Notes on the Troubleshooting and Repair of Microwave Ovens


Chapter 5) Principles of Operation

  5.1) Instant (2 minutes on HIGH) microwave oven theory

A typical microwave oven uses between 500 and 1000 W of microwave energy
at 2.45 GHz to heat the food.  This heating is caused mainly by the vibration
of the water molecules.  Thus plastic, glass, or even paper containers will
heat only through conduction from the hot food.  There is little transfer of
energy directly to these materials.  This also means that the food does not
need to be a conductor of electricity (try heating a cup of distilled water)
and that electromagnetic induction (used elsewhere for high frequency
non-contact heating) is not involved.

What is significant about 2.45 GHz?  Not that much.  Water molecules are not
resonant at this frequency.  A wide range of frequencies will work to heat
water efficiently.  2.45 GHz was probably chosen for a number of other reasons
including not interfering with existing EM spectrum assignments and convenience
in implementation.  In addition, the wavelength (about 5 inches) results in
reasonable penetration of the microwave energy into the food.  The 3 dB (half
power) point is about 1 inch for liquid water - half the power is absorbed in
the outer 1 inch of depth, another 1/4 of the power in the next inch, and so

Since the oven chamber cavity is a good reflector of microwaves, nearly all
the energy generated by the oven is available to heat the food and heating
speed is thus only dependent on the available power and how much food is being
cooked.  Ignoring losses through convection, the time to heat food is roughly
proportional to its weight.  Thus two cups of water will take around twice as
long to bring to a boil as one.

Heating is not (as popularly assumed) from the inside out.  The penetration
depth of the microwave energy is a few cm so that the outside is cooked faster
than the inside.  However, unlike a conventional oven, the microwave energy
does penetrate these few cm rather than being totally applied to the exterior
of the food.  The misconception may arise when sampling something like
a pie filling just out of the microwave (or conventional oven for that
matter).  Since the pie can only cool from the outside, the interior filling
will appear to be much hotter than the crust and will remain that way for a
long time.

One very real effect that may occur with liquids is superheating.  It is
possible to heat a pure liquid like water to above its boiling point
if there are no centers for bubbles to form such as dust specks or container
imperfections.  Such a superheated liquid may boil suddenly and violently
upon removal from the oven with dangerous consequences.  This can take place
in a microwave since the heating is relatively uniform throughout the liquid.
With a stovetop, heating is via conduction from the burner or coil and there
will be ample  opportunity for small bubbles to form on the bottom long before
the entire volume has reached the boiling point.

Most metal objects should be excluded from a microwave oven as any sharp
edges (areas of high electric field gradient) may create sparking
or arcing which at the very least is a fire hazard.  Microwave safe metal
shelves will have nicely rounded corners.

A microwave oven should never be operated without anything inside as the
microwave generator then has no load - all the energy bounces around
inside an a great deal is reflected back to the source.  This may cause
expensive damage to the magnetron and other components.

  5.2) Why don't microwaves leak out from through the glass?

"I am trying to find out what the glass on a microwave consists of 
 exactly.  i have not been able to get a better answer than
 'a wire mesh'.  if you can help, i would greatly appreciate it."

There *is* a wire mesh embedded in the glass panel.  Since the holes
in the mesh are much much smaller than the wavelength of the 2.45 GHz
microwaves (about 5 inches or 12.5 cm), it is essentially opaque to
microwaves and essentially all the energy is reflected back into the
oven cavity.

(From: Filip (I'll buy a vowel) Gieszczykiewicz (filipg@repairfaq.org)).

Greetings. Did you ever see a "mesh" satellite disk up close? You will
note that it looks much like it's made out of simple wire mesh that
you can get in a hardware store (in the USA, it's called "chicken fence"
:-). The reason this works is that the wave that the dish picks up
is longer than the hole in the mesh. Consider bouncing a tennis
ball on the "wire mesh" in the microwave - it WOULD work because
the ball is bigger than the holes. The wave in the microwave is
about 2.5cm "long" ... as long as the holes are smaller than that
(actually, you want them as small as possible - without affecting the
"watching the food" - to minimize any stray and harmonic waves
from escaping... like bouncing tennis and golf and ping-pong balls and
marbles off the mesh - you want to catch all the possible sizes - yet
still be able to see through it) they will not let anything out of the

BTW, it's not really "glass" but rather a 'sandwich' of glass, from
the outside, wire mesh (usually a sheet of metal which is either stamped
or drilled with a hole pattern - like a color TV CRT mask!), and followed
by a sheet of glass or plastic to make sure that food splatters and
vapor condensation are easy to clean - imagine scraping the mesh!

