Author: Samuel M. Goldwasser
For contact info, please see the Sci.Electronics.Repair FAQ Email Links Page.
Copyright © 1994-2008
Reproduction of this document in whole or in part is permitted if both of the
following conditions are satisfied:
1. This notice is included in its entirety at the beginning.
The power supplies for even the smallest microwave ovens operate at extremely
lethal voltage and current levels. Do not attempt to troubleshoot, repair, or
modify such equipment without understanding and following ALL of the relevant
safety guidelines for high voltage and/or line connected electrical and
electronic systems.
We will not be responsible for damage to equipment, your ego, county wide
power outages, spontaneously generated mini (or larger) black holes, planetary
disruptions, or personal injury or worse that may result from the use of this
material.
For a long time, there was controversy as to whether microwave ovens were
safe - in terms of microwave emissions and molecular damage to the food.
Whether these issues have been resolved or just brushed aside is not totally
clear. Nonetheless, the microwave oven has taken its place in virtually
every kitchen on the planet. Connoisseurs of fine dining will turn up
their collective noses at the thought of using a microwave oven for much
beyond boiling water - if that. However, it is difficult to deny the
convenience and cooking speed that is provided by this relatively simple
appliance.
Microwave ovens are extremely reliable devices. There is a good chance
that your oven will operate for 10 years or more without requiring repairs
of any kind - and at performance levels indistinguishable from when it
was first taken out of the box. Unlike other consumer electronics
where a new model is introduced every 20 minutes - some even have useful
improvements - the microwave oven has not changed substantially in the
last 20 years. Cooking is cooking. Touchpads are now nearly universal
because they are cheaper to manufacture than mechanical timers (and also
more convenient). However, an old microwave oven will heat foods just
as well as a brand new one.
This document provides maintenance and repair information applicable to
most of the microwave ovens in existence. It will enable you to quickly
determine the likely cause and estimate the cost of parts. You will be
able to make an informed decision as to whether a new oven is the better
alternative. With minor exceptions, specific manufacturers and models will
not be covered as there are so many variations that such a treatment would
require a huge and very detailed text. Rather, the most common problems
will be addressed and enough basic principles of operation will be provided
to enable you to narrow the problem down and likely determine a course of
action for repair. In many cases, you will be able to do what is required
for a fraction of the cost that would be charged by a repair center - or - be
able to revive something that would otherwise have gone into the dumpster
or continued in its present occupation as a door stop or foot rest.
Should you still not be able to find a solution, you will have learned a great
deal and be able to ask appropriate questions and supply relevant information
if you decide to post to sci.electronics.repair. In any case, you will have
the satisfaction of knowing you did as much as you could before taking it in
for professional repair. You will be able to decide if it is worth the cost
of a repair as well. With your new-found knowledge, you will have the upper
hand and will not easily be snowed by a dishonest or incompetent technician.
It is quite possible your problem is already covered at the Microtech site.
In that case, you can greatly simplify your troubleshooting or at least
confirm a diagnosis before ordering parts. My only reservation with respect
to tech tips databases in general - this has nothing to do with Microtech
in particular - is that symptoms can sometimes be deceiving and a solution
that works in one instance may not apply to your specific problem. Therefore,
an understanding of the hows and whys of the equipment along with some good
old fashioned testing is highly desirable to minimize the risk of replacing
parts that turn out not to be bad.
More detailed explanations are provided elsewhere in this document.
However, if you can do the repair yourself, the equation changes dramatically
as your parts costs will be 1/2 to 1/4 of what a professional will charge
and of course your time is free. The educational aspects may also be
appealing. You will learn a lot in the process. Many problems can be
solved quickly and inexpensively. Fixing an old microwave for the dorm
room may just make sense after all.
Make sure the outlet is in good condition in either case. Check that the
plug (or adapter) fits tightly and that there is no appreciable heating
of the outlet during use of the microwave oven. If there is, spread the
metal strips of each of the prongs apart if possible and/or replace the
outlet.
A grounded outlet is essential for safety. Microwave ovens are high
power devices and a separate circuit will eliminate nuisance fuse blowing
or circuit breaker tripping when multiple appliances are being used at
the same time. It will also minimize the possibility of Radio Frequency
Interference (RFI) between it and any electronic equipment which might be
on the same circuit. A GFCI is not needed as long as the outlet is properly
grounded and may result in nuisance tripping with some microwave ovens.
Inexpensice outlet testers are available at hardware stores, home centers,
and electrical parts distributors, to confirm that the outlet is properly
wired and grounded.
If it is too late and you have a recurring problem of cockroaches getting
inside the electronics bay, tell them to get lost and then put window screen
over the vents (or wherever they are entering). Such an open mesh should
not affect the cooling of the electronic components significantly. However,
the mesh will likely clog up more quickly than the original louvers so make
sure it is cleaned regularly. If possible, clean up whatever is attracting
the unwanted tenants (and anything they may have left behind including their
eggs!!). WARNING: See the section: SAFETY before going
inside.
CAUTION: Do not spray anything into the holes where the door latch is inserted
or anywhere around the touchpad as this can result in internal short circuits
and costly damage - or anywhere else inside, for that matter. If you do this
by accident, immediately unplug the oven and let it dry out for a day or two.
WARNING: This only applies to a *working* microwave oven! If there is no
heat, the magnetron may not be drawing any current from the HV power
supply and the HV capacitor can remain charged for a long time. In this
case, there is a very real risk of potentially lethal electrical shock even
after several minutes or more of being unplugged! See the section:
SAFETY if you will be troubleshooting a microwave oven.
Please see Typical Microwave Oven Electronics Bay
for parts identification.
WARNING! WARNING! WARNING! WARNING! WARNING! WARNING! WARNING! WARNING!
Microwave ovens are probably the most dangerous of consumer appliances
to service. Very high voltages (up to 5000 V) at potentially very high
currents (AMPs) are present when operating - deadly combination. These
dangers do not go away even when unplugged as there is an energy storage
device - a high voltage capacitor - that can retain a dangerous charge
for a long time. If you have the slightest doubts about your knowledge
and abilities to deal with these hazards, replace the oven or have it
professionally repaired.
