Okay, I finally found the problem. See problem below. The interlock monitor switch was not having its contacts opened as it should with the door closed. The nylon lever that operates the switch when the door is closed had relaxed just a tiny bit. If the door was closed normally the contacts opened, if closed very softly the contacts stayed closed.

With the contacts closed, and the primary interlock switch closed, when relay contacts powering the high voltage transformer closed on call for heat the oddly arranged circuit put a direct short across the incoming line. Bye Bye 20A fuse.

A shim of a few thousandths of an inch attached to the end of the lever which is not adjustable solved the intermittent operation of the interlock monitor switch.

A very odd problem, hope this helps someone else. L D





My microwave starting kicking the breaker on a 20 Amp ckt occasionally but without blowing the fuse in the unit. 20 A ABC fuse.

Replaced the magnetron with new one after the filament tested dead shorted after the unit started kicking the breaker immediately each time the unit was started.

It then went back to occasional breaker kicking. Plugged it into a different ckt and it started occasionally blowing the fuse. It would blow the fuse every five to ten starts.

Replaced the transformer, diode and capacitor with new. Problem is still the same.

Bought Buss brand Microwave 20A fuses in case I had gotten a bad batch of fuses. No help.

I am at a loss as to what may be causing this, wiring all looks excellent, no burned smells anywhere and unit heats normally if it starts without blowing the fuse.

I'm a lifetime electronic tech repairing to the component level and this is the most puzzling trouble I've seen in a long time. Help appreciated. :-)

L D