Recently, while in an adjacent room, I've been hearing for the kitchen
an occasional "snapping" noise -- like that from heat
expansion/contraction. At first I assumed it was from my electric
hotpot, which I plug and unplug as needed rather than running
continuously. But sometimes I've heard the sound twice within the same
hour after unplugging -- unlikely within the same cool-down period, I
think.
Last night, while actually in the kitchen, I heard it again, and think
it might have come from the microwave.
The microwave functions normally, except that it seems to have lost
power within the past year; things take longer to heat at the same
power setting.
I now realize that this "snapping" may have gone on longer than
originally thought: the floorplan of my "new" (5 months) apartment is
much more conducive to _hearing_ kitchen noises.
I've heard that microwaves deteriorate over time (this one is at least
5 years old, a Hitachi MRO-M71S bought used here in Tokyo). Does this
"snapping" mean that I am in immediate danger now? Enough to urgently
and summarily get rid of it?
I'd like as much information as possible before resorting to this.
Getting product information in English in Tokyo on domestic products
is chancy at best; there just aren't enough foreign customers to
warrant the effort. As for replacement, new-product prices are
wincingly high (though of course I could buy another used one from a
departing foreigner). And the City trash pickup procedure is
troublesome.

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Request for Question Clarification by ukiguy-ga on 23 Jul 2002 21:45 PDT

When your microwave is running, do you detect a "burning" scent? When
you hear this "snapping" how long does it last?

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Clarification of Question by evergreen-ga on 24 Jul 2002 07:24 PDT

ukiguy, there hasn't been any untoward smell such as scorching, at any
time. I've heard the >SNAP< only when the microwave is _OFF_. It's
brief, like a single finger snap, almost like someone tsk-ing but with
no sizzle. Or maybe >CRACK< without the >LE< .
Following weisstho's suggestion I checked the interior; found no
defects -- though they may be masked by airborne grease! Will check
again after cleaning.
Like journalist, I suspect the snap is a sign of an element failing. 
My situation seems to be like crabcakes's (sound loud enough to be
heard in adjacent living room) and may well have been going on for
more than 5 months; may have been missed in previous dwelling (3 yrs),
where kitchen was less centrally located. But snap occurrence does not
seem to correlate to recent prior device use or heat. Weather in Tokyo
is currently fairly humid.

I have repaired dozens of microwave ovens and can tell you from direct
experience how they work and what to be (or not to be) concerned
about. If the cabinet is intact and the door closes properly there is
absolutely no personal danger. Aside from the brain (timer,
temperature controls, etc.) there are four major components in the
cooking mechanism. They are:
1) step-up transformer
2) diode
3) storage capacitor
4) magnetron
The step up transformer supplies about 3000 volts ac which is
rectified to dc by the diode and stored in the capacitor. The
magnetron converts the high voltage dc to radio frequency energy in
the 3000 Mhz range which induces many small currents in the moist, and
conductive, food which causes internal heating by resistance.

Failure modes usually involve #2 or #4.
If the diode fails there is no heat at all and somtimes a loud hum or
buzz.
If the magnetron fails, the heat is reduced, or absent, and again can
produce a loud hum.

Snapping results from the microwave energy exciting some conductive
particles that can be due to residues in the cooking cavity, or
breakdowns within the magnetron itself. If the cooking cavity is clean
and there are no loose metal objects within it, you problem is most
likely to be a failed magnetron.

These can be purchased new for about $50 but there are tons of usable
parts from discarded appliances. A local technician should be able to
replace this part with a used one for less than $50 total. They come
in about 3 or 4 variations (physical mounting screws, etc.) but can
often be altered to fit safely and effectively.

If you want me to coach a technician who is knowlegeable about
electricity, I am more than willing to advise in follow-up
clarifications.

Chris

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Request for Answer Clarification by evergreen-ga on 27 Jul 2002 05:48 PDT

Chris, your information seems very to-the-point. From what I've been
reading, if a part is failing here, it's probably the magnetron. The
heat _is_ reduced. However, there is no loud hum; only the normal
soft, seemingly continuous hum during the timed cooking period.)
The door fits snugly and no holes or cracks are evident. As for
snapping, there are no loose metal parts. But I must admit it needs
cleaning inside; food residue _is_ present. (I'll clean it today and
see if the snap stops!) But I've heard the snapping sound only when
the unit was OFF. If it has snapped while ON, I never noticed.

CLARIFICATION QUESTION: Are maverick microwaves bouncing around inside
while the oven is OFF??? (and therefore causing the snapping noise
while OFF?) In winter I sometimes use the microwave for short-term
storage; is my bread getting irradiated while stored in there when the
microwave is OFF?

I really appreciate your offer to coach a technician, but I'm in
Tokyo. This economy is geared more for constant new consumption and
junking of inoperative things. Repair sources are fewer and more
expensive. And for us there's always a language problem. All in all
the easiest, cheapest fix is usually to buy another second-hand one
from a home-returning foreigner. (Traditional Japanese don't like to
buy second-hand, so the supply is much smaller.)

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Request for Answer Clarification by evergreen-ga on 29 Jul 2002 07:19 PDT

Editors -- I'm waiting for an answer to my Request for Answer
Clarification of July 27 before I rate the answer and close it.

