Microwave oven arcing

Folks,

I'm not sure if this isn't a bit banale for this group, so please direct me elsewhere if I'm in the wrong place.

My microwave oven has lasted quite a long time (it must be heading for its

10th birthday). In the top of it, there is what appears to be a mica window through which the UHF arrives. Intermittently (but getting more frequently) it produces, on the magnetron side of this window, what looks, smells and sounds like arcing - like a welding set. The light comes through the mica window and lights up the inside of the oven quite brightly. It arcs for two or three seconds then stops, and may start again after 10 seconds. The problem has been there, then disappeared for three weeks, but it's back at the moment.

I haven't taken it to bits yet, but can anybody tell me if they have had the same experience, and whether it is even worth dismantling. If it's the magnetron, how much do they cost? Ovens aren't very expensive, so would it be worth repairing? It's quite a good one (Matsui 170TC, if you know it), so it is worth more to me than a bottom-of-the-range replacement.

Incidentally, I started my carreer as a microwave engineer (S-band), though I haven't been near that side of the business for a long time (in computer storage now), but I'm aware of the consequences of running it with the covers off, so please don't let that danger steer you to recommending a replacement.

Many thanks,

MikeC

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Reply to
MikeC
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"MikeC" wrote in news:HqBgh.10260$ snipped-for-privacy@newsfe5-win.ntli.net:

This is covered in Sam's FAQ:

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Reply to
Jim Land

Intermittently (but getting more frequently)

The inside of the plastic cover gets dirty over time. When there's enough gunk built up, arcing can and does occur. Remove, clean, replace.

Reply to
Dave

I've seen this before, a couple of times on my Panasonic over-the-stove model microwave oven, only not on the mica window. The arcing I've seen always happens at the HV anode of the magnetron, and it it usually fatal. Since the microwave cost more than 2 times the replacement cost (even in today's dollars), I have the magnetron replaced (total cost for part + labor around US$75) versus a new 1200Watt over-the-stove model starting at US$199. The problem is the buildup of grease and cooking aromatics in-and-around the magnetron. It usually arcs when the humidity gets high (that's the variable factor you might be looking for). If it happens to me again, I'll ask my repairman to put alot of HV silicone around the anode terminal.

If your microwave is a cheap ($50 - $70) microwave, it's a throw-away. If it is a $150+ high power fancy model, consider replacing the magnetron.

good luck Tom P.

Reply to
tlbs

The OP said its mica, these tend to de-laminate so they're impossible to clean adequately to prevent future occurrence of arcing.

Often the only solution is to remove the mica and throw it away - then hope the oven lasts long enough without any food splattering into the waveguide to save up for a new oven.

Reply to
ian field

Folks,

Thanks to all who responded. I read Sam's FAQ (and got pretty interested in his work on lasers, and will probably order Dr. Shawn's science CD), and took the "take the cover off and clean it" advice. In fact, I found (going by the burn marks) that it had been arcing from the top of the waveguide to the bottom. There is a U-section welded to the top of the oven to form the box-section waveguide, which is angled down into the oven at about 45 degrees at its end. The end of this 45 degree section meets the far end of the aperture where the UHF goes into the oven (the aperture that is covered by the mica plate), and above the leading edge of the aperture, there is a round bump on the inside top of the waveguide about 1/2" high and with a diameter of an inch or so. It has been pressed in from the outside of the waveguide, and presumably, it disperses the microwaves to fill the inside of the oven. The highest point of the bump is directly above the leading edge of the aperture, and it appears that the arcing was between those two points. The mica cover was half-way burned through at the point where the arc was hitting the edge of the aperture hole.

I've cleaned it up - though there didn't seem to be much to clean. I was expecting that the arcing would be through a film of muck - but it appears to have been across half an inch of open space.

Regards to all,

MikeC

Reply to
MikeC

If the waveguide window is some sort of plastic it shouldn't de-laminate so take it out and scrub it clean and inspect it for any carbonised points - these can be scraped off with a sharp blade.

Reply to
ian field

I've just measured the distance, and in fact, it's a little over an inch. Would you expect to see an arc over this distance? The top of the bump had the paint burned off, as had the edge of the aperture hole, so it suggests an arc between those points. As I said originally, it made a light like a welding set, though, of course, the mica window was in place, so I didn't see the arc itself.

Incidentally, it isn't mica. It's some sort of manufactured material, but it's grey like Mica.

Regards,

MikeC

Reply to
MikeC

On Sat, 16 Dec 2006 19:02:40 GMT, "ian field" put finger to keyboard and composed:

Here in Australia we can buy sheets of the stuff:

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- Franc Zabkar

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Reply to
Franc Zabkar

well it was initially, but arcing burnt it away.

You'll need to replace this bit of sheet, if you run it without or with a hole, muck will get into the waveguide and it'll screw up, only this time you may have great difficulty trying to clean it out.

Any sheet plastic that can handle microwaves and steam will do. Self extinguishing is a real plus.

NT

Reply to
meow2222

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