How to Troubleshoot OTR Microwave: Blows Fuses

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Old 02-27-20, 08:30 AM
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How to Troubleshoot OTR Microwave: Blows Fuses

I have a Frigidaire FGMV176NTW over-the-range microwave - two years old - s/n KG81304618

It blew the 20A line fuse upon starting a cook cycle. The fuse appeared to have had a violent demise, so I didn't expect much when I tried a new fuse. The new fuse blew immediately upon an attempt to start a cooking cycle.

Is there any way I can troubleshoot what the cause is?

Thank you for any help
 
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Old 02-28-20, 12:47 AM
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I'm away from my shop but will be back tomorrow.
Do you have a test meter ?

See if your door has two or three switches. If it has three..... one is used as a safety kill switch. It's job is to short the high voltage section in case it keeps running when the door is opened,
 
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Old 02-28-20, 07:24 AM
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It has three switches - I tested them with the wires disconnected. Two of them are open with the door open and close with the door closed. The third one is closed with the door open and opens with the door closed. Does that sound right?

I also tested the two thermal devices (cut-outs?) on the 'floor', accessible behind the control panel. One is closed and one is open.
 
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Old 02-28-20, 12:06 PM
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The third one is closed with the door open and opens with the door closed.
That sounds like the safety switch.

Be careful servicing as there is high voltage in the microwave section.
When I service I use a Variac where I can vary the incoming AC voltage. Saves on blowing fuses.

The switches can be seen on the wiring diagram.
You'll see the monitor interlock (safety) switch is directly across the AC line. If the primary switch doesn't open.... that safety switch creates a short and will blow the fuse. Door switches are a typical problem.

Look to the right on the diagram and you'll see the microwave section. Disconnect one of the transformer leads and see if the fuse still blows.

Wiring diagram (pdf)
 
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Old 02-28-20, 01:04 PM
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Disconnect one of the transformer leads
Should I disconnect one of the leads on the line side or the load side?
 
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Old 02-28-20, 01:32 PM
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Line side. You want to isolate the entire high voltage section.
 
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Old 02-28-20, 01:36 PM
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With one line side lead disconnected, the unit does not blow the fuse. Display, fan, lights appear to work.
 
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Old 02-28-20, 01:46 PM
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Check the HV diode next. Use a meter and a 9v battery inline to test it.
There are many you tube video's illustrating the method.
 
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Old 02-28-20, 05:03 PM
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Check the HV diode next.
One direction I get 5.55VDC, the other direction I get 00.0

I also ran the unit with the line side connected to the transformer but with the outputs disconnected and the fuse did not blow.

The capacitor test showed the meter (on 20M) slowly raise to ~10.25 and level off there.

Will it hurt anything to run the unit with everything connected except the magnetron (to see if the fuse blows)?
 
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Old 02-29-20, 07:03 AM
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Had too many cases of running out of odd sized fuses when trouble shooting.

For this type of situation temporarily replace fuse with circuit beaker. Connect breaker with same rating as fuse using wire pig tails. When problem is solved install new fuse.
 
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Old 02-29-20, 12:02 PM
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Will it hurt anything to run the unit with everything connected except the magnetron (to see if the fuse blows)?
No. Just be VERY careful as you will have very high voltage there that needs to be bleed off before touching anything.
 
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