Contents:
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. 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.
"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 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!
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).
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.
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.
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.
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.
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 operation.
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).
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 plate. 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 cylinder. 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 chassis.
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.
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 |
|____________________________________________|
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.
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.
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.
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.Go to [Next] segment
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