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Microwave fuse blows when closing door

Panasonic NE-1054C microwave is 3 years old.

Came to me with a blown fuse. Disconnected power, checked HV capacitor was discharged, and began troubleshooting.

- Tested the 3 door switches and power relay connected to PCB. Switches tested correctly with door open and closed, multiple times.

- Checked door latch alignment. Nothing loose or out of alignment.

- Checked for signs of loose connections, wiring issues, burns or signs of overheating. Everything looks good.

- Checked all other major components for possible issues, shorts. Nothing found.

Replaced fuse, button-up cabinet, connect to power, open and close door multiple times, place a cup of water inside cabinet, run for 30 seconds (do this multiple times switching out cups of water) and it works.

Return microwave to owner, owner uses for one evening several times, no issues. Tries to use in morning - dead. Assumption is that last operation of microwave in evening blew fuse without owner noticing.

Pick up microwave, find blown fuse, test switches again. Switches test proper. No visible signs of excessive heat or problems, no components test failed or shorted. Close up microwave, connect to power, run for 30 seconds with cup of water inside, open door, close door, run for 30 seconds again, open door, close door - FUSE BLOWS.

Has to be a door switch - test all switches, again. This time I use an analog meter to see in real time when switch opens and closes. ALL switches work without delay. Power relay contacts are in OPEN position, as they should be.

3 mechanical switches and one electrical relay involved with the door function:

- Primary interlock switch

- Secondary interlock switch

- Monitor switch

- Power relay on main PCB

​I am at the point now where I am assuming that the circuit board is intermittently glitching causing the power relay contacts to CLOSE when the door is closing. This would cause a direct short and blow the fuse (I tested this by disconnecting the power, disconnecting the leads form the power relay, putting a jumper across the leads to simulate CLOSED contacts of power relay, and then closing the door with a meter connected between L1 and N - meter goes to 0 ohms/dead short). But I can't prove it under power which is why I am hesitant to just throw parts at it to attempt a fix.

3 switches = $60.79 CDN before tax and shipping

PCB = $128 CDN, out of stock

Parts found here:

https://www.partstown.ca/panasonic/ne-10...

I could risk $60.79 on switches that appear to be working just fine or $128 on a PCB ( if I can actually get one) that I can't guarantee is the source of the problem. No smoking gun and no desire to waste money.

Any suggestions for troubleshooting this issue further? Thank-you

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Hi @dman32658

Unplug the oven's power cord from the power outlet and connect an Ohmmeter directly across the Active and Neutral pins of the power cord plug.

Close the door and then slowly open it and check if the meter shows a short circuit at any time.

If the door switches operating sequence it correct it shouldn't as the primary interlock should be open before the monitor switch is closed.

Worn/bent switch actuator?

Just what I'd try

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@jayeff

I have tried that with meter set to resistance and measurements are correct. Compared it to my own Panasonic microwave and resistance readings across L1 and N from the cord end are similar with door open and closed.

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@jayeff

Door actuator has been inspected. Solid contact is being made on all switches. Nothing loose or out of alignment.

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Does anybody know the sequence of operation for the switches for this model?

It appears that closing is Monitor, Secondary, Primary. Opening is Primary, Secondary, Monitor. Is that correct???

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@dman32658 First thing I do is replace the switches. No reason to replace that entire $60 switch assembly when the individual switches are dirt cheap. For example, you can get 10 of them for $13.

https://www.amazon.ca/RuoFeng-Microwave-...

These will work for both the NO and NC switches; just plug in the appropriate terminal as needed. If you want name brand parts, search up Omron micro switches; that'll give you good solid parts (not that there's anything wrong with the ones I linked to - they did get pretty good ratings on Amazon).

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PROBLEM SOLVED - maybe....

I had the switches in the plastic assembly loosely connected to the metal bracket and then closed the door. Once the door was closed I adjusted the position of the plastic assembly so that the door was tight to the frame but with an ever so slight gap that if you pulled on the door it would barely move (maybe less than a 1/16th of an inch). Tightened down the 2 screws that secure the plastic switch assembly to the metal bracket. I attached a separate electrical meter to each of the 3 switches. The Monitor switch had a meter connected set to continuity so that I could hear it tone when closed. Did the same thing with the Primary Interlock switch (which should tone opposite to the Monitor switch). To keep things easy to identify I used an analog meter set to resistance for the Secondary Interlock switch.

Next, I opened and closed the door as slowly as possible to verify that the switches worked in the following sequence:

Door closes - Monitor switch (open), Secondary Interlock (closed), Primary Interlock (closed)

Door opens - Primary Interlock (open), Secondary Interlock (open), Monitor switch (closed)

I tried adjusting the plastic switch assembly to see if I could deliberately interfere with the sequence. I couldn't say for certain that when operating the door as slowly as I was that the sequence ever changed but I wouldn't say that it would have been impossible when operating the door at a "normal" speed. Unfortunately, the operation of the switches would be too fast to see an error in real time even with the meters connected. But I can say that the Monitor switch definitely gets a slight head start being activated when CLOSING the door and similarly is the last to be released when OPENING the door. The timing between the Primary and Secondary Interlock switches is very, very close. There is definitely a different sequence (as shown above) but I do not know how crucial it is vs the Monitor switch. How fast the signal gets to the PCB from each switch when operating the door at a "normal" speed would be interesting.

Today I have tested the microwave numerous times, running at different time settings, opening and closing the door, and NO BLOWN FUSE.

In hindsight, I can definitely say that the door did not latch as tight as it does now. There was a little more wiggle room if you pulled gently on the handle. Maybe enough that the "out of sequence" theory could happen. This makes sense since the fuse did not blow every time I closed the door, it was random.

I will post back with an update after the microwave gets put back into service.

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