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Microwave Runs at Half Power [SOLVED]
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jpanhalt:
Kenmore Elite 1200 W microwave approximately 2003 vintage.  Inverter type. Came with house and worked fine until Friday morning, May 24.  It is an above the range, built-in model and a bit heavy for me alone to take off the wall.

Tore into it yesterday and checked what I could without opening the inverter section.  The magnetron is Panasonic, and I presume inverter board is too.  Magnetron is made in Korea.  Attached is a schematic from Panasonic.  It appears generic.  Magnetron filament tested OK with very low resistance and no short to ground. 

After being without power for more than an hour, it was reconnected to power and seemed to work better but quickly returned to half power based on time to warm cream in my coffee cup.

I have no prior experience repairing microwaves and read that magnetrons do age, but assumed that aging is gradual.  There are 2 diodes in the half-wave doubler.  What is the chance one of those failed open and is causing the reduced power?  There are some service tests in the manual from which the schematic came, but they require opening the inverter section.

Thanks, John
Gyro:
We had a Panasonic inverter microwave. Similar symptoms, full power initially and then tailing off. It turned out to be cracked magnets on the Magnetron causing reduced field and overheating as it slowly melted its internal anode structure...  https://www.eevblog.com/forum/projects/will-a-magnetron-draw-480-watts-without-radiating-energy/msg3210292/#msg3210292

The-tell tale sign (apart from cracked magnets!) is loose contact between the cooling fins and the Anode. It's possible that it might be an inverter problem, but given the symptoms I doubt it - and you really don't want to be messing with it! A replacement magnetron is an option but from the sound of it, it doesn't owe you anything.
soldar:
I find that schematic interesting. I am familiar with older microwaves which worked at mains frequency. This one seems to have a flyback transformer to provide the high voltage and I assume it varies the power by changing the working frequency.

I would start by checking the components on the high voltage side of the transformer, capacitors, diodes, resistor. Needless to say, be very careful and make sure capacitors are discharged although such low values are much less dangerous than the higher value, low frequency ones.

If everything checks out then I would try to check the frequency at the base of Q701 but it would be better if you knew what frequency to expect.

I will be following this thread. Good luck.
jpanhalt:
@Gyro
Buying a replacement is not prohibitively expensive.  Finding one that is front vented with a vent fan for the electric range below is, which I didn't mention, is hard.  Black & Decker make a 1000W, sold by Home Depot for about $250.  That's the only one I Have found so far.  I don't know who really makes it.  I will check out the fins when I reopen it.

@soldar
Its circuit is similar to current ones.  According to Panasonic, line voltage is rectified, then an IGBT chops it to something between 20 kHz and 40 kHz followed by a small transformer and half-wave rectifier with voltage doubler.  An MCU on another board controls the IGBT for power control using PWM.
soldar:

--- Quote from: jpanhalt on May 28, 2024, 04:08:29 pm ---@soldar
Its circuit is similar to current ones.  According to Panasonic, line voltage is rectified, then an IGBT chops it to something between 20 kHz and 40 kHz followed by a small transformer and half-wave rectifier with voltage doubler.  An MCU on another board controls the IGBT for power control using PWM.
--- End quote ---

That has to be a flyback. You cannot put DC into a direct transformer.

So if it works between 20 and 40 KHz then the highest frequency would be the highest power. If the frequency is low then the power will be low. maybe something to check if you can do it easily.
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