On Wednesday, March 11, 2015 at 8:46:18 AM UTC+13, Cydrome Leader wrote:
Rouke wrote:
Hi,

Following Samuel Goldwasser's Great(!) Notes on the Troubleshooting and Repair of Microwave Ovens I tested:
* Defective interlock switches or misaligned door. There are four switches, three use only the two terminals to make a contact. The fourth uses all three terminals, to break and make contact. Switch 4 was a bit dodgy so I swapped it with Switch 1:
o Switch 1: 1-2 closed: 0.0 ohm, open: infinite (swapped with Switch 4)
1-4 closed: 0.0 ohm, open: infinite
o Switch 2: 1-2 closed: 0.0 ohm, open: infinite
1-4 closed: 0.0 ohm, open: infinite
o Switch 3: 1-2 closed: 0.0 ohm, open: infinite
1-4 closed: 0.0 ohm, open: infinite
o Switch 4: 1-2 closed: 0.2 ohm, open: infinite (swapped with Switch 1)
1-4 closed: 0.0 ohm, open: infinite
o The door is not visibly misaligned.

* Shorted HV capacitor.
o Tested: ~10M ohm, but not open - good?

* Shorted HV diode
o Tested: 6.5V - good.

* Defective HV transformer.
o Primary: 2.5 ohm - good enough??? (should be 0.2 to 0.5 ohm, 0..2 typical)
o White to frame: ~ 100 ohm - if this is HV, then OK
o Red to red: ~0 ohm - if this is filament, then OK

Would the 2.5 ohm of the primary winding definitely be way too high and cause the fuse to blow when I start the cook cycle? I would expect the opposite to be true, however:

After putting everything back together but leaving the transformer primary winding disconnected I switched it on. The conventional oven worked, and heated. When I pressed the microwave button the fan worked and the fuse didn't blow . So I assumed the transformer was faulty. I then reconnected the transformer and tested the microwave function. It worked for three seconds and then blew the fuse, 'confirming'(?) that the transformer was faulty. I replaced the fuse, disconnected the transformer again and tested again. The conventional oven worked again, and when I pressed the microwave button the fan worked, but the fuse didn't blow. 'Confirming'(?) again that the transformer was faulty. Then I put the microwave oven back in the kitchen cabinet and demonstrated that it worked, partially, but the fuse blew...... suggesting again that the switches are (may be) faulty.

Am I overlooking something still?

I'd be wary of the cap or diode. Can you power it up again with just the magnetron disconnnected to
see what happens? If it doesn't blow a fuse your magnetron is what is bad..

The fuse blew with the transformer disconnected, but not immediately. If it blew immediately I would have thought the door switches were at fault, but now I'm not sure.