Microwave schematic

Most microwaves have a schematic on the inside of the metal outside cover.

H. R. (Bob) Hofmann

What's wrong with it? Microwaves are pretty much all about the same inside.

Unless it's one of the inverter designs, in which case may be no schematic available. Troubleshoot like a switchmode power supply.

Or, of course, the control board.

So, what is wrong with it? :)

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I've looked at just about every conceivable location inside and outside the cabinet but have not located a schematic.

It's my daughter's microwave. Complaint was that it keeps shutting down shortly after starting to heat something. Problem has been around awhile but kept getting progressively worse until now it's just about useless to cook something.

Brought it home with me. Tried it out. Worked perfectly quite a number of times. Took it apart just to look for something that could be causing an intermittent. Found nothing. Put it back together. Oops. Problem back with a vengeance.

Tore it apart again. Checked lots of connections and TCOs with my ohmmeter. Examined circuit board with a high intensity light and a magnifying glass. Nothing turned up.

Unplugged the HV transformer primary and put it through its paces. Everything worked flawlessly(except the heat of course).

Connected the HV transformer again. Voila. It's working normally again! My concern is that it will go back into the failure mode sooner or later, because no smoking gun has shown up.

When it was acting up it made me suspect that the board was losing power because when it shut off the digital display required me to enter the time and date just like it does after it's unplugged.

An aside to Mr. Goldwasser:

Your excellent article on microwave repair has been an enormous help to me. Thanks for sharing your extensive knowledge with the rest of us.

How is the source voltage?

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Underneath I assume? It does sound like an intermitten connection but an ohmmeter probably won't find it. Only a really close examination.

While highly unlikely, check or replace the fuse as well.

Thanks. :)

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Mr. Plow,

The source for my experimentati>

The source where the oven had problems is what I was after. And with the oven running at full power. I've been working with these things for just about as long as they were affordable to the general populace and I've seen plenty of problems that were caused by a fault in the source voltage. Mind you that's not my diagnosis but I just don't put the cart before the horse in things that consume considerable current when operating.

One thing I've found very interesting about this problem. During my Googling I came across more than a few posts on various sites mentioning that their same model microwave was acting the same way as mine but would work flawlessly when the screw is removed from the top of the control panel and it is kind of cantilevered outward being supported by the two plastic tangs at the bottom. All the wiring is still connected, but the panel is tilted out at about a 45 degree angle from the case if you can picture this. My microwave behaves exactly the same way. It does not fail to work in this position.

Some have posited that a bad magnetron causes this behavior.

That brings a few questions to mind.

If it is a bad magnetron, what is bad about it? Does it leak microwaves onto the board which then disrupt the operation of the solid state components, but only when the board is in its normal position?

Is the "bad" magnetron causing a disturbance in the power supply which gets conducted back into the board and disrupts the functions?

Are the wires acting as some kind of microwave antenna when the board is in the normal position?

If there is stray microwave energy bombarding the board is there a simple way to shield it?

What explanations do you have for the observed behavior?

Noticed something peculiar while wrestling with the microwave this evening. My assumption is that the digital display is a vacuum fluorescent type.

With the board dismounted for close visual inspection I can see two little metallic fingers connected to a one of the display's "pads" on the right side. The fingers seem to be touching the inner surface of the glass. There appears to be an identical arrangement on the left side of the display; however, there is a circular blackened area on the inside of the window where the left side fingers make contact.

Would this indicate a problem with the display, perhaps an internal flashover, that could be causing the microwave to act as it does?

Every VFD display I've seen has a dark area like that. I think it's where a 'getter' was used to remove any remaining free oxygen inside the sealed glass envelope. You see them a lot on other vacuum tubes, too. Basically a metal that oxidizes easily is placed inside the tube before it's sealed, then once everything is sealed up it's heated somehow until it vaporizes. It combines with any remaining oxygen inside the tube and leaves a deposit on the inside of the glass.