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Clarification of Answer by chris2002micrometer-ga on 29 Jul 2002 10:08 PDT

As the magnetron deteriorates, the microwaves that produced the heat
will diminish but not necessarily produce a loud hum. The load hum
results when something shorts out completely in any of the four parts,
but usually the diode or the mag. In this case no heating takes place.

When the oven is off, there is no microwave energy present, just as
there is no light from a lamp that is turned off. The microwaves
instantly cease. Nothing reverberates around after power is removed.

Questions: 
1)Does the snapping continue if you unplug the unit altogether? 
2)Can you move the oven out of the room and still hear snapping
noises.
  
A bad light fixture or some other thing may be to blame for the
sounds.
I usually test a microwave by warming an 8oz glass of water. If it
boils in two minutes or less, it is working normally.

Not to suggest poor house keeping, but I live in Florida where it is
common for cockroaches to get into anything that has food morsels.
They, or some other creature, can make audible noises. Storing bread
in the cavity will not irradiate it but it may attract pests as well
as other food morsels.

A Google search for "test microwave oven turned up:
http://www.gallawa.com/microtech/ 
http://www.gallawa.com/microtech/mag_test.html
http://www.repairfaq.org/REPAIR/F_micfaq.html

The second link has a picture of a magnetron which would be
immediately recognized if you removed the cabinet. The only precaution
needed when replacing it is to discharge the capacitor which may
harbor a residual charge when power is removed. It cannot possibly
create additional microwave energy with the power absent.

I see absolutely no personal risk in continuing to use your oven. If
you clean the cavity and can heat a glass of water quickly, it will
continue to work until it fails completely.

If I can help more, please request more clarification.
Chris

Many thanks to you, Chris, for an excellent, focused, comprehensive,
well-organized, well-informed, and informative answer and offer of
help. The mystery snap is now gone, and I learned useful stuff about
the care and feeding of microwave ovens!

Microwave Oven Safety Precautions
http://www.gallawa.com/microtech/safety.html

http://www.physiology.ucsf.edu/eshop/html/ws_tech.htm

“Listen for signs of arcing or corona - snapping or sizzling sounds. A
component on the brink of failing due to overheating may provide
similar audible clues”

I will leave the scientific answer to someone else and pass on an
observation to you.  I noticed in an old microwave that we had that
there was a "screen" that was in the door glass, and there was a
scratch or some other minor defect that would generate the spark or
snap, similar to the noise you describe.

Perhaps a careful inspection of the surfaces within the microwave will
reveal some small defect (such as a scratch, nick, or loose screw)
that could generate the noise.

good luck!  weisstho-ga

I can offfer only this: my coffeemaker made a peculiar snapping noise
right before the heating element failed.  I would guess that's the
sanp in the microwave as well.  I was in no danger from the
coffeemaker snap: in retrospect, I believe it was just the element
dying.

For what it's worth, MY microwave snaps too, when NOT in use. We hear
it into the living room (It's close to the kitchen). We determined, in
a non-scientific manner, that it was the plastic housing changing
temperature, and nothing like arcing or anything electrical.(Similar
to our roof beams creaking at night...we live in Arizona and wonder if
the lack of humidity has something to do with all these snaps/creaks)
We have a 2-3 year old Panasonic Inverter, and it's been
snapping/popping for most of the time we have had it, to no harm. Does
yours snap when NOT in use, or when using it?

Based on what I read here, it sound like a overheating problem. The
magnatron (the part the converts the electrical energy into high
frequency electromagnetic waves-microwaves) gets hot while in use.
There should be a fan inside the unit to cool it. If the fan has died,
this would definatly cause the magnatron to overheat (which will cause
it to die). I would suggest putting the microwave on the counter and
cooking something in it. then you could feel around the inside and
outside of the unit for hot spots. You should also be able to here the
fan running. If the fan is dead, you might be able to take it to a
small appliance shop or your local mechanical guru, one of them may be
able to repair the fan.

Here I will go out on a limb as I am not an electrical engenear.
Microwaves have a large capacator in them which could definatly hold
enough of a charge to cause a rather large spark even if the unit is
unpluged. It could be arcing accros a short in the transformer or
magnatron. Either of these situationd mean your microwave will die
soon.

Here is some gereral microwave info:
http://www.howstuffworks.com/microwave.htm

anotherbrian, thanks for the link. howstuffworks is now on my
Favorites list. There didn't happen to be a direct answer to my
question, but in reading the various bits I think it's possible that
the magnetron is failing. I will try checking for hotspots, etc. as
you recommend.

I will be out of town Aug 3 - 11 and unable to post responses during that time.
Chris

FINAL REPORT Well, first I unplugged the oven for most of a day. No
snap. Then I cleaned the oven's interior (it had some film but wasn't
horrible or gorpy). Haven't heard a snap out of it. Our bestbets had
been the presence of food residue or a deteriorating magnetron. But
since the snap (and no other untoward sound) occurred only when the
oven was Off, the actual cause is still up for grabs. Anyway, it
hasn't snapped since, and I learned a lot about microwave ovens! Thank
you, Chris, for your excellent help.