  5.3) How a microwave oven works

The operation of a microwave oven is really very simple.  It consists
of two parts: the controller and the microwave generator.

A schematic diagram of the microwave generating circuitry and portions of
the controller is usually glued to the inside of the cover.

The controller is what times the cooking by turning the microwave energy
on and off.  Power level is determined by the ratio of on time to off time
in a 10-30 second cycle.

The microwave generator takes AC line power. steps it up to a high voltage,
and applies this to a special type of vacuum tube called a magnetron - little
changed from its invention during World War II (for Radar).

  5.4) Controller

The controller usually includes a microcomputer, though very inexpensive
units may simply have a mechanical timer (which ironically, is probably
more expensive to manufacture!).  The controller runs the digital clock
and cook timer; sets microwave power levels; runs the display; and in high
performance ovens, monitors the moisture or temperature sensors.

Power level is set by pulse width control of the microwave generator usually
with a cycle that lasts 10-30 seconds.  For example, HIGH will be continuous
on, MEDIUM may be 10 seconds on, 10 seconds off, and LOW may be 5 seconds on,
15 seconds off.  The power ratios are not quite linear as there is a 1 to 3
second warmup period after microwave power is switched on.

The operating voltages for the controller usually are derived from a stepdown
transformer.  The controller activates the microwave generating circuitry
using either a relay or triac.

  5.5) Sensors

More sophisticated ovens may include various sensors.  Most common are
probes for temperature and moisture.  A convection oven will include a
temperature sensor above the oven chamber.

Since these sensors are exposed to the food or its vapors, failures of the
sensor probes themselves are common.

  5.6) Cooling fans

Since 30 to 50 percent of the power into a microwave oven is dissipated as
heat in the Magnetron, cooling is extremely important.  Always inspect the
cooling fan/motor for dust and dirt and lubricate if necessary.  A couple of
drops of electric motor oil or 3-in-One will go a long way.  If there are any
belts, inspect for deterioration and replace if necessary.

An oven that shuts off after a few minutes of operation could have a cooling
problem, a defective overtemperature thermostat, a bad magnetron, or is being
operated from very high AC line voltage increasing power to the oven.

One interesting note: Since 30 to 50 percent of the power goes out the vents
in the back as heat, a microwave oven is really only more efficient than
conventional means such as a stovetop or gas or electric oven for heating
small quantities of anything.  With a normal oven or stovetop, wasted energy
goes into heating the pot or oven, the air, and so on.  However, this is
relatively independent of the quantity of food and may be considered to be a
fixed overhead.  Therefore, there is a crossover point beyond which it is more
efficient to use conventional heat than high tech microwaves.

  5.7) Microwave generator

This is the subsystem that converts AC line power into microwave energy.  It
consists of 5 parts: high voltage transformer, rectifier diode, capacitor,
magnetron, waveguide to oven chamber.

* High Voltage Transformer.  Typically has a secondary of around 2,000 VRMS
  at .25 amp - more or less depending on the power rating of the oven.
  There will also be a low voltage winding for the Magnetron filament (3.3 V
  at 10 A is typical).

  You cannot miss this as it is the largest and heaviest component visible
  once the cover is removed.  There will be a pair of quick-connect terminals
  for the AC input, a pair of leads for the Magnetron filament. and a single
  connection for the HV output.  The HV return will be fastened directly to
  the transformer frame and thus the chassis.

* Rectifier - usually rated 12,000 to 15,000 PRV at around .5 amp.  Most
  commonly, this will be rectangular or cylindrical, about .5 inch long
  with wire leads.  Sometimes, it is a box bolted to the chassis.  One
  end will be electrically connected to the chassis.

