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Electronics Repair (sci.electronics.repair) Discussion of repairing electronic equipment. Topics include requests for assistance, where to obtain servicing information and parts, techniques for diagnosis and repair, and annecdotes about success, failures and problems.

Sharp microwave arcs *through* front door



 
 
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  #11  
Old December 16th 07, 02:31 AM posted to sci.electronics.repair
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Posts: 533
Default Sharp microwave arcs *through* front door

William Sommerwerck wrote:
"Eeyore" wrote in message
...

Chris wrote:


I've got a ~2 year old microwave that has shown no previous signs
of trouble and is to all appearances in good working order. During
normal operation (melting butter on low power)...


That isn't a suitable load for a microwave, is it? You need some water
in there. No surprise it arced.


I don't think that's correct. Butter contains water and fat, both of which
absorb microwaves. I've melted butter without problems.

Arcing usually occurs at a sharp metal edge. I once put a plastic jar of
Adams peanut butter in my microwave to soften it up, and got serious arcing
on the teensy bits of aluminum foil that had been left behind when I peeled
off the inner seal.


I've had that happen recently. Same product (different brand)--some
surprisingly SERIOUS fireworks immediately as I hit the button. If the
lights had been off, it would have lit the entire kitchen from really
minuscule bits of foil.

I'm with the other poster, in that I'm not sure I'd ever trust that unit
again.

jak
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  #12  
Old December 16th 07, 02:35 AM posted to sci.electronics.repair
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Posts: 6
Default Sharp microwave arcs *through* front door



I would thoroughly clean everything, inside and out. Also check to see that
there are no bits of foil, or anything metallic, in the oven, on or around
the seals, etc.


Yeah, I'll double check. Also noticed a second burn mark closer to
the hinges, but same vertical position. Just to clarify my
description of the problem, I'm not talking about an arc like you'd
see from a little metal in the microwave. I'm talking about a beefy
pink arc about the diameter of a pencil.
  #13  
Old December 16th 07, 04:00 AM posted to sci.electronics.repair
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Posts: 2,770
Default Sharp microwave arcs *through* front door



James Sweet wrote:

"Eeyore" wrote
Chris wrote:

I've got a ~2 year old microwave that has shown no previous signs of
trouble and is to all appearances in good working order. During
normal operation (melting butter on low power)


That isn't a suitable load for a microwave is it ? You need some water in
there.

No surprise it arced.


Even an empty microwave should never arc to the outside,


I agree it shouldn't have arced to the *outside*.


I've never, ever seen that before, and I too have used microwaves to melt
butter hundreds of
times over the decades.


Ok. I have heard of other stories where people have done things with their
microwave ovens the makers don't recommend and been puzzled why it broke after
say the hundredth time though.

Graham


  #14  
Old December 16th 07, 04:02 AM posted to sci.electronics.repair
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Posts: 2,770
Default Sharp microwave arcs *through* front door



William Sommerwerck wrote:

"Eeyore" wrote
Chris wrote:


I've got a ~2 year old microwave that has shown no previous signs
of trouble and is to all appearances in good working order. During
normal operation (melting butter on low power)...


That isn't a suitable load for a microwave, is it? You need some water
in there. No surprise it arced.


I don't think that's correct. Butter contains water and fat, both of which
absorb microwaves. I've melted butter without problems.


Maybe the quantity wasn't enough ?

Graham

  #15  
Old December 16th 07, 04:33 AM posted to sci.electronics.repair
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Default Sharp microwave arcs *through* front door

Nah, it was a half a stick, well within the pre-programmed range, so
that's not it. In any case, some bit of foil of just the right size
or shape can lead to gradients high enough to get sparks, but this was
not like this. This was some large fraction of the entire current
output of the transformer, like the fireworks you see on youtube when
people have pulled the guts out of the microwave and using it just to
make an arc.
  #16  
Old December 16th 07, 12:16 PM posted to sci.electronics.repair
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Posts: 3,777
Default Sharp microwave arcs *through* front door

"Eeyore" wrote in message
...

William Sommerwerck wrote:
"Eeyore" wrote
Chris wrote:


I've got a ~2 year old microwave that has shown no previous signs
of trouble and is to all appearances in good working order. During
normal operation (melting butter on low power)...


That isn't a suitable load for a microwave, is it? You need some water
in there. No surprise it arced.


