Microwave gone dark

[DaveC426913]

TL;DR Summary

Should I try anything to fix it before I ditch it?

Just doing my husbandly due diligence.

My Panasonic "science oven" was just minding its own business. I went to put something in it and the display went dark. Never even touched a button.

I tried it in another socket and it's still dead.

Should I even try to diagnose it before going out to buy a new one? Even to check the fuse, I'd have to crack it open.

 

[berkeman]

How old is it? What brand?

 

[DaveC426913]

My Panasonic says 'Panasonic' on it.

It's at least ten years old.

 

[berkeman]

My Panasonic says 'Panasonic' on it.

Here's your sign...

J/K.

 

[Tom.G]

I've found two common failure modes in μwave ovens:

1) An interlock switch that when the door is opened during operation, puts a direct short across the incoming power line. After a a few occurences, the switch contacts weld together -- this of course blows fuse/trips circuitbreaker whenever power is applied. It does avoid a cooked hand or eye though. Here in the US, the switch is often located at the bottom latch of the door (needs a fair amount of disassembly though).

2) The circuit board fails, either partially or totally. Sometimes a few of the buttons or display elements will still work.

A third possibility is the magnetron may have an internal short, thus blowing an internal fuse or the circuit board. I've never seen this occurence, only heard about it.

For case 1), the switch is a common one readily available from electronics suppliers for a few dollars. If bought from the oven manufacturer, be ready to pay 3 times as much for it.


Have Fun!
Tom

 

[DaveC426913]

Here's your sign...

The sign's all yours, my friend:
Post 1:

My Panasonic "science oven"

 

[Baluncore]

If it went dark with a loud crack, give up now.

If it went dark quietly, check the fuse on the internal PCB. Replace the fuse if blown.

If the fuse blows again immediately, give up, because you need someone expensive, in money or time, to diagnose and service the unit. Cut and run.

Replace the unit with a new one, and get on with your life and relationship.

 

[DaveC426913]

If it went dark with a loud crack, give up now.

If it went dark quietly, check the fuse on the internal PCB. Replace the fuse if blown.

No, it was silent and uneventful.

 

[Averagesupernova]

I have found that the thinnest of material pinched in the door will cause the microwave to not start. No blown fuses in my case but that is not to say it wouldn't happen in other cases. There are usually two switches, maybe even three on the door. As said previously in this thread, one is set up to cause a short and blow a fuse. This only happens if one of the other switches fail. I would definitely not toss it.

 

[DaveC426913]

have found that the thinnest of material pinched in the door will cause the microwave to not start.

You are ignoring the part about the display going. Also the buttons do not respond. That's not a door problem.

The device is effectively catatonic. (I'm avoiding saying 'dead' because that's a diagnosis - as opposed to a presentation of symptoms.)

 

[Baluncore]

Also the buttons do not respond. That's not a door problem.

Well, it can be. Many microwave ovens will ignore the keypad while they detect a door-open interlock, but they should not be dark.

If there was a breakdown of the 3 kV capacitor, stick rectifier, or there was a heater filament to anode short in the magnetron, then it would have made a noise when it died.

Since it failed silently, that normally suggests a fuse has died of old age, or a door switch has bounced and mistimed, so taken out the fuse.

You are overthinking this. You have little choice but to examine the fuse. Replace the fuse if blown, then cross your fingers and try it once more.

 

[Averagesupernova]

You are ignoring the part about the display going. Also the buttons do not respond. That's not a door problem.

And you are ignoring the part:

This only happens if one of the other switches fail.

Soooo, if the door was opened or something was in the way you could potentially blow the fuse.
-
Your description:

I went to put something in it and the display went dark. Never even touched a button.

Indicates to be it might have happened when the door was opened.
-
The thing to do here is crack it open and look. Keep in mind the large capacitor in the high voltage magnetron circuit could have residual charge.
-
The fact that you bothered to ask about it here on pf indicates your time is not so valuable that you cannot spend some time doing a physical examination.

 

[DaveC426913]

Indicates to be it might have happened when the door was opened.