Careless troubleshooting of a microwave oven can not only can fry you from
high voltages at relatively high currents but can microwave irradiate you as
well. When you remove the metal cover of the microwave oven you expose
yourself to dangerous - potentially lethal - electrical connections. You
may also be exposed to potentially harmful levels of microwave emissions if
you run the oven with the cover off and there is damage or misalignment to
the waveguide to the oven chamber.
There is a high voltage capacitor in the microwave generator. Always ensure
that it is totally discharged before even thinking about touching or probing
anything in the high voltage power circuits. See the troubleshooting sections
later in this document.
To prevent the possibility of extremely dangerous electric shock, unplug
the oven from the AC outlet before removing the cover and do not plug it in
to operate it with the cover off if at all possible. If you must probe
live, remove the connections to the magnetron (see below) to prevent the
inadvertent generation of microwaves except when this is absolutely needed
during troubleshooting. Discharge the high voltage capacitor (with the oven
unplugged) and then use clip leads to make any connections before you plug
it in and apply power. Then after removing power and unplugging the oven
discharge the HV capacitor once again.
WARNING: Experienced technicians have been electrocuted deader than a brick
from even careful probing of the HV circuits of a powered microwave oven.
Therefore, I highly recommend avoiding any probing of the HV circuits - nearly
everything can be determined by inspection and component tests with the oven
unplugged.
The microwave oven circuitry is especially hazardous because the return for
the high voltage is the chassis - it is not isolated. In addition, the HV
may exceed 5000 V peak with a continuous current rating of over .25 AMP at
50/60 Hz - the continuous power rating of the HV transformer may exceed
1,500 W with short term availability of much greater power. Always observe
high voltage protocol.
There is one additional non-electrical safety concern that is *probably* not
present in consumer microwave ovens but still needs to be mentioned.
That is the pretense of beryllium oxide
or BeO, an extremely toxic material in dust or powder form. (Solid BeO is
not particularly hazardous.) A ceramic made of BeO is an excellent heat
conductor and for this reason may be present in the insulating parts of radar
magnetrons as well as high power laser tubes and the like. If BeO is present,
there should be at least one prominent warning label. However, there is
always the possibility of a really old microwave oven having a magnetron
containing BeO with no warning label or where it fell off. So, it is
good practice to NOT attempt to break, smash, grind, pulverize, or otherwise
attack the ceramic insulator on the top of the magnetron.
These guidelines are to protect you from potentially deadly electrical shock
hazards as well as the equipment from accidental damage.
Note that the danger to you is not only in your body providing a conducting
path, particularly through your heart. Any involuntary muscle contractions
caused by a shock, while perhaps harmless in themselves, may cause collateral
damage - there are many sharp edges inside this type of equipment as well as
other electrically live parts you may contact accidentally.
The purpose of this set of guidelines is not to frighten you but rather to
make you aware of the appropriate precautions. Repair of TVs, monitors,
microwave ovens, and other consumer and industrial equipment can be both
rewarding and economical. Just be sure that it is also safe!
For the microwave oven in particular, use a 25K to 100K resistor rated for
at least 5 kV and several watts with
a secure clip lead to the chassis. Mount the resistor on the end of a well
insulated stick. Touch each of the capacitor terminals to the non-grounded
end of the resistor for several seconds. Then, to be doubly sure that the
capacitor if fully discharged, short across its terminals with the blade of
a well insulated screwdriver. I also recommend leaving a clip lead shorting
across the capacitor terminals while working as added insurance. At most,
you will blow a fuse if you should forget to remove it when powering up the
microwave.
As noted, a GFCI (Ground Fault Circuit Interrupter) will NOT protect you
from the high voltage since the secondary of the HV transformer is
providing this current and any current drawn off of the secondary
to ground will not be detected by the GFCI. However, use of a GFCI is
desirable to minimize the risk of a shock from the line portions
of the circuitry if you don't have an isolation transformer.
An isolation transformer is even limited value as well since the chassis IS
the HV return and is a large very tempting place to touch, lean on, or brush
up against.
And, of course, none of these devices will protect fools from themselves!
Take extreme care whenever working with the cover off of a microwave oven.
There's little point to using an isolation transformer with a microwave
for testing the high voltage circuitry. It would have to be HUGE due to the
high power nature of a microwave oven and since the high voltage return is the
chassis which is grounded, it won't be terribly useful as noted above.
However, an isolation transformer can and should be used to test the primary
side circuitry if necessary including interlocks, motors, triac/relay, etc.
Disconnect the HV transformer to eliminate the possibility of high voltage
shock and to reduce the load.
Actually, the best policy is to NEVER EVER attempt to measure anything
in the HV section while the oven is powered - it's almost never needed in
any case. Failures are usually easily found by performing test with the
oven unplugged. If you insist on making live measurements, connect the
meter before power is applied and disconnect or move its probes only
after power is removed AND the HV cap has been discharged (even if the meter
catches fire or explodes!). Qualified service people have been electrocuted
using proper test equipment on microwave ovens!
If you get stuck, sleep on it. Sometimes, just letting the problem
bounce around in your head will lead to a different more successful
approach or solution. Don't work when you are really tired - it is both
dangerous (particularly with microwave ovens) and mostly non-productive
(or possibly destructive - very destructive).
If you need to remove the cover or other disassembly, make notes of which
screw went where - they may not all be identical. More notes is better
than less.
Pill bottles, film canisters, and plastic ice cube trays come in handy for
sorting and storing screws and other small parts after disassembly.
Select a work area which is well lighted and where dropped parts can
be located - not on a deep pile shag rug. Something like a large plastic
tray with a slight lip may come in handy as it prevents small parts from
rolling off of the work table. The best location will also be relatively
dust free and allow you to suspend your troubleshooting to eat or sleep or
think without having to pile everything into a cardboard box for storage.
