech solution was doomed from the
start.  The high frequency inverter approach would not seem to provide any
important benefits in terms of functionality or efficiency yet created many
more possibly opportunities for failure.  The one major advantage - reduced
weight - is irrelevant in a microwave oven.  Perhaps, this was yet another
situation where the Marketing department needed something new and improved!

  8.16) Dangerous (or useful) parts in a dead microwave oven?

A microwave oven with its power cord cut or removed AND its high voltage
capacitor safely discharged is an inanimate object.  There are no particularly
hazardous parts inside.  Of course, heavy transformers can smash your feet
and sharp sheet metal can cut flesh.  And, the magnets in the magnetron may
erase your diskettes or mess up the colors on your TV.

Some may feel there is nothing of interest inside a microwave oven.  I would
counter that anything unfamiliar can be of immense educational value to
children of all ages.  With appropriate supervision, an investigation of
the inside of a deceased microwave oven can be very interesting.

However, before you cannibalize your old oven, consider that many of the parts
are interchangeable and may be useful should your *new* oven ever need repair!

For the hobbiest, there are, in fact, some useful devices inside:

* Motors - cooling fan and turntable (if used).  These usually operate on
  115 VAC but some may use low voltage DC.  They can easily be adapted to
  other uses.

* Controller and touchpad - digital timer, relay and/or triac control of the
  AC power.  See the section: "Using the control panel from defunct microwave oven as an electronic timer".

* Interlock switches - 3 or more high current microswitches.

* Heavy duty power cord, fuse holder, thermal protector, other miscellaneous

* High voltage components (VERY DANGEROUS if powered) - HV transformer (1,500
  to 2,500 VRMS, .5 A), HV rectifier (12,000 PRV, .5 A), and HV capacitor
  (approximately 1 uF, up to 2,500 VAC, perhaps 3,000 V peak).

* Magnetron - there are some nifty powerful magnets as part of the assembly.
  Take appropriate precautions to protect your credit cards, diskettes, and
  mechanical wristwatches.  See the section: "The magnets in dead magnetrons".

DOUBLE WARNING:  Do not even think about powering the magnetron once you have
removed any parts or altered anything mechanical in the oven.  Dangerous
microwave leakage is possible.

  8.17) The magnets in dead magnetrons

The dead magnetron you just replaced is fairly harmless.  There is no residual
radiation but it does contains a pair of powerful ferrite ring magnets.  These
can be removed without extensive disassembly and make really nice toys but
should be handled with care.  Not only can they pinch flesh (yes, they are that
powerful) but they will suck all the bits right off your tapes, diskettes, and
credit cards.  If you do want to save the magnets:

* Disassemble the magnetron assembly as follows:

  - Remove the top portion of the magnetron - it is either fastened with screws
    or some metal tabs which are easily bent out of the way.

  - Remove the cover over the box where the filament connections are located.
    This usually requires peeling off the sheet metal around the edges.

  - Cut the thick copper connections to the filament near the tube itself.
    (The thick copper coils are RFI chokes and prevent any microwave energy
    from escaping via the filament circuit.)

  - Spread the frame apart just a bit and lift out the tube with heat sink
    fins.  CAUTION: the sheet metal fins may be sharp!

  - The magnets can now be pulled off.  They may need cleaning :-(.

  - The magnetron tube itself can be disassembled by grinding off the welds
    around the edges of the large cylinder or cutting around it outer edge
    near one end with a hack saw but it takes quite a bit of curiosity to make
    this a worthwhile exercise.  There is a slight chance that the coating on
    the filament is poisonous so don't take chances.  You don't need to get
    inside to remove the magnets.

* Keep the magnets a safe distance away from any magnetic media including what
  might be in your back pocket, mechanical wrist watches, and color computer
  monitors and TVs.

* Paint the magnets with plastic enamel or coat them with the stuff used on
  tool handles to reduce their tendency to chip.  The chips are as magnetic as
  the overall magnet.  The ferrite is basically a ceramic and fragile.
  Smack them too hard and they will shatter.

* Take care not to get your skin between the magnets when you bring them
  together since the attractive force when nearly touching is substantial.

* Store the magnets in a box packed in the center of another box with at least
  4 inches on all sides.  Clearly mark: powerful magnets with appropriate

Having said that, these magnets can be used to demonstrate many fascinating
principles of magnetism.  Have fun but be careful.

Also see the section: "Magnetron construction - modern microwave oven".

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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]