Author: Samuel M. Goldwasser
E-Mail: sam@stdavids.picker.com
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Copyright (c) 1994, 1995
All Rights Reserved
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To prevent the possibility of extremely dangerous electric shock, do not operate the oven with the cover off if at all possible. If you must probe, remove the connections to the magnetron (see below) to prevent the inadvertent generation of microwaves except when this is absolutely needed during troubleshooting. Use clip leads to make any connections before you plug in the oven and turn it on.
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 1500 W with short term availability of greater power. Always observe High Voltage protocol:
General Safety Guidelines when working on line powered equipment including: TVs, monitors, and microwave ovens.
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!
None of these devices will protect fools from themselves!
Since the oven 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.
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 stove-top, 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 damage.
A schematic diagram of the microwave generating circuitry and portions of the controller is usually glued to the inside of the cover.
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 1-3 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.
Other possible causes: bad LV power supply, controller chip, display panel. The most common way that the controller can be harmed is by a power surge such as from a lightning strike. Assuming that the main fuse checks out, then check the power supply for the controller next. Also check for bad solder connections.
Erratic behavior: suspect the controller power supply or bad connections.
Some of the keys on the touchpad do not function or perform the wrong action: 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.
Incorrect operation for some or all types of cycles: check touchpad as above and interlocks. 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.
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 over-temperature thermostat, a bad magnetron, or is being operated from very high AC line voltage increasing power to the oven.
One interesting note: Since 30-50 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 stove-top or gas or electric oven for heating small quantities of anything. With a normal oven or stove-top, 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.
__________________________
|| ( 3 VAC, 4 A, typical |
Relay or || (____________________ |
fuse I Triac || | - | -- | --
_______-- ________/ ______/ _____ || ______| |____| : |_ _| :
-- | _)|| ( Cap | | | : \/ Magnetron
Line \ I=Interlock _)|| ( __|__ : ___ :
| _)|| ( 2000 VAC _\_/_ :--- | ---:
_____________|____________________)|| ( .25 A | Diode |
|| ( typical | |
(Controller not shown) || (_____________|_________|
Note the unusual 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.
There is also usually a bleeder resistor as part of the capacitor, not shown.
The bleeder may be defective and open as this does not effect operation of oven and the time constant may be long - minutes.
In addition, there will likely be an over-temperature thermostat somewhere in the primary circuit, often bolted to the Magnetron case. Other parts 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. 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 interlock switches, being electro-mechanical 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.
Tracing the circuitry of the interlocks is straightforward, if a bit tedious to keep track of where the wires connect. It is a simply matter to check each of the switches with an ohmmeter. Contact resistance of more than a tiny fraction of an ohm when in the 'on' position is a failure as is anything less than infinity ohms when open. You may need to temporarily remove the quick connects to assure that the measurements are not being confused by other circuitry. Measure across the fuse and neutral while opening and closing the door with various amounts of jiggling thrown in. Any indication of continuity means that the fuse will blow if the oven is powered up and the door is opened or closed. If a switch needs to be replaced due to it having blown the fuse, replace any other switches upstream of it as well since it is likely that the momentary current surge has damaged them also. Generic replacement interlock switches are readily available.
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.
Failures in the HV circuits can cause various symptoms including:
A loud abnormal hum which may be followed by the fuse blowing or not, 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. Arcing within the Magnetron case (visible through ventilation holes in the bottom section) is usually an indication of a bad Magnetron. There may be a burnt odor associated with this behavior.
Arcing in the oven chamber with a normal load (a cup of water, for example), often just indicates that a thorough cleaning of the oven chamber is needed, particularly around and inside/above the waveguide cover. Any food that gets trapped here will eventually burn and carbonize resulting in a focal point for further arcing. Usually, the waveguide cover is designed to be removable without taking the cover off of the oven. However, burnt food and carbon often make this difficult so that some disassembly will be required. Clean the waveguide cover and clean inside the waveguide as well. If the waveguide cover is broken or damaged seriously, replacement material is available. The oven will work without it but replacement will prevent contamination of the waveguide with food vapors or splatters.
The cause for a blown fuse can be:
Put an ohmmeter across the AC input just before the interlocks but beyond the power relay or triac. Open and close the door several times - there should be no significant change in resistance and it should be more than a few ohms. If it approaches zero while opening or closing the door, the interlock switches and door alignment should be checked. (You may need to disconnect one side of the transformer primary since its resistance is a fraction of an ohm. Refer to the schematic pasted inside the cover.)
Assuming it passes the above test for interlocks and door alignment:
With power disconnected and after waiting ample time (a couple of minutes, but check with a meter to be sure, or discharge it with a resistor) for the capacitor to discharge, an ohmmeter can be used to quickly determine if the capacitor, diode, or magnetron are a dead short. Testing the transformer may be more difficult. However, if the other components test out, remove the high voltage connections to the transformer, power up the oven, and initiate a cook cycle. If the fuse still blows, then there is a good chance that the transformer is bad.
Use an ohmmeter to test the diode and capacitor. While connected in circuit, the resistance in at least one direction should be several M ohms. (try in both directions, use the higher reading). Test the magnetron from the filament to chassis - it should be high in at least one direction. If you disconnect one end of each component, then:
A convection oven which shuts down after a couple of minutes during the pre-heat cycle with the temperature display (if any) stuck at LOW (even though the oven is hot when opened) may have a bad thermister temperature sensor. The overtemperature protection sensor is shutting the oven down. The termister will usually be accessible after removing the oven cover. It will be located centrally just above the oven ceiling duct. It is a two terminal device that looks like a tiny resistor or diode and may be mounted on a metal header fastened with a couple of screws. Remove and test with an ohmmeter. An infinite reading means it is bad. As a test, jumper a 50 K ohm potentiometer in place of the thermister. During preheat, as you lower the resistance of the pot you should see the temperature readout climb. The oven will then indicate READY when the simulated temperature exceeds the setpoint. Replacement thermisters are available from the oven manufacturer - about $20.
If the convection preheat cycle never completes and the oven is cool when opened, then either the heating element is bad (test with an ohmmeter) or the relay controlling the heating element or the controller itself is bad. If the circulating fan runs off of the same relay and it is operating, then the problem must be the heating element.
The heating element will be either a Calrod type (GE tradename?) which is a steel tube enclosing a Nichrome wire coil embedded in ceramic filler or a coiled Nichrome element strung between ceramic insulators. The former is probably only available from the oven manufacture, though it is worth trying an appliance parts distributor or a place like MCM electronics first. It may be possible to find a replacement Nichrome coil and form it to fit. Make sure the wire gauge and length are identical.
The circulating fan is probably driven by a belt, which may break or deteriorate. Inspect the belt. If it is loose, cracked, or does not return to its normal length instantly after being stretched by 25% replace it. Check the fan motor and fan itself for adequate lubrication. Check the fan blades for corrosion and damage.
Touchpads and controller parts like the microprocessor chip are usually only available from the manufacturer of the oven. Prices are high - a touchpad may cost $20-30 or more.
Sensors and other manufacturer specific parts will be expensive.
While the HV transformers are fairly standard, they are not readily available from the common replacement parts sources. However, they do not fail that often, either.
With the prices of microwave ovens dropping almost as fast as PCs, a few year old oven may not be worth fixing if the problem is a bad magnetron or touchpad. However, except for a slight decrease in power output as the oven is used over the years and the Magnetron ages, there is little to go bad or deteriorate. Therefore, you can expect a repaired oven to behave just about like new.
Available at your public library - 621.83 if your library is numbered that way.