Bought in 1999 it is a microwave/oven unit. I have checked the following parts & they have readings on my ohm meter. Door switch sensors, upper & lower. There are two separate sensors for the bottom switch. They read totally different from each other depending if door is open or shut. HV Capacitor Replaced Diode (connecting to capacitor) could not get a reading on ohm meter & guy at parts store said this is normally why they stop heating. Magnatron Fan runs Heating sensors (one on top of magnetron & other just inside unit)
Bad: Turntable motor, I took it out & it had an infinity reading.
Question what else is there to check to see why the microwave is not cooking? Thanks for any insight Garry
The Magnetron has probably died. Sometimes it is the diode or capacitor, but since these were checked, i think it is time for you to buy a new microwave
The Magnetron has probably died. Sometimes it is the diode or capacitor, but since these were checked, i think it is time for you to buy a new microwave
You could check the INPUT to the high voltage transformer to see if the microwave generator is getting power when it should be on.
WARNING: Microwave ovens are by far the most dangerous home appliance or electyronic equipment to be troubleshooting!!! Make sure you understand ALL the safety issues!!!
Like Sam says, check the input to the primary of the transformer. If you don't have line voltage there (when it should be running) nothing will heat. If you don't have line voltage it means one of the safety devices (over heat, fuse, interlock switches etc is open) or the controller isn't calling for heating.
1999 means it probably has a micro processor controlled clock, timer, heat modulating controller. Generally cheaply made, but reasonably reliable.
How did it fail? Just quit or died over a period of time? Quit suddenly - check the magnetron filament for an open (check the primary to the HV transformer FIRST for line voltage). Keep your eyes open... any evidence of lightning strikes? (clock and microwaves seem to be the first to go)
If you can find that the controller isn't calling for heat when it should be, then you might (if you have the skills to stay safe) bypass the controller and see if it heats a cup of water.
Dying over a period of a year or two - probably a bad magnetron (rarely a problem - but possible - with the cap or diode) That's where current and voltage measuring come in handy.
I've fixed a few microwave ovens - it is frustrating having to deal with symptoms when you can't actually check things (like current to the magnetron or high voltage).
Most of the problems I've had are safety interlock problems (door switches, or over temp fuses that opened because some food occluded the mica window that lets the waves in), two controller problems (one that I could fix - the other in a proprietary chip that was too expensive)
Like Sam says - worry about your safety. My own practice is to keep one hand in my pocket, my feet on an insulator, and one hand to work the test probe. The very idea of discharging a cap in a microwave give me the willies, and I worked on megawatt CW transmitters in the Navy, and play with Tesla coils at home.
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Or even a bad connection at the magnetron filament terminals.
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I know safety is the first concern. I have bought a new dioade & a new motor ( I thought it was bad because I got an infinity reading on the old one) the new one still has an infinity reading too so probably is always open even thought eh book says there should be a reading & if infinity is bad. My capacitor dissipates w/i 30 seconds of being off, thank goodness. I have checked the transformer for incoming power & it reads 115 vac, cqn not get a reading for outgoing voltage. I disconnected the leads going to the mag & it had ohm readings & not infinity. I checked the interlocks & they all seemed to reading correct ohms. I had the guy at the parts store check the capacitor & he said it was good. I bought a new diode. Bought new turntable motor. Still NOT turning after replacing with new one. It seems that I have checked everything I know. I can't buy a new one because this microwave is in the same cabinet as the oven as one unit. I did buy a cheap replacement but I was hoping to repair this one so the entire unit would work & we could clear up shelf space. Thanks Garry
The capacitors usually incorporate a bleeder resistor so they don't hold a lethal charge indefinitely.
Regarding the transformer . . . you can't really have zero out for 115 in, so there's a couple of things to check there:
The output should be on the order of 2,000 volts on the secondary side
- so your meter has to be able to read that high without damage.
