GMC305PRB01 Whirlpool microwave blew fuse. Replaced fuse, HV transformer and diode - still no heat.

[pfe999]

Model Number

GMC305PRB

Brand

Whirlpool

Age

6-10 years

The microwave of my GMC305PRB01 blew the 15AMP fuse (a 20AMP MDA was actually installed from factory).

While replacing the fuse and noticed the red lead to HV transformer had come lose and touched the transformer case. I suspected this might be way the fuse blew. I reconnected the lead. Turned power back on. Everything works except microwave doesn't heat.

Checked all the micro switches and thermal fuse for continuity. All appear to be ok. Did OHM check HV capacitors appear to be ok. Did OHM check on the magnetron, appears to be ok.

Ordered new HV transformer and diode. Installed new parts - still no heat.

Checked main filter, it appears to be ok. I couldn't get the leads of though so I have to measure with lines connected.

I running out of options. Anybody have any suggestions?

Thanks
Pat

 

[Jake]

Hi Pat,

What was the Mag ohms?

Normally should show LESS THAN 1 OHM.

Jake

 

[pfe999]

Jake, Mag is 0.2 ohms.

Interestingly, my unit has this 3rd T.O.D attached to a strange component above the microwave cabinet that's not included the tech sheet wiring diagram.

 

[pfe999]

FYI - the HV Capacitor omhs out starting at 800 OHMs to then goes to infinity. The spec sheet says it should "momentarily indicate several OHMs then go infinity". 800 is not several.

I wonder if this is the problem?

 

[Jake]

That is odd, I need to ask Rick.

Jake

 

[bigbuck]

Tech sheet if needed Rick. https://drive.google.com/file/d/1osf__D9FqMzLqKgUam1ybcQentXNRFci/view?usp=sharing

 

[rickgburton]

Set the machine to cook on high and measure the voltage at the primary side of the transformer. Let's see if the problem is on the high side or low side.

 

[pfe999]

OK, I'll try that Sunday. Don't have time today or tomorrow.

Also, I ordered capacitor because the measurement was so different the spec sheet.

Thanks Guys.

 

[rickgburton]

My guess is the short probably damaged a relay on the control board and the voltage reading on the primary side of the transformer will be 0 VAC

Set your meter on the 10K ohms scale. Touch the capacitor terminals with the meter leads. Your meter should show several thousand ohms then return to infinity. Some of the less expensive meters won't have the ability to check the capacitor. If that's the case, use a 9 VDC battery and a couple jumper wires. Here's a diagram:


The diode is actually a bridge rectifier. Use the diode check function on your meter. Touch the meter leads to the diode then reverse the meter leads. One direction will show continuity and the other, open. The diode check uses the 9 Volt battery in your meter. If your meter doesn't have a diode check function, use a 9 volt battery and a jumper wire.


To check the transformer be sure power is removed. Measure the resistance of the coils. Between the two primary terminals: less than one ohm. Between the two filament wires: less than one ohm. Between the secondary wire and transformer ground 60 to 120 ohms. Between the two filament wires and transformer ground: infinite resistance.


It's always best to check a component before replacing it. Otherwise your repair will get very expensive, very fast.

 

[rickgburton]

Were you able to find your problem??