Weird (to me). To be clear, the device had not been in use. I had just come in to the kitchen to heat some leftovers, but hadn't started. I am 90% sure I saw the display working, and when I went to open the door to put food in, that's when the display went dark.

The thing to do here is crack it open and look. Keep in mind the large capacitor in the high voltage magnetron circuit could have residual charge.

Yes, this is why
a] I am asking here, and
b] why I am exercising an abundance of caution, up to and possibly including replacement.

The fact that you bothered to ask about it here on pf indicates your time is not so valuable that you cannot spend some time doing a physical examination.

True. I am prepping for retirement by starting to tinker with things in my shed.

I'll check the fuse and be very careful.

 

[Averagesupernova]

Way back when I went to school they told us that the failures in electronics in general were mechanical. Switches, potentiometers, relays, connectors, anything that moved. None of us believed it at the time. I come to realize after a short amount of time troubleshooting in the field that this was quite true. I'm betting on a switch in your case.

 

[DaveE]

I would open it (unplugged and after waiting a couple of minutes and do a visual inspection, especially of the PCB, given your description. Stay away from the big chassis mounted capacitor with the high voltage warning/label.

Look at each IC and any big transistors, if they look burned in any way then you need to replace that board. Look at each fuse, if they are removable, and check continuity with a DMM (out of the circuit). If any are melted, then replacing it probably won't work; something else caused that. Rarely, they can open from mechanical stress and old age, then replacing it probably will work. You can also check switches with an ohmmeter (disconnected is best).

If you don't see anything wrong, then remove and reinsert any small connectors to the PCB, then try it again. Some times their contacts get corroded and reseating or wiggling them can fix that. Switches, relays, connectors, and power transistors are the likely suspects. If you see any relays, you can gently tap them to free stuck contacts or connections (if they are in a socket) for a small chance of a temporary repair.

I've had good luck repairing appliances with eBay used PCBs. However, microwaves aren't THAT expensive, a repair isn't guaranteed, finding the correct parts isn't trivial, and you have the rest of your life to live. The last time my microwave died I didn't even take the cover off, I was ready for a new one anyway.

 

[DaveE]

they told us that the failures in electronics in general were mechanical. Switches, potentiometers, relays, connectors, anything that moved.

100%. I was on a crusade at my last company to remove as many electromechanical parts as we could. Pots were replaced with EEPOTS*, relays with MOSFETS, SCRs, or ICs, etc. Sockets for ICs were verboten (except ROMs sometimes).

Our field service people didn't like the idea. They said "we use that stuff to repair things". My response was "exactly, that's why we have to get rid of them". I would bet there's not a single potentiometer orbiting the Earth in a satellite.

* Off topic, but... EEPOTS are great for calibrations because you have to know what your doing to change them and they are somewhat separated from the rest of the SW for bugs, weird transients, memory leaks, etc. You can have SW routines that support proper recalibrations. Everyone that can hold a tiny screwdriver thinks they can fix things by twisting a potentiometer without actually knowing why it's really there.

 

[Baluncore]

I would bet there's not a single potentiometer orbiting the Earth in a satellite.

I would take you up on that bet. The old ISS probably has a couple.

 

[Averagesupernova]

Everyone that can hold a tiny screwdriver thinks they can fix things by twisting a potentiometer without actually knowing why it's really there.

Truer words, or very few, were ever spoken here on pf.

 

[DaveE]

I would take you up on that bet. The old ISS probably has a couple.

You're probably right. I don't know anything about the Russians, for example. Plus there's no telling what's in Cube-Sats and Starlink. That simply has to be different than my experience. I wouldn't put a pot in those but they might.

When I did power supplies for DoD satellites in the 1980's, they were verboten.
I suppose you could have used them, but only if you had several design reviews with your customers and proved that:
1) You had no other choice; literally, no other option.
2) They would perform as needed given the environmental specifications. Key among them were shock, vibration, temperature, outgassing, and proven reliability.
3) Spent a small fortune on getting some built to Mil-Spec S-level requirements. Because no one else was using them.

