microwave riddle?

On Fri, 19 Sep 2008 20:05:27 -0700 (PDT), poster put finger to keyboard and composed:

I can't find anything in the Microwave Repair FAQ that discusses the high voltage protector diode:

I did find a datasheet for your HVR6x2 diode, though:

This device consists of two back-to-back HV diodes, one drawn larger than the other, the smaller one with a 1500V reverse breakdown voltage, the larger with 6000V. AFAICS, if your HV capacitor were to go open, then the smaller diode could see a reverse voltage close to the peak secondary voltage of the power transformer (~2000V). This would exceed its rating and presumably it would fail.

So IMO the capacitor is one possible suspect, although I would think that anything which allowed a good capacitor to charge in the reverse direction may also cause the same failure. Therefore it may be wise to replace the HV diode as well, just in case it is leaky. Also check that the magnetron's filament is not shorted to the case.

- Franc Zabkar

though:

Thanks for the answer however you are assuming the anode of the bigger diode is connected to the transformer but it was the other way around because this was the original part

On Mon, 22 Sep 2008 07:45:19 -0700 (PDT), poster put finger to keyboard and composed:

though:

No, I was thinking that the smaller diode connects to the transformer. That's how it's drawn in the schematic diagram.

If the dual diode were reversed, then the smaller diode would always see the voltage of a fully charged capacitor, which would presumably exceed its breakdown voltage.

If the capacitor is open, then, during the negative half cycle of the mains, you effectively have two reverse biased diodes (HVR and small).

though:

thanks again What is actually the curved line close to the transformer and directly connected to magnetron? What if upon disconnecting the both diodes fast fuse blows again when either microwave or grill cooking is started? What is it exactly connected between a8-a7?

though:

also here-

Problem: Fuse blows when initiating cook cycle. Possible causes:

1. Defective interlock switches or misaligned door. 2. Shorted HV capacitor. 3. Shorted HV diode. 4. Shorted magnetron (probably won't blow main fuse but HV fuse if used). !5. Defective triac. 6. Old age or power surges. 7. Defective HV transformer. !8. Short in wiring due to vibration or poor manufacturing.

On Wed, 24 Sep 2008 19:31:52 -0700 (PDT), poster put finger to keyboard and composed:

That's the transformer winding that supplies 3VAC to the magnetron's filament.

According to Sam's Microwave FAQ ...

"An open HV diode will result in AC instead of DC across the magnetron with a peak negative value (the only one that matters) about 1/2 of what it should be. The result will likely be little or no detectable heat but no other symptoms".

If the fuse blows again, then I'd be looking at the power transformer or the magnetron.

That's the transformer (and metal oxide varistor) that is part of the low voltage DC supply for the control board. One secondary winding supplies the +5V (?) logic, the other probably supplies the display filament.

- Franc Zabkar

thanks for input if the transformer secondary was shorted inside it would not be able to supply voltage high enough to burn the rectifier diode when it was replaced so maybe wiring is a good suspect since magnetron seems ok- that could be tested with turning on grill operation only ?

On Fri, 26 Sep 2008 23:00:09 -0700 (PDT), poster put finger to keyboard and composed:

I agree that the transformer is probably OK. All I can add is that whenever I've seen a failure in the protection diode, I've always replaced it and the rectifier diode as a pair, and I've tested the magnetron and cap. I can't see what else could cause the protector to fail.

BTW, I don't understand how you can confidently state that the magnetron is good unless you've tried it in another oven.

- Franc Zabkar