So my folks have this over the range microwave. The light underneath quit working, so dad got a new bulb. When he took the old bulb out it came apart, with the metal sticking inside the fixture. So when he took the metal threaded part out of the socket, he didn't unplug it and blew the breaker.

Now the new bulb is in but doesn't work... breaker is back on and microwave works fine, all lights work fine, but not the bulb underneath.

When testing the wire leads, there is 120v there constantly, no matter what buttons you push but dad thinks it may have blown a diode or something as the light switch on the panel controls off/high/low.

Any ideas?

https://www.reliableparts.com/lookup...682497#diagram

 

Unfortunately the lamp brightness is controlled from the PCB A.
It's the board where power enters and has the relays on it.
On an old style microwave with a turn knob or slide switch... there would be a diode.
Not on an flat panel electronic control.

I'm trying to figure out if the front board where the buttons are is separate from the control board.

That uses tech sheet: W10844811. That may/should be inside the unit.
It's also covered in service training document: W11107277
The training manual is not terribly useful but here it is..... w11107277 - pdf

 

Well that's what all the youtube videos seem to indicate too... control board. I see a used one on ebay for $99. I think dad is content to get a $15 led strip light and wire it up like undercabinet lighting. They are so cheap.

He should have had the brains to unplug it.

 

The problem there... based on what you said about power.... that socket was always live and they switched the neutral. Fairly common in appliances. If they had switched power.... there wouldn't have been power there,

 

I saw a lot of microwave videos that were entitled "light won't go off". Must be a similar problem.

 

They typically use a triac to do the dimming and switching. When it gets a spike... like from when a bulb blows out... it can short on. Almost the same thing that happens with ceiling fans.... especially when LED's are used.