Microwave buzzing a vibrating

[acctforjunk]

Samsung MW8490W, about 6.5 years old. Tried microwaving something today, and the microwave just vibrates and buzzes. It's definately a

60Hz-based buzz. No arcing or odd smells. Everything appears OK except the metal enclosure vibrates violently like there's a loose transformer in there. Oh, it won't heat water, and is drawing only about 350W from the outlet when running (rear label says 1.5KW).

I can tell you everything you want to know about electronics (well, almost), but microwaves are somewhat of a mystery to me.

Any ideas on what's wrong, or where to start?

[Sam Goldwasser]

Shorted high voltage diode or magnetron.

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[acctforjunk]

Any quick checks that can be made? Should the two terminals to the mag have some low resistance? (I'm not sure what they connect to)

I'm also going to read up some more on how these things work.

[Percival P. Cassidy]

Stay out of that thing. And if you could buy a new magnetron, it would probably cost more than a new microwave.

Perce

[acctforjunk]

Stay out, why? I know there's a HV capacitor in there, and I know electronics plenty good enough to know what not to do.

And yes, you're right. Magnetr> Stay out of that thing. And if you could buy a new magnetron, it would

[Sam Goldwasser]

First you MUST read up on the SAFETY issues. Even though this one appears quite dead, you don't want to take chances.

Complete troubleshooting quide at the Web site, below.

With the unit unplugged, make sure the HV capacitor is discharged.

The two terminals on the magnetron are the filament and should be low resistance, close to zero ohms. But they ahould read open to the chassis.

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[mm]

The most important thing is to either not mess with the cage around the microwave part, or to put it back EXACTLY the way you find it. At first the Amana girl refused to send me a schematic (back when I didn't realize how simple they are) because she thought I'd radiate myself or my eyes to death. Apparently the metal side of the oven, where the electronics are, is not enough to keep the waves inside.

You might want to get a microwave tester (10 dollars at Radio Shack) to make sure there is no leakage. I calibrated mine by pulling the door open a little but less than it took to have the oven turn off. now that the ovens latch instead of just close, this might be harder to do. The RS tester was calibrated fine to begin with afaict.

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[mm]

Don't know about that part but for Amana model #2, they wanted 700 dollars iirc for the transformer. After I whined (and pointed out that no one would pay that price, they lowered it to the wholesale price of 400 dollars. I don't think anyone will ever pay that price either for an oven that had no special features, one power, and two timers. And weighed so much it was hard to carry. All that was good was that it was big and antique.

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[acctforjunk]

There's no cage around the microwave parts. It's in the open once the outside shell has been removed. Nothing that I'm messing with here looks real critical....just high voltage stuff. I'm not remove the mag or anything.

[Sam Goldwasser]

Right. Just try not to electrocute yourself in the process. If you don't remove the magnetron, there is no critical seal you'll be affecting.

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[mm]

Interesting. I tguess this is part of how they lowered the weight of these things, like they lowered the weight of tv's in part by removing the cage around the high voltage parts. Of course if you don't touch the tv hv, there is no problem at all, but microwaves go zooming around the room. Maybe it was never the danger they thought it was, or they figured out how not to generate excess microwaves without and thus didn't need a cage.

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[Sam Goldwasser]

I've never seen a cage around the magnetron in a microwave oven. It's all metal that's exposed outside the waveguide except for the filament feedthroughs. As long as holes are a small fraction of the wavelength, very little microwave energy will get through. And, the metal cover over the entire oven adds another layer of protection.

TVs in the old days had high voltage rectifier tubes and flybacks with exposed terminals. Modern CRT-based equipment has the flyback entirely potted so there is no exposed HV.

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[PipeDown]

Usually when the magnatron goes out, the unit appears to run normally but the food don't get hot. If there is a short such that the HV circuits are making a lot of noise, then the unit is probably trash (for ex. the HV transformer is shot or there are multiple component failures). At the very least, you need basic troubleshooting knowledge of the circuit. Unless you are working on an expensive built in or over the range model, I would just replace it. As you probably know, small ovens cost way less than $100.

The OP described a loud buzzing which could be a stuck motor. There are several in an oven. Notably the exhaust fan and the one that turns the blades that "mix" the microwaves around the oven cavity. A frozen motor will,often make a lot of noise. These moving parts have lower reliability than the HV circuits

You may be able to operate the oven with the magnatron disconnected. In this manner, you can confirm the other functions (timer relay, fans, interlocks, no corona around HV circuits etc). Without an HV probe, you can often tell if HV is present by holding a fluorescent tube near (but not touching) the circuit while you ground the other end with your hand, it will light at the end near the HV. You will not know if it is the correct voltage.

[PipeDown]

Hope that's all it is. In my experiance, HV failures tend to cascade to several components.

[Sam Goldwasser]

There aren't that many HV components: HV diode, HV cap, magnetron.

So, conceivably, a bad magnetron took out the diode (though the exact mechanism isn't clear). The cap is still good or else the main fuse would have blown. If it was the HV transformer, that wouldn't damage the HV diode. So, you're left with the HV diode.

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[mm]

I have.

Who says a cage can't have holes? AFAIK, almost all cages have holes.

Above, I'm referring to a separate removable cage, with a woven metal gasket where the cage attaches to a piece of sheet-metal.

True.

I'm pretty sure I've seen flybacks with no normallly exposed HV that still had a cage, but I don't have anything that old anymore to check.

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[mm]

I had one where all that was wrong was the wiring to the diodes. Some of the insulation was crumbling off. And I think the HV was sparking to an internal metal part. A heavy glob of GE silicone cement around the wire, and everything was fine for years.

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[Steve B]

Buzzing is the last thing a microwave does before detonates.

HTH

Steve

[mm]

Don't they have a red light timer inside that counts down to zero? All the bombs on tv have that.

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[Sam Goldwasser]

And unmarked red and blue wires..... :)

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