A basic set of high quality hand tools will be all you need to work on a
microwave oven. These do not need to be really expensive but poor quality
tools are worse than useless and can cause damage. Stanley or Craftsman
are fine. Needed tools include a selection of Philips and straight blade
screwdrivers, needlenose pliers, wire cutters and wire strippers.
A medium power soldering iron and rosin core solder (never never use acid
core solder or the stuff for sweating copper pipes on electronic equipment)
will be needed if you should need to disconnect any soldered wires (on
purpose or by accident) or replace soldered components.
However, most of the power components in microwave ovens use solderless
connectors (lugs) and replacements usually come with these as well.
See the document: Troubleshooting and Repair of
Consumer Electronics Equipment for additional info on soldering and
rework techniques and other general information.
An assortment of solderless connectors (lugs and wirenuts) is handy when
repairing the internal wiring. A crimping tool will be needed as well but
the $4 variety is fine for occasional use.
Old dead microwaves can often be valuable source of hardware and sometimes
even components like interlock switches and magnetrons as these components
are often interchangeable. While not advocating being a pack rat, this
does have its advantages at times.
A DMM or VOM is necessary for checking of power supply voltages (NOT
the high voltage, however) and testing of interlock switches, fuses,
wiring, and most of the components of the microwave generator. This does
not need to be expensive but since you will be depending on its readings,
reliability is important. Even a relatively inexpensive DMM from Radio
Shack will be fine for most repair work. You will wonder how you ever
lived without one! Cost: $25-50.
Other useful pieces of 'test equipment':
There are special magnetron and microwave test instruments but unless you are
in the business, these are unnecessary extravagances.
The technique I recommend is to use a high wattage resistor of about 5 to
50 ohms/V of the working voltage of the capacitor. This will prevent the
arc-welding associated with screwdriver discharge but will have a short enough
time constant so that the capacitor will drop to a low voltage in at most a
few seconds (dependent of course on the RC time constant and its original
voltage).
The reason for specifying the resistor in this way is for voltage hold-off.
Common resistors only are rated for 200 to 500 V, but there may be as much
as 5 kV on the HV cap. You don't want the HV zapping across the terminals
of the resistor. Special high voltage resistors are available but they are
expensive and not readily available from common electronics distributors.
WARNING: DO NOT use a DMM for checking voltage on the capacitor unless you
have a proper high voltage probe. If your discharging did not work, you may
blow everything - including yourself.
A suitable discharge tool can be made as follows:
This discharge tool will keep you safely clear of the danger area. The
capacitor discharge indicator circuit described in the document:
Capacitor Testing, Safe Discharging and Other Related
Information can be built into the discharge tool if desired.
Again, always double check with a reliable high voltage meter or by shorting
with an insulated screwdriver!
Reasons to use a resistor and not a screwdriver to discharge capacitors:
Unplug the unit! Usually, the sheet metal cover over the top and sides
is easily removed after unscrewing 8-16 philips head or hex head sheet
metal screws. Most of these are on the back but a few may screw into the
sides. They are not usually all the same! At least one of these includes
a lockwasher to securely ground the cover to the case.
Note that on some ovens (I've heard that some Sharp models do this), there
may also be one screw that is slightly longer than the others to engage a
safety case interlock switch and prevent the oven from getting power if it
is not present or one of the shorter screws is used in its place.
So, with the cover removed, nothing is powered inside (which is a good
thing for safety!). But when the cover is
replaced with the screws in random locations, there's a high probability
that the oven no longer works at all. Kind of like Russian Roulette.
And, if it's then taken to a service center, they will know someone has
been inside. If less than entirely honest, they can make any sort of
claim they want as to what might have been damaged even if all you did
was remove and replace the cover without touching anything inside.
"The repair will be $195 because you blew out the touch panel by removing the
cover."
Therefore, it is essential to make note of any differences
in screw types so they can be put back in the same place. The cover will
then lift up and off. Note how fingers on the cover interlock with
the main cabinet - these are critical to ensure prevention of microwave
leakage after reassembly.
Please see Typical Microwave Oven Electronics Bay
for parts identification. Not all ovens are this wide open. If yours is a
compact unit, everything may be really squeezed together. :) Details will
vary depending on manufacturer and model but most of the major components will
look fairly similar to those depicted in the photo. Note that for this model,
the oven lamp is actually inside the electronics bay right next to the high
voltage on the magnetron filament - light bulb changing here is really best
left to a professional if you would otherwise not go inside!
Discharge the high voltage capacitor as described in the section:
Safe discharging of the high voltage capacitor
before even thinking about touching anything.
A schematic showing all of the power generation components is usually
glued to the inside of the cover. How much of the controller is included
varies but is usually minimal.
Fortunately, all the parts in a microwave can be easily replaced and most of
the parts for the microwave generator are readily available from places
like MCM Electronics, Dalbani, and Premium Parts.
Reassemble in reverse order. Take particular care to avoid pinching any
wires when reinstalling the cover. Fortunately, the inside of a microwave
is wide open and this is not difficult. Make sure ALL of the metal fingers
around the front edge engage properly with the front panel lip. This is
critical to avoid microwave emissions should the waveguide or magnetron
become physically damaged in any way. Confirm that the screws you removed
go back in the proper locations, particularly the one that grounds the
cover to the chassis.
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
forth.
From: Barry L. Ornitz (ornitz@tricon.net).)
Water has numerous resonances over the entire spectra range, but the lowest
frequency resonance is the rotational resonance is around 24 GHz. Other
resonances occur in the millimeter wave range through the infrared.
For references, check books on microwave spectroscopy by Townes and Gordy."
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.
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
oven.
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!
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).
Power level in most microwave ovens 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.
However, some models use finer control, even to the point of a continuous
range of power. These are typically "inverter" models which use a more
sophisticated type of power supply than the simple high voltage transformer,
capacitor, rectifier, system described below. However, there have been
some back in the 1970s that did this with a 1 second or so pulse width
modulated cycle, fast enough to have the same effect as continuous control
for all practical purposes.
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.
