Help with a strange microwave oven behavior please

[berkeman]

TL;DR

Is it time to replace this microwave oven, or is it a simple problem?

The last couple days at work, one of the microwave ovens in the kitchen has been acting strangely. Today it seemed to get potentially dangerous, so I marked it "Out of Order" for now, and would like to figure out if it is hopeless, or if it can be recovered easily.

I first noticed a couple days ago that the display was dark, despite being plugged in. Normally there will be something in the display, I think, even if there has been a power loss and the clock needs to be reset. We have two other microwave ovens in that kitchen -- one that matches this model, and one that is different. They both are acting fine.

I hadn't tried this microwave since its display was dark, but I got the impression that it was probably working, since I thought other employees were using it okay. But today I walked into the kitchen and noticed that the microwave was displaying "0" and was running. It kept running like that for a few seconds before I couldn't take it anymore and opened the door to cancel it. Closing the door again did not restart it, but the inside of the oven and its interior walls were pretty hot, and there was nothing inside the oven.

So, either it turned itself on for no reason, or somebody turned it on with nothing inside it (unlikely), or somebody used it to cook some food, and closed the door and it restarted on its own. All seem to be bad possibilities.

I searched at the Mfg website for Troubleshooting tips, but no joy. I guess I'll try to call them tomorrow, but if anybody has seen anything like this before, that would be helpful.

Link to the model: http://www.sharpusa.com/ForHome/HomeAppliances/Microwaves/Models/SMC1132CS.aspx

Pic of the 2nd oven with a normal display:

 

[AZFIREBALL]

I had one that acted the same way. I got up at night to get a drink of water and it was running with its light on! Very hot inside (nothing inside). No one else here but me. Opened the door and it stopped running. Closed the door, punched a few numbers and it seemed to be working OK.

The next day it started on its own again, during the day. I thought I had a spook in the house.

Opened the door...it stopped running. Closed the door. Put in some numbers...seemed to work fine. The next few days it would start at odd times on its own. Figured it had lost its mind, so I replaced it. I am going to use its parts to make something some day. (Wishful thinking.)

 

[anorlunda]

That sure sounds hinky. I don't think I would trust it to be left alone.

 

[256bits]

Spooky.
It might be wise to check the refrigerator too when its not looking to see if the light is on when the door is closed.

 

[Wrichik Basu]

But today I walked into the kitchen and noticed that the microwave was displaying "0" and was running. It kept running like that for a few seconds before I couldn't take it anymore and opened the door to cancel it. Closing the door again did not restart it, but the inside of the oven and its interior walls were pretty hot, and there was nothing inside the oven.

I would never keep this thing plugged in all the time. It is, after all, a fire hazard.

One website explains why this may happen:
https://www.doityourself.com/stry/5-reasons-why-a-microwave-might-turn-on-by-itself

 

[Tom.G]

It's just a dedicated microcontroller scanning the keypad, counting down, and triggering a relay or two. Sounds like it has gone schizophrenic and broken out of its straightjacket (program).

The controller boards, when they are available from the manufacturer, cost around 1/3 the price of a new oven. The replacements are not very difficult once you remove the too-many security screws holding the cabinet on.

Some (many?) ovens have troubleshooting guide folded up and inserted in the U-channel frame near the controller.

If it has seen heavy use for a few years and you are bored, it could be that the electrolytic caps on the controller board are no longer caps.

Otherwise take up a collection for a new oven and ask if anyone wants parts.

Irrelevant memory:
Back when the ovens used a line voltage transformer, a friend decided that the 1+kW transformer was ideal for making an electric furnace. After he removed the secondaries, he found some wire just small enough to make one layer in the freed-up space and wound a few turns. The heating element was a couple Carbon rods about the diameter of you thumb. As I recall it was mostly used in arc-furnace mode .

 

[Guineafowl]

@Wrichik Basu ’s link has some interesting suggestions, especially the membrane keypad sticking. In light of the dark display you saw, I would go for a control panel fault, particularly on the power supply. Most mfrs are decent enough to label the voltage rails. A google of the model no and fault may also throw up some useful info.

Disconnect the main transformer primary and stick a bulb across it, so you know when the magnetron should be running, rather than risk getting zapped.

On a related note, while many a busted microwave has passed through my hands, my parents still have their 40 year-old one. Stainless interior (so no peeling enamel and arcing), rotary timer (no silly keypad/controller faults) and a chunky defrost/full power switch

 

[anorlunda]

Another irrelevant memory:
I once owned one of the original Amana Radar Ranges (version 1.0). Twice, it was mistakenly turned on with no food inside. Both times, it melted a hole in the glass tray leaving a 3cm molten glass marble under the tray. So, twice I had to buy a new tray.

I spoke to Amana engineers about that. They insisted that I was a crackpot who made up the story and that no such thing could happen. But thereafter, I always thought of a microwave oven with no food inside as a potential death ray or at the very least something very hazardous.

 

[Guineafowl]

^^^ One way to measure the speed of light, by measuring the hot spots produced by a microwave(!)

Many diagrams of how a microwave works show a spinning ‘wave stirrer’. This would eliminate the melting glass tray/hot spot problem, but I am yet to see an oven fitted with one.

 

[berkeman]

Looks like we're just going to replace it. At about $150 they are inexpensive, and not worth any fire hazard risk.

Thanks for the very helpful replies, folks. It's good to hear that some others have had this same thing happen to them.

 

[Guineafowl]

Looks like we're just going to replace it. At about $150 they are inexpensive, and not worth any fire hazard risk.

Thanks for the very helpful replies, folks. It's good to hear that some others have had this same thing happen to them.

Please tell me you’re going to take it home and post-mortem it.

 

[jim hardy]

I had a Panasonic that was intermittently losing time of day.
It never self started(to my knowledge) but i suppose anything is possible with a computer that's seeing intermittent power interruptions.
But i get ahead of myself.

I fixed it once maybe twelve years ago by re-soldering the little power transformer joints on the keypad computer PC board . I noticed then somebody else had done the same, so figured it was probably a factory rework.

Two or three years ago the problem returned.
Having completely forgot my prior repair i was surprised to find the "Tattletale LED" i'd wired into the 12 volt supply - i'd placed it where i could peek in through a vent hole and see it from the outside.
Sure enough, now banging on the case would make my tattletale light flicker.
Opening the oven i found that just touching the little control transformer made the light flicker too. That was the most sensitive spot.
So i removed the little transformer and voila - on the transformer itself one of the fine magnet wires was fractured right where it attaches to the solder pin.
There was barely enough slack to scrape off some varnish and resolder it.

It's worked fine since.

Point being - intermittents are hard to find.
Look for something upsetting the computer.
Good programmers would write a power fail sequence immune to a failure like you describe - but who knows?
I'd install a tattletale LED on the computer board's power supply trace and physically push on every component on the board . If you find a 'touchy' spot that's a mighty strong clue.

If it's an inverter type oven -
hobbyists make heat-treating induction furnaces out of those inverter boards, try Youtube... so don't throw it out.

Environmentally speaking -
The magnetron has just enough Thorium on its cathode to make scrap metal dealers reject a truckload with as few as three microwave ovens in it. So if you discard it to the landfill yank the magnetron first . It has nice soft aluminum heatsink fins that come in handy for small sheet metal projects,
and two dandy donut shaped magnets of perfect strength for holding your calendar to the fridge.
The magnetron body on mine was thick copper worth recycling.

Waste not want not ?

old jim

 

[Guineafowl]