* Capacitor - .65 to 1.2 uF at a working voltage of around 2,000 VAC.  Note
  that this use of 'working voltage' may be deceiving as the actual voltage
  on the capacitor may exceed this value during operation.  The capacitor
  is metal cased with quick-connect terminals on top (one end).  Always
  discharge the capacitor as described below before touching anything inside
  once the cover is removed.

* Magnetron - the microwave producing tube includes a heated filament
  cathode, multiple resonant cavities with a pair of permanent ceramic ring
  magnets to force the electron beams into helical orbits, and output antenna.
  The magnetron is most often box shaped with cooling fins in its midsection,
  the filament/HV connections on the bottom section, and the antenna (hidden
  by the waveguide) on top.  Sometimes, it is cylindrical in shape but this is
  less common.  The frequency of the microwaves is usually 2.45 GHz.

  5.8) Magnetron construction and operation

The cavity magnetron was invented by the British before World War II.  It is
considered by many to be the invention most critical to the Allied victory
in Europe.

The story goes that shortly after the War, a researcher at the Raytheon
Corporation, Dr. Percy Spencer, was standing near one of the high power radar
units and noticed that a candy bar in his shirt pocket had softened.  In the
typical 'I have to know why this happened' mentality of a true scientist, he
decided to investigate further.  The Amana Radarange and the entire future
microwave oven industry were the result.

Here are two descriptions of magnetron construction.  The first is what you
will likely find if you go to a library and read about radar.  (Some really old
microwave ovens may use the classic design as well.)  This is followed by my
autopsy of a dead magnetron of the type that is probably in the microwave oven
in your kitchen.  (Items (1) to (6) in the following sections apply to each
type while items (7) to (9) apply to both types.)

For more detailed information with some nice diagrams, see the articles at the
Microtech Web Site.  Topics include basic microwave theory as well as a
complete discussion of microwave oven magnetron construction and principles of

  5.9) Magnetron construction - basic textbook description

This is the description you will find in any textbook on radar or microwave
engineering.  The original Amana Radarange and other early microwave ovens
likely used this design as well.

1. A centrally located cylindrical electron emitting cathode.  This is supplied
   with pulsed or continuous power of many thousands of volts (negative with
   respect to the anode.

2. A cylindrical anode block surrounding but separate and well insulated from
   the cathode.

3. Multiple cylindrical resonator cavities at a fixed radius from the cathode
   bored in the anode block.  Channels link the cavities to the central area
   in which the cathode is located.

   The wavelength of the microwave energy is approximately 7.94 times the
   diameter of the cavities.  (For the frequency of 2.45 GHz (12.4 cm) used
   in a microwave oven this would result in a cavity diameter of approximately
   .62" (15.7 mm).

4. An antenna pickup in one of the cylindrical cavities which couples the
   microwave energy to the waveguide.

5. The entire assembly is placed in a powerful magnetic field (several thousand
   Gauss compared to the Earth's magnetic field of about .5 Gauss).  This is
   usually supplied by a permanent magnet though electromagnets have been also
   used.  The original designs used huge somewhat horseshoe shaped permanent
   magnets which were among the most powerful of the day.

6. Cooling of the anode block must be provided by forced air, water, or oil
   since the microwave generation process is only about 60 to 75 percent
   efficient and these are often high power tubes (many kilowatts).

  5.10) Magnetron construction - modern microwave oven

This description is specifically for the 2M214 (which I disassembled) or
similar types used in the majority of medium-to-high power units.  However,
nearly all other magnetrons used in modern domestic microwave ovens should be
very similar.

The item numbers are referenced to the diagram in the section: "Cross section diagram of typical magnetron".

Also see this photo of the Typical Magnetron Anode and Resonant Structure.
This is a view looking up through the anode cylinder from the filament end of
the tube.  See the text below for parts names and dimensions.

1. The filament and cathode are one in the same and made of solid tungsten
   wire, about .020" (.5 mm) diameter, formed in a helix with about 8 to 12
   turns, 5/32" (4 mm) diameter and just over 3/8" (9.5 mm) in length.  The
   cathode is coated with a material which is good for electron emission.

   Note: this coating is the only material contained in the microwave oven
   magnetron that might be at all hazardous.  Beryllium, a toxic metal, may
   be used in large radar magnetrons but should not be present in the types
   found in domestic microwave ovens.

