• ** 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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FIXED FGMV176NTW Frigidaire Microwave blows fuses...

divide_by_zero

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Unit had lights and display, then upon pressing START the line fuse blew. A second fuse, same thing.

The magnetron shows 00.0 between the terminals on the DMM 200M setting. I measured the diode and got inconsistent readings but I figured if I'm replacing the magnetron, I'll replace the diode, too.

Is the reading of 00.0 enough evidence to replace the magnetron?

Thank you
 

Dan O.

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The filament of a magnetron tube should measure less than 1 ohm so I would say your reading is likely normal.

I doubt a shorted magnetron tube would cause a fuse to blow anyway but if it did, it wouldn't be immediately. It would take at least some time after being powered.

Dan O.
 

divide_by_zero

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Thank you, Dan O. I just saw I wrote "magnetron shows 00.0 between the terminals on the DMM 200M setting" and it should have been 00.0 on the 200 ohm setting.

I understand the reading should be less than one ohm but isn't 00.0 too low?
 

Dan O.

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As long as the resistance is less than 1 ohm (0 is less) and there is no short between either of the filament terminals and the body of the magnetron, it is unlikely to be defective.


Dan O.
 
Last edited:

Dan O.

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If the fuse is blowing immediately when the microwave is activated, I'd suggest someone check operation of the door switches. All 3 door switched must operate in sync or the fuse will blow if started, as a safety precaution.



Dan O.
 

divide_by_zero

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The door switches check okay. I also started the unit with one line-side lead disconnected from the high-voltage transformer and the fuse did not blow.
 

rickgburton

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.... but isn't 00.0 too low? .... At 20K, 200K, and all scales, the reading is 00.0
Everything has resistance. Even wire has a small amount of resistance. If you measure the resistance of 10' coil of wire your meter would indicate 0.0Ω. If you measure the resistance of a 1000' coil of wire your meter might indicate 0.2Ω. On the lowest ohms scale your meter can measure tenths. The more resistance you have the higher the ohms scale is needed to measure it. So if you have 0Ω on the lowest scale you'll definitely have 0Ω on the higher scales.

Here's how to check the high voltage components in your microwave:


C-HV Testing.jpg
 

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divide_by_zero

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Are any of the transformer windings shorted?
My meter on the 200 ohm setting reads 00.1 across the filament windings

The filament to the core at 20M reads open

The single HV lead to the core reads 118 ohms

Question: my meter shows 00.0 or 00.1 (it alternates between the two) on the magnetron. So it's less than one. But could it be shorted?

rickgburton, thank you for the reference document - that answers some questions I had.
 

divide_by_zero

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Measure from each terminal to the mag tube body
Each mag tube terminal to the body shows open.

I've run the unit with the secondary leads of the HV transformer disconnected and the fuse did not blow. Would it hurt anything to run the unit with everything connected except have the magnetron unplugged?
 

divide_by_zero

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On the 20M range, the capacitor starts around 5 and stops climbing around 10.20

On the DMM diode test, both directions show open. Using the 9V battery test with the meter set to 20VDC, I get 00.0 one direction and 5.55 the other direction
 

rickgburton

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On the 20M range, ......stops climbing around 10.20
Set the ohmmeter to the R x 10K scale. Touch the ohmmeter leads to the capacitor terminals. The meter should indicate several ohms, and gradually return to infinity. Set the meter on R x 1Touch one ohmmeter lead to the capacitor case. Touch the other lead to each of the terminals. The meter should indicate infinity at both terminals.
C-HV-Capacitor Test.jpg

On the DMM diode test, both directions show open.
That's correct
Using the 9V battery test with the meter set to 20VDC,
The microwave diode is actually a bridge rectifier, four diodes orientated in a specific direction. A battery can be used to check forward and reverse bias through the bridge rectifier. Forward Bias decreases the resistance through the diode and Reverse Bias increases the resistance. You're still measuring resistance not DC voltage so keep the meter on the ohms scale.



C-HV-Diode Test.jpg
 

divide_by_zero

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Set the ohmmeter to the R x 10K scale. Touch the ohmmeter leads to the capacitor terminals.
My meter doesn't have a 10K scale so I used the 20K. I got an open reading on this test.

On the 200 ohm scale, each terminal to the capacitor case showed open.

I did the 9V battery test as illustrated and got 0V for the terminals and 0V for the terminal to case.

[diode test] keep the meter on the ohms scale
I did the diode test as illustrated and the meter showed open in both directions.
 

rickgburton

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