Microwave oven repair

So, another round of posts to relieve the backlog of ones that I have pictures for.  We had a wonderful microwave at the house I lived at while in college.  It had all the features I could ask for: a temperature probe.  I know some people want power levels, digital interface, or even a rotating tray, but I really really liked the idea of a temperature probe.  I wonder why modern microwaves dropped this fantastic feature.  My instinct says that morons think “I’m not supposed to put metal in the microwave, therefore I can’t use this feature supplied by the manufacturer”.  The ability to put metal in the microwave has been proved as a fantastic way to heat a solder pot or melt pewter for molds (and it is not bad for the microwave).

Now, I mentioned this microwave didn’t have a rotating tray.  That’s not true, it came with an aftermarket one, but no rotation mechanism.  My theory is that the mechanism had an electric motor on it and it turned quite well until someone lost it before we bought it.  I also mentioned it didn’t have a power level selector.  That’s also not true, it had one, past tense.  This time it had one when we bought it and it doesn’t now.  I’m the one who removed it and I don’t feel bad about it.  Originally the switch was sparking and eventually died as seen here:

The fix I originally had for this was a set of switches that replaced this one (I’m NOT trying to source an equivalent to that).  That worked for a while, but eventually we had a problem of complete failure of the power system.  That is shown here:

The method this microwave uses to change power levels is literally by dissipating it through a huge power resistor and switching parts of it in and out of the circuit.  This method is less efficient than modern methods of sending pulses of full power followed by no power to the magnetron, but much more regular and produces a smoother pattern of heating in the microwave.  A nice big 120W wirewound resistor would be the best idea for setting power level, but that’s prohibitively expensive and I’m just too lazy to look.  This microwave got shorted to full power for the remainder of the time it was under my care.  I’m not sure what happened to it, but I’ve only seen one better than it and that one had digital controls, a temperature probe and a rack!

Inside this microwave I found schematics and testing procedures, as it turns out our magnetron is operating within normal parameters of full power.

Schematic

Manual

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