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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!

This is an excerpt of the full split-phase household power supply webinar held on June 6, 2016. In this excerpt, I explain why antiphase sine waves (meaning 180 degrees out of phase with each other) cancel each other out in a sound mixer but not in a center tapped transformer. Just because an AC voltage can be represented or modeled as a sine wave does not mean all sine waves behave the same way everywhere regardless of the device-- you have to know what you're measuring! References: Summing Amplifier Basics How Sine Waves are Used to Model things in the Real World Using an Oscilloscope to Understand 120 VAC Household Power Supplies