Microwave oven arcing

[MikeC]

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

--
Mental decryption required to bamboozle spam robots:

mike_best$ntlworld*com
$ = @
* = dot

 

[Jim Land]

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.

This is covered in Sam's FAQ:

http://www.repairfaq.org/sam/micfaq.htm#micarcaoc

 

[Dave]

Folks,

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.

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.

 

[tlbs]

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

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.

 

[ian field]

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.

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.

 

[MikeC]

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

 

[ian field]

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

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.

 

[MikeC]

Folks,

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.

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

 

[Franc Zabkar]

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.

Here in Australia we can buy sheets of the stuff:
http://www.wagner.net.au/Catalogue/10_10.pdf

- Franc Zabkar

 

[[email protected]]

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