GE microwave no longer heats, but otherwise operates normally

[Mancolt]

I'm once again back to this forum for help. It's been almost exactly 18 months since you all helped me identify a $7.50 part that, once replaced, got my microwave working again. This time the issue is different. The microwave screen still works, as do all the buttons. It turns on, the plate spins, the exhaust fans spin up, but it won't actually heat anything. I'm not sure where to start in terms of diagnosing this problem. For reference, the microwave is a GE PVM1970SR1SS.

I'm happy to pull it down, disassemble, and get to testing with a multimeter, I'm just not sure where to start.

Also, anything to be extra careful with in terms of safety? I stumbled across a video that mentioned the source of the issue could be the diode responsible for converting A/C to D/C and that it's capable of storing high voltage. Any way to safely discharge this before I'm sticking my fingers in those tight spaces?

[Vance R]

Setup to measure amperage draw of the microwave. Put a cup of water into the microwave and start the microwave for a 2 minute test cycle. Report the find back here.

[Mancolt]

Sorry for the dumb question, but is there a way to check the amp draw without an amp clamp? Or can that be done with a voltmeter?

I did a 2 minute test with a cup of water in the microwave prior to your message and then again just now, and both times it came out after 2 minutes at the same temperature it went in at.

[Vance R]

1 hour ago, Mancolt said:

but is there a way to check the amp draw without an amp clamp? Or can that be done with a voltmeter?

A kilowatt meter or other inline meter.  Some meters allow for inline amperage measurements, would just have to make an adapter. Need to handle at least 15 amps.

[Mancolt]

Would this device work? Looks to be rated up to 15A and easy enough to just plug the microwave into and then turn on and tell you what happens. https://www.amazon.com/Century-Energy-Voltage-Electricity-Monitor/dp/B06XH73LJB/ref=sr_1_2_sspa?keywords=plug+in+voltmeter&qid=1651283963&sr=8-2-spons&psc=1&spLa=ZW5jcnlwdGVkUXVhbGlmaWVyPUEzMVVTSFIxUlg0MlJGJmVuY3J5cHRlZElkPUEwNTE5OTkyMkVFNFRCRVEyUkZDVSZlbmNyeXB0ZWRBZElkPUEwNzcyMTgwMVZSNUtCS1JQMzNPRiZ3aWRnZXROYW1lPXNwX2F0ZiZhY3Rpb249Y2xpY2tSZWRpcmVjdCZkb05vdExvZ0NsaWNrPXRydWU=

[vee8power]

You can take the cover off and check for 120 volts on the transformer primary side. It may be risky for a diy'er, but it tells us some of what an amp reeadiig would. 120 volts on the transformer primary indicates a high voltage component is faulty. Never measure transformer secondary voltage!

[Vance R]

1 hour ago, Mancolt said:

Would this device work?

Look like it should work for this test.

[Mancolt]

Here are the readings I got. I'm not sure which is most relevant, so I measured every instant value that this device is capable of, when at rest and when running at default cook power for 2 minutes with a mug of water in the microwave. First number is when plugged in but not running, and 2nd number is when "cooking".

Instant Amperage: 0.08amps; 0.70amps

Instant Active Power: 5.3watts; 78.1watts

Instant Apparent Power: 10.0VA; 84.1VA

Instant Voltage: 125 volts; 125 volts

[Vance R]

The amperage when cooking is less than 1 amp. There is an issue in the 120 vac side of the HV transformer.  Microwaves draw about  9 to 13 amp when working.  If you feel comfortable need to check to see if there is  120 vac on the primary of the HV transformer as @vee8power suggested. Just to double check the watt meter you can check it using a mixer or cooker where the wattage is know from the model tag.  The "instant" may just mean it is measuring the select parameter.  

[vee8power]

11 hours ago, Vance R said:

There is an issue in the 120 vac side of the HV transformer.

So either there is no voltage to the primary side or the primary side is bad.

@MancoltJust for kicks, swap the position of the 2 fuses on the noise filter and plug the unit back in.

If the microwave still has lights, fans, etc., then check for 120 volts AC at the primary side of the transformer while running a cooking cycle. No voltage means the control board is bad. 120Vac at the primary side of the transformer but still less than 2 amps use means a bad transformer.

[Mancolt]

Thanks @Vance R I'll check the diagnostic tool on a known working device to see what it does. 

@vee8power Is the highlighted part (above #43) the 2 fuses you're referring to? I just pull them out and swap them around? I don't have the microwave apart yet, so I can't look inside my unit. And is the location I highlighted in the microwave where I can find it. The quality of the schematic doesn't make it easy to see where the dashed line for #43 leads to within the microwave. Schematic link

[vee8power]

12 minutes ago, Mancolt said:

just pull them out and swap them around?

Yes, swap the 2 fuses on part#43

 

[Mancolt]

On 5/1/2022 at 9:32 PM, Vance R said:

 Just to double check the watt meter you can check it using a mixer or cooker where the wattage is know from the model tag.  The "instant" may just mean it is measuring the select parameter.  

I did as you suggested, and checked the meter on a known appliance. Using the "instant amperage" function, it read 0.00 at rest on an air fryer, and 14.10 when running. So I think the unit is measuring the draw at that moment. So "instant" = live reading and seems to be working. Next step is to unmount the microwave, open it up, find the fuses @vee8powermentioned and swap those around. I'll try to do that today and post the results.

[Mancolt]

23 hours ago, vee8power said:

If the microwave still has lights, fans, etc., then check for 120 volts AC at the primary side of the transformer while running a cooking cycle. No voltage means the control board is bad. 120Vac at the primary side of the transformer but still less than 2 amps use means a bad transformer.

Can you elaborate how to test for 120 volts AC at the primary side of the transformer? Can I use a multimeter and where am I putting the probes to check this?

[Mancolt]

@vee8power Swapped the fuses, and when in the original location of each, the microwave screen powers on and I have some functions (clock, fan, etc). When I swap them, nothing happens. 0.00amp draw when plugged in, no vampire drain.

I'm guessing that just means 1 of the 2 fuses is bad? Is there anything else I should check before ordering replacement fuses, to save me from potentially paying shipping for yet another part?

[vee8power]

It's a crapshoot; order a few fuses and we can diagnose further when/ if we can get the thing to heat. This is pretty far down the rabbit hole for a microwave oven.

[Mancolt]

The last time was a TCO magnetron issue. This one was just a fuse. After I bought a replacement pair of 20A fuses from Home Depot, and swapped them in, and did a mug full of water test, it all looks to be functioning again. I could also see on one of the fuses a bit of black/gunk that looks like it may have shorted. Hopefully these next ones last as long as the first ones and I'm not taking this microwave apart again for a few more years.

Thank you @vee8power and @Vance R for all your help! I was able to fix this quickly thanks to both of you.