Since these sensors are exposed to the food or its vapors, failures of the
sensor probes themselves are common.
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.
The most common microwave generator consists of the following:
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.
These transformers are designed with as little copper as possible. The
primary for 115 VAC is typically only 120 turns of thick wire - thus about 1
turn per volt input and output (this is about 1/4th as many turns as in a
"normal" power transformer. (It's usually possible to count the primary
turns by examining how it is wound - no disassembly required!) So there
would be about 3 turns for the magnetron filament and 2080 turns for the
high voltage winding for the transformer mentioned above. The reason they
can get away with so few turns is that it operates fully loaded about 90
percent of the time but is still on the hairy edge of core saturation.
The HV components are actually matched to the HV transformer characteristics.
Performance will suffer if the uF value of a replacement HV capacitor is not
close to that of the original.
There is also generally a "magnetic shunt" in the core of the transformer.
This provides some current limiting, possibly to compensate for various
magnetron load conditions. However, it's not enough to provide any reduction
in the likelihood of electrocution should you come in contact with the
HV winding!
When salvaging parts from dead microwave ovens, save the HV components
(transformer, capacitor, and diode) as a group (assuming all are still
good). Then, if a repair is needed to another oven it may be better to
replace all 3 both because this eliminates uncertainty if more than 1
part failed or is marginal, and they will have been designed to have
the best compatibility.
The transformer goes by several names, depending on where you are. Variable
reluctance, leakage flux, stray flux, etc. It is exactly the same
construction and operating principle as a neon transformer, some kinds of HID
light ballasts and some series streetlight constant current transformers.
The core is an almost standard "E" core (or "H" core if you prefer) with one
exception. The center leg has an air gap. The windings are on the end legs
of the "E" instead of the center leg.
There are two magnetic paths around the core for the field set up by the
primary to travel. Around the periphery and across the secondary and around
the center leg and across the air gap. The field that travels along the center
leg does not cross the secondary and induces no voltage.
With no load applied, the bulk of the field travels the peripheral, very much
lower reluctance solid iron path, inducing full secondary voltage proportional
to the turns ratio. As current flows in the secondary, counter-MMF raises the
reluctance of the peripheral path so that some of the flux travels through the
center leg. With less flux traveling around the periphery and cutting across
the secondary, the secondary voltage drops as the current remains about the
same. At the limit, if the secondary is shorted, the peripheral path has so
much reluctance that most of the flux travels the center leg and across the
air gap. The same current as before flows through the secondary but at zero
volts.
When the dimensions of the core and gap are set up correctly, the transformer
behaves as an almost perfect constant current device. That is, the secondary
voltage varies as necessary to keep the same current flowing through a varying
load. Just what the doctor ordered to keep the magnetron happy.
The secondary current can be increased by opening up the air gap. This raises
the reluctance of that path and forces more field through the secondary leg.
Closing the gap has the opposite effect.
The center leg is often called the magnetic shunt and frequently it is a
separate piece of laminated iron stuck between the coils and TIG welded in
place. It is a common trick for Tesla Coilers to open up a neon transformer
and either knock out the shunt entirely or grind it down to open the air gap.
This modification causes the transformer to output much more current than it
is designed for - for a little while, at least :-) The same thing works with
microwave oven transformers (MOT).
This design in a microwave oven is a vital part of keeping the magnetron anode
current within spec. The magnetron is electrically a diode. A diode that
isn't emission-limited would draw destructive current if not externally
limited. With this design, the filament can be heated good and hot for long
life and not have the tube run away. The design also is vital for protecting
the magnetron from potentially damaging conditions such as operating the oven
empty, arcing, etc.
It's popular to use several MOTs to build an arc welder. This works quite
well specifically because these transformers are constant-current devices -
exactly the characteristic stick welding needs. If they were conventional
transformers, the first time the rod touched the work and shorted the
secondary, fault current would flow and the breaker would trip or blue smoke
would leak out.
Along similar lines, one can cut off the high voltage secondary and replace it
with a suitable number of turns of heavy wire, connect a bridge rectifier and
have a nice constant current battery charger. Select the turns carefully and
it'll do the bulk/absorption stages of the smart 3 stage charging algorithm.
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
operation.
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).
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.
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 the form of a ceramic of beryllium oxide (BeO) in large radar
magnetrons due to its excellent heat conductivity. But should not be
present in modern domestic microwave ovens. However, see the section:
SAFETY.
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.
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
plate.
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
cylinder.
The anode and magnetron case are at ground potential and connected to the
chassis.
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.
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.
Note that there is a difference in the labels on the filament connections of
the magnetron. Functionally, it probably doesn't matter which way they are
connected. However, the typical schematic (as above) shows FA going to
the node attached to the Anode of the HV diode, while F goes to the lone
Filament terminal on the HV transformer.
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.
HOWEVER: DO NOT ASSUME THAT THIS IS SUFFICIENT TO DISCHARGE THE CAPACITOR -
ALWAYS DO THIS IF YOU NEED TO TOUCH ANYTHING IN THE MICROWAVE GENERATOR AFTER
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.
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. This makes it more difficult for
an ignorant consumer to just bypass the door interlocks should they fail or
to run the oven with an open door as a room heater - and protects the
manufacturer from lawsuits. (That interlock may be known as a "dummy switch"
for obvious reasons and is often not even mentioned in the schematic/parts
manifest.) Of course, should that switch ever actually be used, not only will
the fuse blow, but the switch contacts will likely be damaged by the high
initial current! This also means it probably wouldn't be a bad idea to
replace the interlock switch which might have been affected if your oven
fails with a blown fuse due to a door problem.
Failed door interlocks account for the majority of microwave oven problems -
perhaps as high as 75 percent. This is not surprising considering that two
of the three switches carry the full oven current - any deterioration of the
contacts results in increased resistance leading to their heating and further
deterioration. And, opening the door to interrupt a cook cycle results in
arcing at the contacts. Complete meltdowns are not unusual! If any defective
door switches are found, it is probably a good idea to replace all of them as
long as the oven is already apart.