What we actually did, when needed, was to use "factory selected values". The engineer (me) would write and perform a test procedure on each unit that was assembled without that resistor and would pick a value to be installed. I'll spare the details, but you basically only got one chance to get it right. DoD satellites in the 1980's wasn't high volume production, LOL.

 

[DaveC426913]

Yup. I jumped the fuse and the lights came on.

So I need a replacement fuse. It's some sort of ceramic. The PCB labels it as (and I'm quoting here) "Special Fuse".

Looks like it's a common fuse for microwaves. Just dunno if it's 10A or 20A.

 

[hutchphd]

Now you are one step closer to your "old geezer" merit badge. Congrat's.

 

[DaveE]

The PCB labels it as (and I'm quoting here) "Special Fuse".

It may be marked on the metal bits.

 

[tech99]

I learned very early on in telecommunications to never place a pot in a direct signal path.
Incidentally, referring to reliability of mechanical or electronic parts, the old cars without electronics and ECUs seem more reliable to me, and are cheaper and easier to diagnose and repair.

 

[Baluncore]

Looks like it's a common fuse for microwaves. Just dunno if it's 10A or 20A.

The mains is 240V, so the wall outlet max current will be about 10 amp.
A 1 kW oven on 240V is about 4.2 amps, so use 250V10A fuse.

The fuse is special, ceramic so it will not shatter, and will extinguish, after blowing with a dead short at 250V.

It may be marked on the metal bits.

The fuse is marked on its metal ends, I can see a bit of text between the PCB fingers.
Go with those numbers. There may be a momentary turn on surge.

 

[renormalize]

The mains is 240V, so the wall outlet max current will be about 10 amp.

@DaveC426913 is in Canada so it's most likely that his microwave oven is connected to 120V. (I've never encountered a 240V consumer microwave in North America.) I'm guessing that the obscured part of the white PCB label under the cord plug says 125V/20A and that's the spec for the fuse.

 

[DaveC426913]

Ah. Thanks. It says 20A 250V.

Man, that's hard to read!

BTW, I am on 120V.

 

[renormalize]

Ah. Thanks. It says 20A 250V.

Man, that's hard to read!

BTW, I am on 120V.

Twenty-amps at 250V is 5kW which is way too powerful for a home microwave. I'd use a 20A 125V fuse. I think the manufacturer spec'd the fuse for 120V-use which is why they blacked-out the label below the 250V marking (probably says 10A underneath) so as to avoid confusing the repairman and/or curious consumer. But they should have also blacked-out the 250V label.

 

[DaveC426913]

Twenty-amps at 250V is 5kW which is way too powerful for a home microwave. I'd use a 20A 125V fuse. I think the manufacturer spec'd the fuse for 120V-use which is why they blacked-out the label below the 250V marking (probably says 10A underneath) so as to avoid confusing the repairman and/or curious consumer. But they should have also blacked-out the 250V label.

OK. That jives with what others were saying, above.

 

[hutchphd]

Unless this is a very unusual situation, only the amperage of the fuse needs to match the specification. The voltage rating only needs to be high enough (so 250V will work fine in this application. Almost all household line fuses are rated to 250V so they will work "anywhere" and there is no need to duplicate the supply chain.

 

[DaveE]

The voltage rating of fuses is separate from the current rating, multiplying them doesn't make much sense in fuse selection. It's about the worst case values, not the combination. The voltage rating is about clearing an arc in the fault condition, i.e. how much voltage is available. The current rating is normally about preventing fire from overheating of wires and such in fault conditions. It is determined by the size of downstream wires, traces, and sometimes components. It's not directly about normal operating currents. So for an appliance that runs at 120V/16A or 240V/8A, a 250V/20A fuse makes sense.

The "special fuse" label is really a work around for safety agency approvals, what it really means is "not user serviceable". Of course that's BS, you or I would replace it. The ceramic envelope is characteristic of fuses with a high interrupting current rating. So, don't get the cheapest fuse available (usually glass).

 

[DaveE]

It says 20A 250V.

Also look for other markings, something like MDA.