Electronics > Repair
GE Microwave oven with burnt connectors on thermal cutoffs [USA]
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scootley:
Hi,

Note: I am generally aware of Microwave internal safety and the giant capacitor risk.  The manual says it has an internal shunt resistor and is designed to discharge within 30 seconds.  And I know there are shock issues if working on it with mains active, and the high voltage magnetron.  And issues if I were to physically change the magnetron or case position/integrity where microwaves could be emitted to the exterior at harmful levels.

I have a GE Microwave oven that shows "PF" on the display and if you try to activate it, it goes dark (light inside goes off, buttons/display do not operate) for a while until PF eventually reappears.
So after taking it apart, I see two connectors that are brown/burnt a bit, one on each of two thermal cutoffs (TCOs) (labeled as "oven/cavity" and "magnetron"). 
With the cord unplugged, the "oven/cavity" TCO reads open circuit and the "magnetron" TCO reads short.  I'm assuming of course these TCOs are normally short and go open circuit when they exceed some temperature.

Nothing else inside has any obvious visual signs of failure.  The troubleshooting flow chart on the wiring diagram says to replace the TCOs that are reading open circuit.
What could have caused this?  Something causing a short in the circuit that flows through the TCOs?  Fan motor failure?  Turntable motor failure?  High voltage transformer failure?  Magnetron failure?  I admit I have not yet fully internalized the wiring layout.

Photo of oven/cavity TCO:

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Photo of magnetron TCO:

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Photo of internals overall:

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Wiring diagram:

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Schematic:

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Troubleshooting:

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Prior history of this unit is that:
1) for years there has been some weird issue with the door interlock where either a) it thinks the door is open when it is shut, and won't activate at times without a wiggle, OR b) when the door is shut but the magnetron is off, the fan actually activate and be blowing out the back, also fixed by a wiggle.  I just assume the interlock switches (there are 3) and/or their alignment with the door was bad.
2) a few months ago, the main relay that activates the magnetron failed and was stuck on.  I replaced it and it has worked fine ever since.  I wonder if the fusing of that relay was caused by issue #1 where the flaky switches would make/break the circuit randomly causing arcing that welded it stuck on?

Thanks
amyk:
One of the interlocks shorts the power when the door is open. If one of the others hasn't opened yet, it is definitely possible to burn something.

Another possibility is a poor connection, leading to heat, burning, and an even worse connection.

(If you have pictures in your post, I can't see them.)
scootley:

--- Quote from: amyk on February 21, 2021, 01:20:03 am ---(If you have pictures in your post, I can't see them.)

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Yes there are 6 pictures in the post.  Do you see the imgur.com links after each picture's title?


--- Quote from: amyk on February 21, 2021, 01:20:03 am ---One of the interlocks shorts the power when the door is open. If one of the others hasn't opened yet, it is definitely possible to burn something.

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Thanks.  Yes I did a fairly crude simulator of the circuit here: https://tinyurl.com/yckgufad
This particular unit has 2 interlock switches that seem to be in direct control of power.  Both of them are depressed when the door is shut, but one is wired "normally open", so it goes short with the door shut (called "primary interlock" in their diagram), and the other is wired "normally closed", so it goes open circuit when the door is shut (called "monitor interlock" in their diagram). 

So my deduction is that if the monitor interlock gets "stuck" as a short when the door is closed AND the main magnetron power relay is active, then there will be a zero resistance loop passing through the TCOs and they will burn out.  Does that make sense?  It could happen if someone wiggles the door while the unit is running, I guess.  The magnetron would deactivate and the whole thing would shut off simultaneously with the burnout happening, since the monitor interlock's purpose seems to be to disable it by shorting the primary winding to it's transformer.

So your theory is correct but only if the magnetron relay is also active, I think. 
Or an alternative scenario is where the door is open, but both the power relay AND the primarily interlock are stuck short, which is less likely because that involves 2 components being broken rather than 1 as above.

(There is a third switch but it's more of an electronic interlock because it's wired to the control board and does not carry current through the circuit powering the main loads inside.  It seems to be yet another backup "door is closed" sensor and shorts when door closed)

So I guess I could get replacement TCOs.  But each TCO seems to have it's push-on terminal connector welded to it, so maybe I would need new connectors too because I fear soldering to the TCO directly would activate it and kill it due to heat, right?  How do I find terminal connectors that fit a given TCO and are rated for appropriate current?
CaptDon:
Those friction fit push on 1/4" spade terminals on the wires were always a piece of shit and GE knew it from day one. Now the insulation is also hard and brittle. 1/4" spades were never meant to carry 11+ amps for more than a minute or two at a time. Cut off the old lugs and both crimp and solder some new lugs on. Scrape the male spades for a shiny connection also. This is normal age and GE saving money. There is really no other problems except poor design.
scootley:

--- Quote from: CaptDon on February 22, 2021, 03:46:00 pm ---Those friction fit push on 1/4" spade terminals on the wires were always a piece of shit and GE knew it from day one. Now the insulation is also hard and brittle. 1/4" spades were never meant to carry 11+ amps for more than a minute or two at a time. Cut off the old lugs and both crimp and solder some new lugs on. Scrape the male spades for a shiny connection also. This is normal age and GE saving money. There is really no other problems except poor design.

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Thanks!  These are actually only 3/16" too.
Wait so do you mean that you think the lugs overheated from carrying too much current during normal magnetron operation, and that heat is what tripped the TCO, rather than a short circuit somewhere?  Interesting.

I definitely need at least one new TCO (the oven cavity one) because that one reads open circuit now and so it doesn't seem to have been the auto-reset type or it was just pushed way beyond its capability and broke.  Thing is that I don't know what the current rating for it was.  The wiring diagram says it's "110", so I assume that's the trip point in degrees Celsius (not Fahrenheit), but I can't figure out who makes the part and it pains me (on principle more than budget) to buy it from a "microwave parts distributor" because it will cost like 5x in price.  The wiring sheet also says 14.3 amps (1650W) "line current" (it has a 20A fuse). Unit is branded as "1150W".  The only parts I see on digikey that seem similar are rated at 15A max.  Maybe that will be ok.
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