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Re: 1800XL microwave hums
A test bench will have known-good power, but disconnectable and on a breaker and metered, for example. The lamp will be disconnected by opening the thing up (hence, the workbench setting) and removing the lamp circuit connections in whatever way is most convenient.
The stirrer is often integrated with the output from the magnetron to the oven cavity; it's a microwave component that literally spins as the thing runs, and since the oven light is commonly on the same side easy to see moving or evidenced by changes in the back reflections from its vicinity. That is, you would notice some periodic change in the internal oven illumination from the stirrer's motion, which might have stopped around the same time the hum started....
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If you have the mag out of the unit. You can take an ohm meter and measure between the terminals on the mag. That is the Filiment. It should show continuity. Then measure from either of the two terminals to the outer metal frame of the mag. You should not see any connection. If you do it is shorted to ground and is bad. The unit would probably hum loudly when it ran. The only other way to really test a mag is under load with full hi volts and filiment power going to the mag and watch an amp meter and see if it fires up and draws around 10- 15 amps. I personally have Hi Voltage trays from stripped down units that I can power up a mag on the bench and see if it fires. Another way to check is to see if you have hi voltage. Unplug the mag and run unit for a couple of seconds, then, once unplugged, you can short across the capacitor with a couple of screw drivers and if you get a really Large Hi Voltage snap, the unit is producing hi voltage, so if it is not heating it is probably the mag. If it makes Hi voltage the timer and interlock circuits are working to send power to the transformer and the other Hi Volt components. If ti doesnt heat probably bad mag. If your unit has an inverter you caannot use this test.
It sounds to me as though your magnetron is shorted. If you have an ohmmeter, DISCONNECT POWER, unplug the connector on the magnetron, and make two tests. From pin to pin should measure less than an ohm. This is the filament.
From either lead to the frame of the magnetron should measure infinite. Do not touch the lead tips while making this measurement (makes false reading). If the magnetron checks okay, perhaps you have a shorted capacitor, or diode.
I did not fix the problem. I ordered a bulb (lamp) and subsequently saw it is beyond my knowledge to install it. I'll communicate with the store where I bought it 3 months ago and if they won't deal with it, I'll pay a repairman to do it.
Perhaps a fluorescent ballast or transformer is letting you know it's going bad; it might just need to be reset (see the manual as well as trying to unplug/wait/replug.)
Failing that, It is simple enough to unplug it, put it on a test bench, disconnect the lamp and see if it still hums; if not the lamp, you will have a better idea what is humming with the lid off, but stay inches away of the (usually well-packed-away) high voltage feeds to the magnetron as you do. It could be that you just have junk inside the door switch and can unplug, clean out the switch interior (crumbs sneak in!) and put it back for a fix; or a stuck stirrer at the RF feed off the magnetron.