The typical door switches and their function:
Note that if the Door Sensing switch should malfunction, peculiar behavior
may occur (like the fan or turntable operating at the wrong time) but should
never result in microwaves being generated with the door open.
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.
The interlock switches, being electromechanical can fail to complete the
primary circuit on an oven which appears to operate normally with no blown
fuses but no heat as well. Faulty interlocks or a misaligned door may result
in the fuse blowing as described above due to the incorrect sequencing of the
door interlock switches. Failed interlocks are considered to be the most
common problems with microwave ovens, perhaps as high as 75% of all failures.
See the section: Testing and replacing of interlock
switches.
No adjustments should ever be required for a microwave oven and there are no
screws to turn so don't look for any!
First, unplug the microwave oven for a couple of minutes. Sometimes, the
microcontroller will get into a whacko mode for some unknown reason - perhaps
a power surge - and simply needs to be reset. The problem may never reoccur.
Note: when working on controller related problems, unplug the connection
to the microwave generator (HV transformer primary) from the power relay
or triac - it is often a separate connector. This will prevent any possible
accidental generation of microwave energy as well as eliminating the high
voltage (but not the AC line) shock hazard during servicing.
If this does not help, there is likely a problem with the controller circuitry
or its power and you will have to get inside the oven.
Clean the circuit board and connectors thoroughly with water and then isopropyl
alcohol. Dry completely. Inspect the circuit traces for corrosion or other
damage. If there are any actual breaks, these will have be be jumpered with
fine wire and then soldered. Hopefully, no electronic components were affected
though there is always a slight possibility of other problems.
If you find the fuse blown or circuit breaker tripped, unplug everything from
the circuit to which the microwave is connected (keep in mind that other
outlets may be fed from the same circuit). Replace the fuse or reset the
circuit breaker. If the same thing happens again, you have a problem with
the outlet or other wiring on the same branch circuit. If plugging in the
microwave causes the fuse to blow or circuit breaker to trip immediately,
there is a short circuit in the power cord or elsewhere.
The microwave oven may be powered from a GFCI outlet or downstream of one and
the GFCI may have tripped. (Removing a broken oven lamp has been known to
happen.) The GFCI outlet may not be in an obvious location but first check
the countertop outlets. The tripped GFCI could be in the garage or almost
anywhere else! Pushing the RESET button may be all that's needed.
Next, try to set the clock. With some ovens the screen will be totally blank
following a power outage - there may be nothing wrong with it. Furthermore,
some ovens will not allow you perform any cooking related actions until the
clock is set to a valid time.
Assuming these are not your problems, a fuse has probably blown although
a dead controller is a possibility.
If the main fuse is upstream of the controller, then any short circuit
in the microwave generator will also disable the controller and display.
If this is the case, then putting in a new fuse will enable the
touchpad/display to function but may blow again as soon as a cook cycle
is initiated if there is an actual fault in the microwave circuits.
Therefore, try a new fuse. If this blows immediately, there may be a
short very near the line cord, in the controller, or a defective triac
(if your oven uses a triac). Or, even a shorted oven lamp - remove and
inspect the light bulb and socket.
If it does not blow, initiate a cook cycle (with a cup of water inside). If
the oven now works, the fuse may simply have been tired of living. This is
common.
If the fuse still blows immediately, confirm that the controller is
operational by unplugging the microwave generator, power relay, and/or
triac from the controller. If a new fuse does not now blow when a cook
cycle is initiated - and it appears to operate normally - then one of
the components in the microwave generator is defective (shorted). See
the section: Microwave generator problems.
Some models have a thermal fuse as well and this may have failed for no
reason or a cooling fan may not be working and the oven overheated (in
which case it probably would have died while you were cooking something
for an important guest - assuming you would use a microwave oven for such
a thing!).
Other possible causes: bad controller power supply or bad controller chip.
Of course, any number of other pre-existing or induced problems can result
in the oven playing dead after it has been "repaired". :
If the controller power supply is working and there is still no sign of life
(dead display and no response to buttons) the microcontroller chip or some
other part may be bad. It could be a simple part like a capacitor or diode,
but they would all need to be tested. At this point, a schematic of the
controller board will be needed - often impossible to get - and replacement
controller or even just the main chip may be nearly as expensive as a complete
new oven.
Also see the section: Some of the keys on the touchpad do
not function or perform the wrong action.
For microwaves to actually be generated with the door still open would require
the failure of all 3 interlock switches. The only way this could really
happen would be for the 'fingers' from the door that engage the interlocks to
break off inside the oven keeping the interlocks engaged. In this case, the
controller would think the door was always closed.
Where no such damage is evident, a failure of this type is extremely unlikely
since power to the microwave generator passes through 2 of the 3 interlock
switches. If both of these failed in the closed position, the third switch
would have blown the fuse the last time the door was opened.
Another more benign possibility is that one or more fans are running as a
result of either a defective sensor or normal operation to maintain air flow
until all parts have cooled off.
First, unplug the oven for a couple of minutes to try to reset the controller.
If this doesn't help, put a cup of water into the oven and let it run for a
minute to check for heating. (You could also note the normal sound change or
slight dimming of lights that accompanies operation of the magnetron.)
Much more must be enabled to actually power the magnetron so this might point
more to the controller as being faulty but not always.
Also see the section: Whacked out controller or incorrect
operation.
Try pulling the plug for a minute or two - for some reason the display portion
of the controller may have been sent out to lunch by a power surge or alpha
particle. It woudn't be the first time.
Check for bad connections between the display panel and the power supply
and solder joints on the controller board.
With everything else operational, a bad microcontroller chip is not that
likely but is still a possibility. If the oven was physically abused,
the display panel may have fractured though it would take quite a bit
of violence. In this case, more serious damage to the door seals may
have resulted as well which would be a definite hazard.
First, try unplugging the oven for a couple of minutes - perhaps the controller
is just confused due to a power surge, lightning strike or the EMP from a
nearby nuclear detonation because it wanted attention.