I don't think that's correct. Butter contains water and fat, both of

which
absorb microwaves. I've melted butter without problems.


Maybe the quantity wasn't enough?


I'm not sure. The "received knowledge" on such things is that if there's
nothing in the oven to absorb the microwaves, the magnetron will eventually
overheat (from the reflected unabsorbed energy). The lower the power
setting, the less likely this is to happen.

But overheating isn't arcing. Arcing requires a sharp metallic edge for the
electric field to build to a point where the air breaks down.


  #17  
Old December 16th 07, 12:19 PM posted to sci.electronics.repair
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Posts: 3,777
Default Sharp microwave arcs *through* front door

"Chris" wrote in message
...

Nah, it was half a stick, well within the pre-programmed range, so
that's not it. In any case, some bit of foil of just the right size
or shape can lead to gradients high enough to get sparks, but this
was not like this. This was some large fraction of the entire current
output of the transformer, like the fireworks you see on youtube when
people have pulled the guts out of the microwave and using it just to
make an arc.


If a thorough cleaning and inspection doesn't resolve the issue, I'd get a
new unit. The potential for damage or injury seems too great.


  #18  
Old December 16th 07, 01:56 PM posted to sci.electronics.repair
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Posts: 244
Default Sharp microwave arcs *through* front door

Chris writes:

Nah, it was a half a stick, well within the pre-programmed range, so
that's not it. In any case, some bit of foil of just the right size
or shape can lead to gradients high enough to get sparks, but this was
not like this. This was some large fraction of the entire current
output of the transformer, like the fireworks you see on youtube when
people have pulled the guts out of the microwave and using it just to
make an arc.


This whole thing doesn't sound right. Regardless of whether the microwave
oven is attached to Earth ground, the return path for the high voltage
IS the chassis of the microwave oven. It would be almost impossible for
that to be disrupted as the magnetron and HV transformer are bonded to
the chassis. So, it's highly unlikely that the HV current (not the
microwave energy) would want to jump from the oven to an external ground.
Looking at the typical schematic, it's hard to come up with any sort of failure
mode where such a potential difference would appear between the chassis
and ground.

I'm not saying something very peculiar didn't happen. Just that an
explanation relating to the HV power doesn't make sense. I'd quicker go
with some combination of load (butter), dirt, and other factors affecting
the microwave distribution.

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  #19  
Old December 16th 07, 07:41 PM posted to sci.electronics.repair
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Posts: 501
Default Sharp microwave arcs *through* front door

Sam Goldwasser wrote:
Chris writes:

Nah, it was a half a stick, well within the pre-programmed range, so
that's not it. In any case, some bit of foil of just the right size
or shape can lead to gradients high enough to get sparks, but this was
not like this. This was some large fraction of the entire current
output of the transformer, like the fireworks you see on youtube when
people have pulled the guts out of the microwave and using it just to
make an arc.


This whole thing doesn't sound right. Regardless of whether the microwave
oven is attached to Earth ground, the return path for the high voltage
IS the chassis of the microwave oven. It would be almost impossible for
that to be disrupted as the magnetron and HV transformer are bonded to
the chassis. So, it's highly unlikely that the HV current (not the
microwave energy) would want to jump from the oven to an external ground.
Looking at the typical schematic, it's hard to come up with any sort of failure
mode where such a potential difference would appear between the chassis
and ground.

I'm not saying something very peculiar didn't happen. Just that an
explanation relating to the HV power doesn't make sense. I'd quicker go
with some combination of load (butter), dirt, and other factors affecting
the microwave distribution.


It could be possible that some part of the door, possibly one of the
hinges has lost it`s bonding to chassis earth and a potential is
building up there then striking to the nearest earthed metal. it`s
unlikely tho in my experience.

There was a model of Phillips oven that could build up a charge on the
thin aluminium cover over the lamp leading to quite spectacular arcing
around the rivet which held it in place and acted as a swivel. A poor
connection there was indicated by quite strong leakage of microwave
radiation through the plastic menu strip on top of the oven.

Ron(UK)
  #20  
Old December 16th 07, 11:06 PM posted to sci.electronics.repair
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Posts: 30
Default Sharp microwave arcs *through* front door

On 15 Dec, 23:09, "William Sommerwerck"
wrote:
Are the door seals clean? Does the door close firmly and evenly?


Mains wont jump 2 inches !!!!!!

 




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