   The filament gets its power via a pair of high current RF chokes - a dozen
   or so turns of heavy wire on a ferrite core - to prevent microwave leakage
   back into the filament circuit and electronics bay of the oven.  Typical
   filament power is 3.3 VAC at 10 A.

   The cathode is supplied with a pulsating negative voltage with a peak value
   of up to 5,000 V.

2. The anode is a cylinder made from .062" (1.5 mm) thick copper with an
   inside diameter of 1-3/8" (35 mm) and a length of about 1" (25.4 mm).

   Steel plates (which probably help to shape the magnetic field, see below)
   and thin steel covers (to which the filament and antenna insulators are
   sealed) are welded to the ends of the cylinder.

   The filament leads/supports enter through a cylindrical ceramic insulator
   sealed to the bottom cover and then pass through a hole in the bottom end

3. Rather than cylindrical cavities (as you would find in most descriptions
   of radar magnetrons), there are a set of 10 copper vanes .062" (1.5 mm)
   thick and approximately 1/2" (12.7 mm) long by 3/8" (9.5 mm) wide.  These
   are brazed or silver soldered to the inside wall of the cylinder facing
   inward leaving a 5/16" (8 mm) central area clear for the filament/cathode.

   Surrounding this space are the .062" (1.5 mm) thick edges of the 10 vanes
   with gaps of approximately .04" (1 mm) between them.

   Copper shorting rings at both ends near the center join alternating vanes.
   Thus, all the even numbered vanes are shorted to each other and all the odd
   numbered vanes are shorted to each other.  Of course, all the rings are
   also all shorted at the outside where they are joined to the inner wall
   of the cylinder.

   This structure results in multiple resonant cavities which behave like
   sets of very high quality low loss L-C tuned circuits with a sharp peak
   at 2.45 GHz.  At this high frequency, individual inductors and capacitors
   are not used.  The inductance and capacitance are provided by the precise
   configuration and spacing of the copper vanes, shorting rings, and anode

4. A connection is made near the middle of a single vane to act as the output
   power takeoff.  It passes through a hole in the top end plate, exits the
   tube via a cylindrical ceramic insulator sealed to the top cover, and
   attaches to the pressed-on bull-nose antenna cap.

5. The entire assembly is placed in a powerful magnetic field (several thousand
   Gauss compared to the Earth's magnetic field of about .5 Gauss).  This is
   provided by a pair of ceramic ring magnets placed against the top and bottom
   covers of the anode cylinder.  For the 2M214, these are about 2-1/8" (54 mm)
   OD, 1-13/16" (46 mm) ID, 1/2" (12.7 mm) thick.

6. A set of thin aluminum fins act as a heat sink for removing the significant
   amount of wasted heat produced by the microwave generation process since
   it is only about 60 to 75 percent efficient.  These are press fit on the
   magnetron anode and also in contact with the magnetron case.  There will
   always be a cooling fan to blow air through this assembly.

   The anode and magnetron case are at ground potential and connected to the

  5.11) Magnetron construction - common features

The following items apply to all types of magnetrons.

7. The gap between the cathode and anode, and the resonant cavities, are all in
   a vacuum.

8. When powered, electrons stream from the cathode to the anode.  The magnetic
   field forces them to travel in curved paths in bunches like the spokes of
   a wheel.  The simplest way to describe what happens is that the electron
   bunches brush against the openings of the resonating cavities in the anode
   and excite microwave production in a way analogous to what happens when you
   blow across the top of a Coke bottle or through a whistle.

9. The frequency/wavelength of the microwaves is mostly determined by the size
   and shape of the resonating cavities - not by the magnetic field as is
   popularly thought.  However, the strength of the magnetic field does affect
   the threshold voltage (the minimum anode voltage required for the magnetron
   to generate any microwaves), power output, and efficiency.

  5.12) Cross section diagram of typical magnetron

The really extraordinary ASCII art below represents (or is supposed to
represent) a cross section of the 2M214 type magnetron (not to scale) through
the center as viewed from the side.

