Microwave oven underside lamp is failing to turn on: is it the main board?

I've got a Whirlpool WMH53521H206 Oven. Two days ago, the underside lamp stopped working. I'm not aware of any electrical shorts that may have occurred. The bulb isn't bad. This is how I diagnosed it:

At first, I thought it was simply the bulb going out. I swapped the bulb with a new bulb and nothing. I then unplugged the oven, pulled the underbody off, disconnected the motor, detached the socket and ran my Sperry multimeter across the spade connectors: 120 volts AC on the nose. I tried another bulb, again: nothing again. I noticed that if I tap the "light bulb" icon on the display panel the voltage doesn't change (I assumed it would so as to deliver different intensities). Something "smart" seems broken.

I then detached the socket itself, reattached the bulb and took it downstairs where I have a variable DC power supply. I hooked the whole thing up to the power supply and the light worked. So, it's not the bulb nor the housing.

I'm starting to think the main board has gone bad. And, of course, I'm about 5 months outside of my limited warranty. The curious thing is this: why am I getting 120 volts across the spade connectors? It feels like an open circuit voltage, but I'm not sure; in the past when I've experienced situations like this, I notice steady voltages across resistors indicating no current, not the connectors themselves. Any idea what might be going on? Any tests I can run before forking out the money for a new main board?



The red / white wires are for the bulb.

 

I'd suggest you post your question in the Small Kitchen Appliance forum.
https://www.doityourself.com/forum/e...appliances-67/

 

This is a highly moderated board. We move threads fairly quickly.
This one is now in the appliance repair section.

Your model number is not in the list and the one you linked to is a different model.

Your picture is a private google user picture. You can make it public or post it directly here.
How to insert pictures.

You must measure with at least one bulb in the socket as it uses triac switching.

 

The (pdf) tech sheet below should be your unit. The lamp wiring is on page 2.
W11175370A

Please be careful working inside the microwave. There are hazardous voltages in use.

 

Wow! It's embarrassing how many things I got wrong in posting that. I apologize.

In terms of the correct group: I swore I spent time looking for the right group for this, I must have been blind.

In terms of the photo:


....I was simply showing a photo of the underside and the wires I was testing. Nothing major.

Thanks. I did connect the socket + bulb to the spade connectors and measure as well as measure the voltage without the bulb and both times received 120 volts

In terms of it being a triac: I've understood them as solid state high/medium voltage AC switching components (like an AC-aware transistor in practice), but I don't have direct experience with them in any projects. I don't mean to turn this into a course on circuitry, but I'm not sure I understand the mechanism that's causing voltages to show up like I'm seeing but with no current to flow.

Thanks again for everything. And I'm sorry you/whomever had to deal with moving my thread around.

 

No worries. Nothing to apologize for. You did fine.

A triac can pass enough current so that you could measure 120v on your meter with no load.
With the load of the lamp(s) that voltage should be near 0v if the circuit is not on.
If you have 120v WITH at least one lamp in the socket.... then there is some problem at the light.

 

I see. I studied the nature of triac switches a bit more and found they have a natural leakage about them (https://www.apogeeweb.net/electron/h...eakage-current). I'm assuming that's what you're referring to.

So, if I'm understanding you correctly, with the bulb in the socket (there's only one bulb), if I'm measuring 120 volts and the bulb is not lighting, the issue is likely not with the main board but with the connection to the bulb itself? I haven't done it yet (and I'm traveling at the moment so I can't make the test): but I suppose I could put the multimeter in series with the light and see what, if any, current is being drawn.

Thanks again. Let it be known at this point I'm mostly trying to draw some knowledge from you . This is an interesting puzzle.

 

That is correct.
If the bulb is not lighting.... there won't be any measurable current flow.

Disconnect the two wires to the light. With the bulb installed.... check the socket terminals for continuity.

 

Hi again.

I'm starting to think maybe there's an issue with the bulb's power source. Here is a photo showing a continuity test on the socket w/ the bulb connected. Looks good to me.



...when I connect the multimeter in series with the bulb / microwave and press the button to "high" for the bulb, I get .08 amps of current:



And as I said earlier, I'm getting 120 volts still, still while in series with the load (I know this is technically an open circuit due to the high impedence of the meter):



...and I think maybe I must have tested wrong earlier because once put in parallel, I'm _not_ getting 120Vac, but instead .01Vac.



Maybe it's time to call a technician. I feel like maybe it is the main board after all?

Edit: the last time I tested for the potential across the connectors "with the bulb connected" I attached to the spade terminals with the bulb connected to them. Perhaps I just tested it wrong. I think maybe I owe you an apology for the bad information previously.