The next thing to check would be the resistance of the primary winding of the transformer - unplug the oven let the cap discharge. Disconnect the primary wires from the transformer (you want to read just the transformer and not the light, motor, fan, etc..) and check the resistance, it should be close to zero ohms (so close as to be indistinguishable from zero with most meters).
You could have a transformer with a shorted secondary or diode to give you zero out with 115 in BUT I've yet to encounter a failed transformer - I'm sure it happens, but it must be rare.
Your turntable motor still doesn't turn? That would lead me to believe you're doing something wrong with your test procedure. It is somewhat unlikely you have two coincident failures in the same appliance.
Most of the motors I've seen are either "shaded pole" or a type of synchronous motor (turns in either direction) - but they run from 115 volts just like the transformer. That's what suggests the problem is upstream to the transformer and your reading of 115 on the primary is incorrect.
Back to square one:
There are a series of safety interlocks that have to work for power to be applied to the microwave - they all have to read zero ohms - that is the "correct ohms." Zero on the lowest ohms range of your meter (zero or as close as the meter reads when the meter leads are shorted together) Zero for switches, zero for fuses, zero for over temp cutouts . . .
A very few microwave ovens also sense airflow past the magnetron tube before HV can be applied. Another switch - with a vane the size of two postage stamps located in the air path or exhaust of the oven. I've only seen them in commercial ovens but some home units may also have them.
Read the switches individually if that can be done easily. Then, if you can, check the whole string - they are usually wired in series and all have to make (close) at the same time for power to get to the oven.
Don't overlook things like a loose mounting or bent arm on a switch that might prevent it from closing when the door (for instance) is closed. Some microwaves incorporate two door switches just in case one fails.
One last caveat - some interlock switches are "micro switches" (a brand now associated with generic snap action switches). Some are single pole two position switches - they only need one position (normally open, as a rule) in a microwave but it is usually cheaper to source two position switches - make sure you're checking the correct switch position. For safety they are normally open and mechanically held closed to operate.
When you think you've checked everything and it isn't working - you must have overlooked something or the test you did was not definitive. Try to approach a problem as if you're looking at it for the very first time.
Microwave ovens are simple as schematics go - not that much to go wrong. You have a lot of safety devices that all have to function, a controller/timer, a nearly indestructible current limited transformer, feeding a diode/capacitor voltage doubler feeding a magnetron tube. The tube just wants to see high voltage and filament voltage to produce microwaves.
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A magnetron will usually fail it two modes: filament opens or emission drops too low. In the former case you can check it easily with an ohm meter, a tube with low emission will work but won't heat as much and it fails over a long period of time.
Bad filament (or filament winding on the transformer, or bad filament connection) and you'd have high voltage DC at the filament, but no microwaves would be produced - and your turntable would probably turn.
The magnetron only has a few internal parts - a filament to emit electrons and a plate with slots/cavities cut in it. Electrons leave the filament and are attracted to the plate, before they get there, the magnets bend their path so they cruise by the cavities. The cavities resonate and produce high frequency radio waves. There's an antenna probe to couple energy out of the tube and guides it into the oven chamber. Very simple; not much to go wrong.
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Lots of good information but NOT the jost important: SAFETY.
Don't even think about trying to probe the output of the HV transforme or anything connected to it when the power is on. Experianced technicians have been electrocuted doing that. Bleeder resistors can fail as well.
Microwave ovens are by far the most lethal of consumer appliances and electronics.
Much more safety and troubleshooting info in the Microwave Oven Repair Guide and other resouces at the Web sites below.
Checking the HV should be left to experts who already have the equipment, experience and knowledge to do it safely.
Likewise anyone who depends on safety devices to keep him alive is (an idiot) tempting fate. Safety devices fail.
A microwave oven is a lot cheaper than one's own life.
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I've seen a transformer failure in a microwave exactly once, it produced plumes of nasty smelling smoke from the winding insulation burning, it was pretty obvious.
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