                               |  ____  |
                               |_|    |_|  Antenna cap
                               / |____| \
                              |  | || |  | Antenna insulator
                              |  | || |  |
                      xxxxxxxx|__| || |__|xxxxxxxx RF sealing gasket
          ____________________|    ||    |____________________ 
         |        |       (5)||    ||    ||       (5)|        |
         |        |   Top    ||    ||    ||   Top    |        |
         |        |  Magnet  ||    ||    ||  Magnet  |        | Outer case
         |        |__________||    ||    ||__________|        |
         |              ______|     \\   |______              |
         |             /____    (7)   \\    ____\             |
         |____________||    \__  ______ \\ /    ||____________|
         |            ||_______ |__  __| _\\ ___||            |
         |____________||       | o || o |  ||(4)||____________|
         |            ||       | o || o |       ||        (6) | Heat sink fins
         |____________|| Vane  | o || o |  Vane ||____________|
         |            || (3)   | o || o |  (3)  ||            |
         |____________||       | o || o |       ||____________| o: Filament
         |            ||_______|(1)|| o |_______||            |    helix
         |____________||     __ |_||||_| __     ||____________|
         |            ||____/   || ||      \____||<-- (2)     |
         |             \______   \\ \\    ______/             |
         |         __________ |  ||  ||  | __________         |
         |        |       (5)||  ||  ||  ||       (5)|        |
         |        |  Bottom  ||  ||  ||  ||  Bottom  |        |
         |        |  Magnet  ||  ||  ||  ||  Magnet  |        |
         |________|__________||  ||  ||  ||__________|________|
             |                |__||__||__|                |
             |                 | ||  || |  Filament       |
             |                 | ||  || |  insulator      |
             |   (RF chokes    |_||__||_|                 |
             |    not shown)     ||  ||  Filament/cathode |
             |                   ||  ||  connections      |

  5.13) Microwave generator circuit diagram

Nearly all microwave ovens use basically the same design for the microwave
generator.  This has resulted in a relatively simple system manufactured at
low cost.

The typical circuit is shown below.  This is the sort of diagram you are
likely to find pasted inside the metal cover.  Only the power circuits
are likely included (not the controller unless it is a simple motor driven
timer) but since most problems will be in the microwave generator, this
schematic may be all you need.

                                      || +------------------------+
                                      ||( 3.3 VAC, 10 A, typical  |
                        TP  Relay or  || +------------+------+    | Magnetron
           _ Fuse   I   __    Triac   ||              |    +-|----|-+
   o------- _---+---/ --  ----/ ----+ || +------||----+    | |_  _| |
                |                    )||( HV Cap      |    |   \/   |
  AC           I \ I=Interlock       )||(           __|__  |   ___  |
 Line           |  TP=Thermal Prot.  )||( 2,000 VAC _\_/_  +----|:--+
   o------------+-------------------+ ||( .25 A       | HV      |'--> Micro-
                                      ||( typical     | Diode   |      waves
       (Controller not shown)         || +------------+---------+
                                                      -  Chassis ground

Note the unusual circuit configuration - the magnetron is across the diode,
not the capacitor as in a 'normal' power supply.  What this means is that the
peak voltage across the magnetron is the transformer secondary + the voltage
across the capacitor, so the peaks will approach the peak-peak value of the
transformer or nearly 5000 V in the example above.  This is a half wave voltage
doubler.  The output waveform looks like a sinusoid with a p-p voltage equal to
the p-p voltage of the transformer secondary with its positive peaks at chassis
ground (no load).  The peaks are negative with respect to the chassis.  The
negative peaks will get squashed somewhat under load.  Take extreme care - up
to 5000 V at AMPs available!  WARNING: Never attempt to view this waveform on
an oscilloscope unless you have a commercial high voltage probe and know how
to use it safely!

The easiest way to analyze the half wave doubler operation is with the
magnetron (temporarily) removed from the circuit.  Then, it becomes a simple
half wave rectifier/filter so far as the voltage acrtoss the capacitor is
concerned - which will be approximately V(peak) = V(RMS) * 1.414 where V(RMS)
is the output of the high voltage transformer.  The voltage across the HV
rectifier will then be: V(peak) + V where V is the waveform out of the
transformer.  The magnetron load, being across the HV diode, reduces the peak
value of this somewhat - where most of its conduction takes place.

