What might be wrong with my microwave?

[Bert Coules]

Working perfectly this afternoon, tonight it runs for twenty seconds and then switches the cooking process off (without the usual end-of-sequence beeping) and there's no evidence that the food has been heated even slightly.

Could this be linked to the fact that I used it earlier to heat up an empty pyrex dish (high setting for two minutes)? That worked perfectly and I've done it before with no ill effects.

This is a Panasonic combi microwave/convenction oven about six years old.

Many thanks for any thoughts.

[meow2222]

no, that ceased to be a problem in the 1960s

NT

[Andrew Gabriel]

Many microwave oven instructions warn of dire consequences if you use them with nothing in them. Some time ago, we pondered on what happens to the magnetron when you do this. My guess is that with nothing absorbing the microwaves, they end up overheating the magnetron, and either distort it until it stops resonating, or burn out the cathode (which would be easy to continuity test, modulo all the dire warnings about opening microwave cases, and the cathode is the very terminal which will retain a very high (-ve) voltage on it, particularly if the magnetron was no longer working). Some of them have a high voltage fuse, which would be worth checking. The current rating of these is usually a strange value, and the value is critical for safe operation, as there's very little overcurrent available on the HV side to be able to blow it.

I know someone who switched theirs on just to use the timer for something else. It lasted about a week (don't know how many times they did this during that period).

[Roger Mills]

More likely that there's something wrong with the interlock micro-switches on the door - which prevent you from microwaving any part of yourself by not letting it operate unless the door is firmly shut. If these fail - or go out of adjustment - the thing won't operate, and may even blow a fuse.

Does anything still work? Grill? Turntable? Light?

[newshound]

But if you try to start with the door open, normally nothing happens, no light, no turntable. Could be that there are two switches of course, a second one which just takes out the magnetron.

[Graham.]

Model number always helps but from your description of how it cuts out after 20 seconds I can be pretty sure it's got an inverter and associated protection circuitry.

With these the fault is usually the magnetron or the inverter PCB, also the failure mode of the magnetron is often one of the ring magnets gets cracked with the heat.

If it's not the magnetron it will probably be the inverter, probably uneconomic to repair. When I was repairing them I was prepared to repair to component level but could not find the IGBT's (transistors) at a reasonable price.

[Graham.]

Can you explain that please? I know Pyrex is not the same material as it used to be, but I can tell you the Pyrex turntable in my microwave is insufficient as a dummy load.

[Bert Coules]

That's an interesting notion: I'll check.

All of those. Fearing that something really fundamental might be amiss I didn't check either the convention oven or the grill, but perhaps I should.

Thanks for the thoughts.

[Bert Coules]

Ah yes, sorry. NN-A554W

It has. Well, the inverter certainly, since it proclaims the fact on the front, and I assume the protection is there too.

Pity, if that is the trouble. The oven's far from new but until this evening it was performing splendidly.

Many thanks, Graham.

[meow2222]

Only early nukes were vulnerable to no load conditions.

NT

[meow2222]

Microwave interlock systems use 4 switches, resistor and fuse.

NT

[Brian Gaff]

Most Microwaves these days protect themselves from excessive feedback of the signal. IE if I put too little into mine it will cut out when its had enough so to speak, but I suppose it could have blown a fuse and monitoring its output sees nothing after a while so just stops. Its very hard to say. Life was far more predictable in the days of mechanical timers.

Brian

[F Murtz]

It could be a fuse, there are usually two fuses, one obvious, and a 5000 volt .75amp not so obvious,often hidden in a plastic tube near transformer. this could be the culprit if the thing lights up but not cooks. These fuses look like the normal glass fuse but longer.

[Graham.]

I can't see why. Early microwave ovens had a substantial length of wave-guide, and the radiation entered at the top of the cooking cavity. The wave guide doubled as an air duct for the fan so the magnetron was more efficiently cooled.

[Graham.]

There won't be a conventional iron cored transformer.

[Graham.]

If you are up to opening it I can show you how you can tell if it's the magnetron that is faulty. Basically you pull off two wires and see if it doesn't cut out after 20 seconds the inverter is OK.

You could then get a cheap generic magnetron or even salvage one out of a scrap unit, although strictly speaking you are supposed to use the correct one which is a lot more expensive.

[meow2222]

I don't know for sure what changed since then, but I'd presume its 2 things: a) enough thermal margin that the magnetron doesnt die when power is reflected back to it b) waveguide that doesnt arc when reflected power is also added

NT

[F Murtz]

Is on mine,dont know about his model

[F Murtz]

OOPS it seems his is an inverter type.

[Graham.]

They also don't have the big oil-filled capacitor, so they are probably safer to work on when unplugged.