If you recently cleaned the oven, some liquid may have accidentally gotten
inside the touchpad or even the controller circuitry (though this is less
likely). See the section: Some of the keys on the
touchpad do not function or perform the wrong action.
If the oven seems to have a mind of its own - running a cycle you didn't
think you programmed, are you sure a previous cook cycle was not interrupted
and forgotten? Try to recreate the problem using a cup of water as a load.
Assuming this does not apply, it sounds like a controller problem - possibly
just a power supply but could also be the controller chip. My guess is that
unless you were to find some simple bad connections or an obvious problem
with the controller's power supply, the cost to repair would be very high
as the custom parts are likely only available from the manufacturer.
The controller's program may be corrupted (unlikely) but we have no real way
of diagnosing this except by exclusion of all other possibilities. Depending
on the model, some or all operations - even setting the clock - may be
conditional on the door interlocks being closed, so these should be checked.
Some ovens will not allow any actions to be performed if the door has been
closed for more than a few minutes - open and close the door to reset.
A controller failure does little to predict the reliability of the rest
of the oven. The microwave generator circuits could last a long time
or fail tomorrow. The output of the magnetron tube may decrease slightly
with use but there is no particular reason to expect it to fail any time
soon. This and the other parts are easily replaceable.
However, unless this oven has a lot of fancy features, you can buy a
replacement (depending on size) for $100-200 so it is probably not worth
fixing unless it is something relatively simple and inexpensive.
The filter capacitor(s) in the controller's power supply may be dried
up or faulty. Check with a capacitor meter or substitute known good ones.
Prod the logic board to see if the problem comes and goes. Reseat the
flex cable connector to the touchpad.
For mechanical timers, the timing motor could be defective or require
lubrication. The contacts could be dirty or worn. There may be bad
connections or loose lugs.
The primary relay may have dirty or burnt contacts resulting in erratic
operation. If the oven uses a HV relay for power control, this may be
defective.
If the times and power levels appear on the display reliably but then become
scrambled when entering the cook cycle or the oven behaves strangely in some
other way when entering the cook cycle, there are several possibilies:
I only service Amana's, but have serviced lot's of them over the years. I've
only found a few that leaked with my expensive leak detector. The most
memorable was the one with the leak that was due to the copper gasket that's
between the magnetron tube and the cavity. I just reformed the gasket and
reseated the magnetron and that fixed the leak.
The symptom was that the Touch Pad timer lights and indicators would change
while the unit was cooking. I thought I had a timer problem. I took it apart
and checked for loose solder joints and even cleaned the glass touch pad
contacts.
For some reason that I don't remember now, I checked for radiation with the
cover off the unit and found it extremely high.
It turned out that the radiation was affecting the controller.
From the outside, with the cover on, the unit didn't leak.
Long ago, I tried one of the cheapie detectors because one of my parts supply
houses suggested it, and it detected leaks on everything. After that I
shelled out the bucks and bought a real detector.
(From: Matthew Sekulic (goatboy@telusplanet.net).)
I have had a similar experience with a Sanyo, similar symptoms, but with the
leakage from the spot welded waveguide inside the unit. Our calibration meter
showed a two watt leakage, with none escaping the outer case when attached.
(My worst case of actual external leakage was from a misaligned door at
.75 watts with the probe's styrofoam spacer placed against the door, of course
dropping off to near zero a few inches away. My clue in was a spark between
the waveguide and the case, when I was messing with the Controller PCB.)
Look carefully for any visible signs of damage or spills. The touchpads
often use pressure sensitive resistive elements which are supposed to be
sealed. However, any damage or just old age may permit spilled liquid
to enter and short the sensors. A week or so of drying may cure these
problems. If there is actual visible damage, it may be necessary to
replace the touchpad unit, usually only available from the original
manufacturer. Also, check the snap type connector where the touchpad
flex-cable plugs into the controller board. Reseating this cable may cur
a some keys dead problem.
Some people have reported at least temporary improvement by simple peeling
the touch pad off of the front panel and flexing it back and forth a few
times. Presumably, this dislodges some bit of contamination. I am skeptical
as this could just be a side effect of a bad connection elsewhere.
With a little bit of effort (or perhaps a lot of effort), the internal
circuitry of the touchpad can be determined. This may require peeling it
off of the front panel). Then, use resistors to jumper the proper contacts
on the flex cable connector to simulate key presses. This should permit
the functions to be verified before a new touchpad is ordered.
Caution: unplug the microwave generator from the controller when doing
this sort of experiment!
If the problem was the result of a spill into the touchpad, replacement will
probably be needed.
However, if you have nothing to lose, and would dump it otherwise, remove the
touchpad entirely and wash it in clean water in an effort to clear out any
contamination, then do the same using high purity alcohol to drive out the
water, and then dry it out thoroughly. This is a long shot but might work.
If there is an alternate way of activating the cook cycle, try it. For
example, Sharp Carousel IIs have a 'Minute Plus' button which will cook
for one minute on HIGH. Use this to confirm the basic controller logic and
interlock circuitry. If it works, then the problem may indeed be a faulty
START button. If it is also ignored, then there may be a bad interlock
or some other problem with the controller.
Check for bad interlocks or interlocks that are not being properly activated.
Next confirm if possible that the START touch pad button is not itself faulty.
If you can locate the matrix connections for this button, the resistance should
go down dramatically (similar to the other buttons). See the section:
Some of the keys on the touchpad do not function or
perform the wrong action. The START button does, after all, sees quite
a lot of action!
Assuming it is not the touch pad, it sounds like the controller is either not
sensing the start command or refusing to cooperate for some reason - perhaps
it thinks an interlock is open. Otherwise, the timer would start counting.
Testing the relay or triac control signal will likely show that it is not
there. Check that there are no missing power supply voltages for the
controller and bad connection.
Most of these are easy to diagnose and the required parts are readily
available at reasonable prices.
Some models may have a separate high voltage fuse. If this is blown, there
will be no heating but no other symptoms. However, high voltage fuses are
somewhat rare on domestic ovens.