WARNING: What this implies is that if the magnetron is not present or is not
drawing power for some reason - like an open filament - up to V(peak) will
still be present across the capacitor when power is removed.  At the end of
normal operation, some of this will likely be discharged immediately but will
not likely go below about 2,000 V due to the load since the magnetron does not
conduct at low voltages.

Other types of power supplies have been used in a few models - including high
frequency inverters - but it is hard to beat the simplicity, low cost, and
reliability of the half wave doubler configuration.  See the section: "High frequency inverter type HV power supplies".

There is also usually a bleeder resistor as part of the capacitor, not shown.
THE OVEN HAS BEEN POWERED.  The bleeder may be defective and open as this does
not effect operation of oven and/or the time constant may be long - minutes.
Some ovens may not have a bleeder at all.

In addition, there will likely be an over-temperature thermostat - thermal
protector - somewhere in the primary circuit, often bolted to the magnetron
case.  There may also be a thermal fuse or other protector physically
elsewhere but in series with the primary to the high voltage transformer.

Other parts of the switched primary circuit include the oven interlock
switches, cooling fan, turntable motor (if any), oven light, etc.

  5.14) Interlock switches

Various door interlock switches prevent inadvertent generation of microwaves
unless the door is closed completely.  At least one of these will be directly
in series with the transformer primary so that a short in the relay or triac
cannot accidentally turn on the microwaves with the door open.  The interlocks
must be activated in the correct sequence when the door is closed or opened.

Interestingly, another interlock is set up to directly short the power line
if it is activated in an incorrect sequence.  The interlocks are designed
so that if the door is correctly aligned, they will sequence correctly.
Otherwise, a short will be put across the power line causing the fuse
to blow forcing the oven to be serviced.  At least that is the most likely
rational for putting a switch across the power line.

Failed door interlocks account for the majority of microwave oven problems -
perhaps as high as 75 percent.

Chapter 6) Troubleshooting Guide

  6.1) Instant troubleshooting chart - most common problems and possible causes

The following chart lists a variety of common problems and nearly all possible
causes.  Diagnostic procedures will then be needed to determine which actually
apply.  The 'possible causes' are listed in *approximate* order of likelihood.
Most of these problems are covered in more detail elsewhere in this document.

While this chart lists many problems, it is does not cover everything that can
go wrong.  However, it can be a starting point for guiding your thinking in
the proper direction.  Even if not listed here, your particular problem may
still be dealt with elsewhere in this document.

Problem: Totally dead oven.
Possible causes:

 1. No power to outlet (overload or fault in microwave or other appliance).
 2. Blown main fuse - likely due to other problems.
 3. Open thermal protector or thermal fuse.
 4. Defective controller or its power supply.
 5. Clock needs to be set before other functions will operate (some models).

Problem: No response to any buttons on touchpad.
Possible causes:

 1. Door is not closed (some models).
 2. You waited to long (open and close door to wake it up).
 3. Controller is confused (pull plug for a minute or two to reset).      
 4. Defective interlock switches.
 5. Faulty controller or its power supply.
 6. Touchpad or controller board contaminated by overenthusiastic cleaning.
 7. Defective/damaged touchpad.

Problem: Oven runs when door is still open.
Possible causes:

 1. Damaged interlock assembly.
 2. Cooling fans (only) running due to bad sensor or still warm.

Problem: Oven starts on its own as soon as door is closed.
Possible causes:

 1. Defective triac or relay.
 2. Controller is confused (pull plug for a minute or two to reset).
 3. Defective controller or its power supply.
 4. Touchpad or controller board contaminated by overenthusiastic cleaning.
 5. Defective/damaged touchpad.

Problem: Oven works but display is blank.
Possible causes:

 1. Defective controller or its power supply.
 2. Broken display panel.
 3. Oven needs to be reset (pull plug for a minute or two to reset).

Problem: Whacked out controller or incorrect operation.
Possible causes:

 1. Previous or multipart cook cycle not complete.
 2. Controller is confused (pull plug for a minute or two to reset).
 3. Defective controller or its power supply.
 4. Touchpad or controller board contaminated by overenthusiastic cleaning.
 5. Defective/damaged touchpad.
 6. Defective sensor (particulalry covection/mirowave combos).

