• ** REMEMBER! **The microwave can still shock you even unplugged!!

    ALWAYS discharge the high-voltage capacitor first if you even think your hands will come close to any HIGH VOLTAGE components.

    Jeff mentions this: Anything in the high voltage ( magnetron, capacitor, diode, wires to and from ):
    ...Use a metal ( not the shiny chrome type ) screw driver with a insulated handle to short across ( touch both at the same time ) the terminals of the high voltage capacitor to discharge it.

    From Jeff's site: http://www.applianceaid.com/component-testing.php

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

pfe999

Premium Member
Joined
Nov 27, 2006
Messages
21
Location
Atlanta, GA
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

Appliance Tech - Admin
Staff member
Joined
Aug 24, 2004
Messages
106,202
Location
McMullen Valley, Arizona
Hi Pat,

What was the Mag ohms?

Normally should show LESS THAN 1 OHM.

Jake
 

pfe999

Premium Member
Joined
Nov 27, 2006
Messages
21
Location
Atlanta, GA
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.

Lk4C4p8pesZDIcvItr9oFjpYe17LT_3HNxSo7fMsHsspX92IB.jpg
 

pfe999

Premium Member
Joined
Nov 27, 2006
Messages
21
Location
Atlanta, GA
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

Appliance Tech - Admin
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Messages
106,202
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McMullen Valley, Arizona
That is odd, I need to ask Rick.

Jake
 

rickgburton

Appliance Tech - Moderator
Staff member
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Messages
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Location
Murray, Utah 84107, United States
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

Premium Member
Joined
Nov 27, 2006
Messages
21
Location
Atlanta, GA
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

Appliance Tech - Moderator
Staff member
Joined
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Messages
37,043
Location
Murray, Utah 84107, United States
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:
C-Capacitor Test.jpg

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.
C-Diode Test.jpg

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.
transformer HV.jpg

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