A number of failures can result in the fuse NOT blowing but still no heat:
A shorted HV diode, magnetron, or certain parts of the HV wiring would
probably result in a loud hum from the HV transformer but will likely not
blow the main fuse. (However, the HV fuse - not present on most domestic
ovens - might blow.)
Depending on design, a number of other component failures could result in
no heat as well including a defective relay or triac, interlock switch(s),
and controller.
(From: Bonita Lee Geniac (bgen@wdl.net).)
When the timer counts down but nothing else works, 99% of the time the lower
door switch is bad or else the door is not closing fully and the latch hooks
are not depressing the upper and lower switches. There is also a slight
possibility that the relay or triac on the control board is not closing but
those usually do not result in these particular symptoms. Most of the
microswitches used in recent production microwaves are very poor quality
and the silicone lubrication used by some of the manufacturers migrates
into the switch contact area and makes the switch fail even faster than
it should.
The cause is almost certainly related to either the door interlock switches
or the door itself. Marginal door alignment, broken 'fingers' which operate
the switches, dislocated parts in the interlock mechanism, or a defective
interlock switch may result in either consistent or erratic behavior of this
type.
On some ovens, this can happen at any time regardless of the control panel
settings or whether the oven is in the cook cycle or not. On others, it can
only happen when interrupting the cook cycle by opening the door or when
initiating the cook cycle from the front panel (if the switches are in the
wrong state).
The rational for this basic design - some form of which is used in virtually
all microwave ovens - is that a defect in the interlock switches or door
alignment, which might result in dangerous microwave radiation leakage, will
produce a hard permanent failure. This will prevent the oven from being used
until it is inspected and repaired.
See the section: Testing and replacing of interlock
switches.
Note that a short on the load side of the HV capacitor will likely result
in the actual wattage drawn from the power line being
much lower than under normal conditions. Although
there will be a high current flowing in the HV transformer secondary
through the HV capacitor (which is what causes the hum or buz), the
real power consumed will be reduced since the
current and voltage will be out of phase (due to the series capacitor)
and the power factor will be low.
A reading on an AC line wattmeter of 300 W compared to the normal 1,200 to
1,500 W would be reasonable.
The following procedure will quickly identify the most likely component if
the problem is not food/spills/carbon related:
(Usually a loud hum that doesn't result in a blown main fuse is caused by a
short in the HV diode, magnetron, or wiring on the load side of the HV
capacitor. The other items listed below would likely blow the main fuse but
possibly not always.)
(Portions from: Tony (tonyb@ramhb.co.nz).)
All Rights Reserved
2. There is no charge except to cover the costs of copying.
DISCLAIMER
Careless troubleshooting of a microwave oven can result in death or worse.
Experienced technicians have met their maker as a result of a momentary lapse
of judgement while testing an oven with the cover removed. Microwave ovens
are without a doubt, the most deadly type of consumer electronic equipment
in wide spread use.
Introduction
Radar Range anyone?
Remember when you actually had to use the real oven to defrost a TV
dinner? Think back - way back - before VCRs, before PCs (and yes, before
Apple computers as well), almost before dinosaurs, it would seem. There
was a time when the term 'nuke' was not used for anything other than bombs
and power reactors.
On-line microwave oven repair database
Microtech maintains a web site with a large amount of information on microwave
oven repair including an on-line Tech Tips Database with hundreds of solutions
to common problem for many models of microwave ovens. There are also an
extensive list of microwave oven related links to other interesting sites
(including this document!). The comprehensive
Safety Info is a
must read as well. Not entirely coincidentally, I assume, some of its
wording appears remarkably familiar! Microtech also offers instructional
videos and books on microwave oven and VCR repair.
The simplest problems
Repair or replace?
With small to medium size microwave ovens going for $60-100 it hardly makes
sense to spend $60 to have one repaired. Even full size microwave ovens with
full featured touchpanel can be had for under $200. Thus, replacement
should be considered seriously before sinking a large investment into an
older oven.
Installation and Preventive Maintenance
Microwave oven installation and use
To assure safety and convenient, follow these recommendations:
Microwave oven maintenance
Most people do not do anything to maintain a microwave oven. Many will go
for 20 years or more without any noticeable decline in performance.
While not much preventive maintenance is needed, regular cleaning at least
will avoid potentially expensive repairs in the future. Most of this
involves things that don't require going inside and anyone can do. A shop
that wants to add on preventive maintenance while doing some other repair
is just trying to pad their wallet - anything that was required to
ensure the health of the oven should have been included. :)
How long does microwave energy hang around?
You have probably been warned by your mother: "Wait a few seconds (or minutes)
after the beep for all the microwaves to disappear". There is no scientific
basis for such a recommendation. Once the beep has sounded (or the door has
opened), it is safe. This is because:
Microwave Oven Troubleshooting
SAFETY
The following applies to microwave oven troubleshooting - once the cabinet
cover is removed. There is also safety information on proper use of the
oven in subsequent sections, below.
Safety guidelines
Isolation transformers and microwave ovens
Troubleshooting tips
Many problems have simple solutions. Don't immediately assume that
your problem is some combination of esoteric complex convoluted
failures. For a microwave oven, there may be a defective door
interlock switch or just a tired fuse.
Test equipment
Don't start with the electronic test equipment, start with some analytical
thinking. Many problems associated with consumer electronic equipment
do not require a schematic (though one may be useful). The majority of
microwave oven problems are easily solved with at most a multimeter (DMM
or VOM). You do not need an oscilloscope for microwave oven repair unless
you end up trying to fix the logic in the controller - extremely unlikely.
Safe discharging of the high voltage capacitor
It is essential - for your safety and to prevent damage to the device under
test as well as your test equipment - that the large high voltage capacitor
in the microwave generator be fully discharged before touching anything
or making measurements. While these are supposed to include internal
bleeder resistors, these can fail. In any case, several minutes may be
required for the voltage to drop to negligible levels.