Problem: Erratic behavior.
Possible causes:

 1. Previous or multipart cook cycle not complete.
 2. Bad connections in controller or microwave generator.
 3. Faulty relay - primary (or HV side, much less commonly used).
 4. Defective controller or its power supply.
 5. Bad contacts/connections on mechanical timers.  Intermittent fuse.
 6. Power surge at start of cook cycle confusing controller.
 7. Microwave (RF) leakage into electronics bay.

Problem: Some keys on the touchpad do not function or perform the wrong action.
Possible causes:

 1. Touchpad or controller board contaminated by overenthusiastic cleaning.
 2. Defective/damaged touchpad.
 3. Controller is confused (pull plug for a minute or two to reset).
 4. Faulty controller.

Problem: Microwave oven does not respond to START button.
Possible causes:

 1. Defective START button.
 2. Faulty interlock switches.
 3. Door is not securely closed.
 4. Faulty controller.
 5. You waited too long - open and close door to wake it up!

Problem: No heat but otherwise normal operation.
Possible causes:

 1. Blown fuse in HV transformer primary circuit or HV fuse (if used).
 2. Bad connections (particularly to magnetron filament).
 3. Open thermal protector or thermal fuse.
 4. Open HV capacitor, HV diode, HV transformer, or magnetron filament.
 5. Shorted HV diode, HV capacitor (will blow a fuse), or magnetron.
 6. Defective HV relay (not commonly used).

Problem: Fuse blows when closing or opening door:
Possible causes:

 1. Defective door interlock switchs.
 2. Misaligned door.

Problem: Loud hum and/or burning smell when attempting to cook.
Possible causes:

 1. Shorted HV diode, magnetron.
 2. Burnt carbonized food in or above oven chamber.
 3. Shorted winding in HV transformer.
 4. Frayed insulation on HV wiring.

Problem: Arcing in or above oven chamber.
Possible causes:

 1. Burnt carbonized food deposits.
 2. Exposed sharp metal edges.

Problem: Fuse blows when initiating cook cycle.
Possible causes:

 1. Defective interlock switches or misaligned door.
 2. Shorted HV capacitor.
 3. Shorted HV diode.
 4. Shorted magnetron (probably won't blow main fuse but HV fuse if used).
 5. Defective triac.
 6. Old age or power surges.
 7. Defective HV transformer.
 8. Short in wiring due to vibration or poor manufacturing.

Problem: Fuse blows when microwave shuts off (during or at end of cook cycle).
Possible causes:

 1. Defective triac (doesn't turn off properly).
 2. Defective relay.
 3. Shorting wires.

Problem: Oven heats on high setting regardless of power setting.
Possible causes:

 1. Faulty primary relay or triac or HV relay (not commonly used).
 2. Faulty controller.

Problem: Oven immediately starts to cook when door is closed.
Possible causes:

 1. Shorted relay or triac.
 2. Faulty controller.

Problem: Oven heats but power seems low or erratic.
Possible causes:

 1. Low line voltage.
 2. Magnetron with low emission.
 3. Faulty controller or set for wrong mode.
 4. Stirrer (or turntable) not working.
 5. Intermittent connections to magnetron filament or elsewhere.
 6. Faulty primary relay or triac or HV relay (not commonly used).   

Problem: Oven heats but shuts off randomly.
Possible causes:

 1. Overheating due to blocked air vents or inoperative cooling fan.
 2. Overheating due to bad magnetron.
 3. Bad connections in controller or microwave generator.
 4. Faulty interlock switch or marginal door alignment.
 5. Faulty controller.
 6. Overheating due to extremely high line voltage.

Problem: Oven makes (possibly erratic) buzzing noise when heating.
Possible causes:

 1. Fan blades hitting support or shroud.
 2. Vibrating sheet metal.
 3. Vibrating transformer laminations.
 4. Turntable or stirrer hitting some debris.

Problem: Oven light does not work.
Possible causes:

 1. Burnt out bulb :-).
 2. Bad connections.

Problem: Fans or turntables that do not work.
Possible causes:

 1. Gummed up lubrication or bad motor bearing(s).
 2. Loose or broken belt.
 3. Bad motor.
 4. Bad thermostat.
 5. Bad connections.

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