New microwave capacitor and now HV transformer is humming

[adam108]

My GE spacesaver (JVM1430BD) stopped working in the middle of microwaving and blew the inline fuse. After some digging and testing, it turns out the capacitor was bad (continuity between the two sides). I ordered a new one with nearly the same specs (2100wvac and 0.86uF on old vs 2100wvac and 0.91uF on new). When the part came it was about 0.5" shorter than the old one but otherwise fit into the holder etc. I installed it and turned the microwave back on...everything worked again. I microwaved some water and it heated it just as expected in the same amount of time as before the capacitor went out. An hour later I was in the kitchen and noticed a very soft hum. It was coming from the microwave and was very soft. The only reason I heard it was because the fridge had stopped its cycle and everything was dead silent in the kitchen.

I took the cover off thinking it was the new capacitor or the diode. The sound actually seems to be coming from the HV transformer. While the microwave still had power to it, I went ahead and unplugged the diode from the capacitor for a few seconds wondering if maybe the old diode was bad and drawing current. Humming stayed the same with the diode attached or not. I re-tested the microwave function and everything still works as expected. Before I installed the new capacitor I had checked resistance for the HV transformer and it was appropriate. I also checked the magnetron for continuity etc. and everything seemed in order. I was convinced that the only thing that went bad was the capacitor.

Any thoughts on why the HV transformer is humming. Again, it is very soft but I am convinced that it is louder than it was before the switch. Could the diode be bad despite the contiued hum when I disconnected it? Is the new capacitor not functioning properly? Is the shortened length or minor discrepancy in uF to blame? Most importantly, is a humming transformer dangerous/a risk? It just sounds like a piece of electrical machinery working but it is louder than before the fix.

Thanks!

[Vance R]

For a transformer too hum it needs to have voltage applied. Usually HV transformer are quite loud when they hum.  Capacitor and diode are on the secondary side and have no control of incoming voltage.  Usually quite hums are coming from the control modules transformer. They have to be power all the time so the module can be ready to receive input from user.  Easy test for HV transformer would be to check for voltage when it is in "off" state. Did the cabinet have any rubber blocks that fell off? These are stuck to the cabinet in various location to dampen noise.