Getting inside a microwave oven
You will void the warranty - at least in principle. There are usually no
warranty seals on a microwave so unless you cause visible damage or mangle the
screws or plastic, it is unlikely that this would be detected. You need to
decide. A microwave still under warranty should probably be returned for
warranty service for any covered problems except those with the most obvious
and easy solutions.
Principles of Operation
Instant (2 minutes on HIGH) microwave oven theory
Please see Typical Microwave Oven Electronics Bay
for parts identification.
"Industrial ovens still often operate at 915 MHz and other frequencies near 6
GHz are also used.
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."
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.
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.
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.
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.
Microwave generator
This is the subsystem that converts AC line power into microwave energy.
The majority of microwave ovens use a brute force approach which
consists of 5 parts: high voltage (HV) transformer running off the AC line,
HV rectifier diode, HV capacitor, magnetron, waveguide to oven chamber.
(A few employ solid state inverter in place of the simple HV transformer.
These will be discussed later.)
High voltage transformer
(From: John De Armond.)
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.
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.
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.
Magnetron construction - common features
The following items apply to all types of magnetrons.
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 |
|____________________________________________|
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.
|| +------------------------+
||( 3.3 VAC, 10 A, typical |
TP Relay or || +------------+------+FA F| 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!
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.
Troubleshooting Guide
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.
Possible causes:
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What can go wrong
The most common problems occur in the microwave generating portion of the
system, though the controller can be blown by a lightning strike or other power
surge. Bad interlock switches probably account for the majority of microwave
oven problems. Also, since the touchpad is exposed, there is a chance that it
can get wet or damaged. If wet, a week or so of non-use may cure keys that
don't work. If damaged, it will probably need to be replaced - this is
straightforward if the part can be obtained, usually direct from the
manufacturer. Unfortunately, it is an expensive part ($20-50 typical).
General system problems
The following problems are likely power or controller related and not in
the microwave generator unless due to a blown fuse or bad/intermittent
connections:
Uninvited guests
Some cockroaches (or other lower life forms) may have taken up residence on
the controller circuit board. It is warm, cozy, safe, and from their point of
view makes an ideal habitat. If you got the microwave oven from a flea market,
garage sale, the curb, a relative, or friend, or if your kitchen isn't the
cleanest in the world, such visitors are quite possible. Creatures with six
or more legs (well, some two legged varieties as well) are not known for their
skills in the areas of housekeeping and personal hygiene.
Totally dead oven
First, check power to the outlet using a lamp or radio you know works. The
fuse or circuit breaker at your service panel may have blown/tripped due to
an overload or fault in the microwave oven or some other appliance. You may
just have too many appliances plugged into this circuit - microwave ovens are
high current appliances and should be on a dedicated circuit if possible. If
you attempt to run a heating appliance like a toaster or fryer at the same
time, you *will* blow the fuse or trip the circuit breaker. A refrigerator
should never be plugged into the same circuit for this reason as well - you
really don't want it to be without power because of your popcorn!
Totally dead oven after repair
On some microwave ovens, there is at least one cabinet screw that is slightly
longer than all the others. This engages a safety interlock which prevents
the oven from receiving power if the correct screw is missing or in the wrong
hole. Check the length of all the screws and locate the interlock switch
behind one of the screw holes. I don't know how common this practice is
but have heard of it on some Sharp models.
Also see the section: Getting inside a
microwave oven.
Dead controller
The most common way that the controller circuitry can be harmed is by a power
surge such as from a lightning strike. Hopefully, only components on the
primary side of the power transformer will be affected.
No response to any buttons on touchpad
There can be many causes for this behavior (or lack of behavior):
Oven runs when door is still open
WARNING: Needless to say, DO NOT operate the oven with the door open! While
extremely unlikely, the microwave be generator could be running!
Oven starts on its own as soon as door is closed
If the oven starts up as soon as the door is closed - regardless of whether a
cook cycle has been selected, the cause could be a shorted triac or relay or
a problem with the controller or touchpad.
Oven works but totally dead display
If all functions work normally including heating but the display is blank
(assuming you can issue them without being able to see the display),
the problem is almost certainly in the controller or its power supply.
Whacked out controller or incorrect operation
The following are some of the possible symptoms:
Erratic behavior
There are three different situation:
Problems with internal microwave leakage
(From: Charles Godard (cgodard@iamerica.net).)
Some of the keys on the touchpad do not function or perform the wrong action
Touchpads are normally quite reliable in the grand scheme of things but can
fail as a result of physical damage (your spouse threw the roast at the oven),
liquid contamination (from overzealous cleaning, for example), or for no
reason at all.
Microwave oven does not respond to START button
While all other functions operate normally including clock, cook time, and
power setting, pressing START does nothing, including no relay action and
the timer digits do not count down. It is as though the START button is
being totally ignored. (However, if there is a momentary response but then
the oven shuts off, see the section: Erratic behavior.
Microwave generator problems
Failures in the microwave generator can cause various symptoms including:
No heat but otherwise normal operation
If the main power fuse is located in the primary of the high voltage
transformer rather then at the line input, the clock and touchpad will
work but the fuse will blow upon initiating a cook cycle. Or, if the
fuse has already blown there will simply be no heating action once the
cook cycle is started. There are other variations depending on whether the
cooling fan, oven light, and so forth are located down stream of the fuse.
Timer and light work but no heat, cooling fan, or turntable rotation
This means the controller thinks the oven is working but the microwave
generator AND motors aren't being powered. Note that these symptoms are
subtly different than just having no heat and eliminates the actual components
of the microwave generator from suspicion in most cases.
Fuse blows when closing or opening door
This means that the main fuse in the microwave (or less commonly, the fuse or
circuit breaker for the power outlet) pops when the microwave oven door is
closed or opened. This may be erratic, occurring only 1 out of 10 times, for
example.
Loud hum and/or burning smell when attempting to cook
A loud abnormal hum is an indication of a short somewhere. The sound may
originate from the HV transformer vibrating and/or from within the magnetron
depending on cause. There may be a burnt odor associated with this behavior:
