chuck@csn.org (Chuck Luciano) (03/20/91)
I am posting this because other people who like myself have had experience
troubleshooting electronic equipment and are competant to use this
information wisely. However I in no way encourage anybody to actually try
anything I am suggesting here and in fact I STRONGLY RECOMMEND you take
your microwave to somebody who gets paid to take the risks involved in
servicing such a piece of equipment.
This is not a 9 volt transistor radio, there are DEADLY voltages, and REAL
RISKS of exposure to HARMFUL RADIATION involved in working on one of these
beasts. You could think everything went fine and a year from now wonder why
you have cancer. There is a capacitor in the microwave that stores enough
energy to KILL YOU even if the unit is UNPLUGGED.
THIS IS NO JOKE. DON'T DO IT. YOU ARE ON YOUR OWN IF YOU TRY. I'M NOT KIDDING.
IF YOU CAN'T TAKE THIS SERIOUSLY STOP READING NOW.
Thanks to everybody who replied and provided valuable insight.
Also netnews has frequently proven an amazing source of information so
thanks for the bandwidth.
I managed to find one microwave service guy who (I believe) gave the
straight stuff on troubleshooting the microwave.
All of these tests are done with the unit unplugged, and the powersupply
capacitor discharged, except the voltage measurement test. Beware of
capacitors, they have "memory", you can discharge them and over time they
can build up a charge just because of the forces involved in the way they
store energy. I don't claim to understand it, just to have been zapped by it.
There is a power transformer, capacitor, diode, and magnetron in the
cooking circuit. The transformer almost never fails (mine seems fine).
The diode is a many (~12) junction diode, so a regular diode checker
will probably not give you any indication. I built a diode checker using
3 9Volt batteries, resistor, and voltmeter (V) :
9V 9V 9V 100 ohm
+------|+|+|+---|+|+|+---|+|+|+---\/\/\/\/-------+-------o o
| | |
| V D
| | |
+------------------------------------------------+-------o o
I used the voltmeter to measure the voltage drop across Diode in the
reversed biased direction and measured no drop in voltage, in the forward
biased direction I measured a drop across the diode of 9 Volts implying
9/.7 juctions operating properly.
I used an ohm meter to test the cap. Set ohm meter to 10 kohm scale. Short
the capacitor to bring voltage to zero. Connect ohm meter to cap and you'll
see a resistance which will start low on the scale and will over several
seconds will go to infinity, reverse the leads and the same indication
(low going to high) will repeat itself.
My cap tested fine.
The last test :
ON HIGH VOLTAGE PROBES : These DO NOT come with meters, so don't think
that the probes you got with your meter qualify. You meter and possibly your
life depend on some judgement being used here. A High Voltage Probe is
about a 15 inches long and have some serious insulation properties, and
cost about 40 bucks. Mine has in it a 1 billion (1000 Meg) resistor that
is about a foot long that used with a 10 meg impedance meter will produce
a 100:1 voltage divider. So all measurements at the meter must be multiplied
by 100 to get the actual voltage.
The next test is done with the unit plugged in, and a cup of water inside
to act as a load. (In my ovens present condition, I could have used a cat :-)
To make the measurements I connected the probe leads to the terminals and
then pressed start let it run a few seconds, and then stopped it.
Using a HIGH VOLTAGE PROBE I measured the AC voltage, and the DC voltage
at the Magnetron while the unit thought it was cooking. I measured a few
hundred volts DC, and a few thousand volts A.C. I suspect this is okay. I am
going to replace the magnetron.
The service guy I spoke with said he could replace it with a new magnetron
for $80-90, or he might have a used one for about half that.
I hope to be nuking by the weekend.
Here is a summary of the original posting and the replys.
I (chuck@csn.org) posted the following article (Some stuff deleted)
> From chuck Mon Mar 18 13:52:40 MST 1991
> Newsgroups: misc.misc,sci.electronics,rec.ham-radio
> Subject: Anybody out there ever fixed a microwave oven
>
>
> Through pilot error I managed to run a microwave oven at full power for
> 70 minutes with nothing in it. It having nothing else to cook, cooked
> itself.
>
> There is apparently a magnetron and a power supply, the magnetron has two
> wires attached to it, it is a sealed unit. I have to assume that the
> oscillator is inside the sealed unit.
>
> Does anybody know if the magnetron is the most likely point of failure,
> or is the supply? Can I use a volt meter to measure the voltage from the
> power supply? Should I disconnect the supply from the magnetron before
> measuring the supply? If I disconnect the supply, should I provide a dummy
> load? Any other suggestions?
>
> Does sams provide photofact folders on microwaves? Does the manufacturer
> put service manuals in the hands of mere mortals.
>
> I have a hard time believing that some sensible precautions, wouldn't
> reduce this to a fairly safe activity, but maybe you know otherwise.
>
> Chuck Luciano
> 303-421-9113
> chuck@csn.org
>
I recieved the following (edited) replys which were very helpful :
>
> From brian@UCSD.EDU Mon Mar 18 10:02:56 1991
>
> 1. A magnetron IS an oscillator - a very high power one. It works by
> circulating electrons in a magnetic field.
>
> 2. It may have arced and fried, but it could also have fried the power
> supply rectifier.
>
> 3. To test the power supply, you need a very high voltage probe for your
> meter - magnetrons commonly run with anode voltages in the 12 to 16
> KILOVOLT range.
>
> 4. Magnetrons aren't cheap.
>
> Good luck!
> - Brian
>
These folks provided insight in several areas :
> From: jpd@pc.usl.edu (Dugal James P.)
> From: Maralee Morado <morado@milton.u.washington.edu>
> From: lpdjb@brahms.amd.com (Jerry Bemis)
> From: Stephen Jacobs <saj@chinet.chi.il.us>
>
1) Any testing should be done with a cup of water in the oven as a load.
2) That there is a fuse that could have blown (It didn't in my case).
3) That repair shops love to charge a fortune to replace a $.30 fuse.
4) That running without a load probably fried the magnetron.
5) That a new magnetron would cost almost as much as the oven. (In my
case the oven is a Sharp convection/microwave with turntable.
This oven cost $325, which is considerably more than a magnetron.
However if you fry a $110 dollar microwave it would seem a good
move to buy a new one)
Included here are several messages that proved interesting and are
included in their entirety.
> From: bob@cobalt.cco.caltech.edu (Robert S. Logan)
>
> There should be a schematic inside the oven someplace. They are really
> pretty simple minded: a bunch of interlock switches, the computer and
> keyboard that do the work, some relays, a HV supply, and the magnatron.
> If the computer still works & all the fuses and switches work, I would
> suspect the magnatron. You should be able to disconnect some wires
> and be able to check the transformer and HV cap with an meter. GROUND
> everything first (is case the bleeder is bad). You should be able to
> buy a magnatron, but I have no idea about prices.
>
> I have a Toshiba that stops working every once in a while. I take it
> apart, check the interlocks which are ok, put it back together & it
> works fine.
>
>
> --
> Robert S. Logan
> Campus Computing Organization, 158-79 Caltech, Pasadena, CA, 91125
> 818-356-4631
> rslogan@caltech.bitnet
> bob@iago.caltech.edu
> bob@tybalt.caltech.edu
>
>
> From: lpdjb@brahms.amd.com (Jerry Bemis)
>
> It has never been a good idea to run a microwave empty. In the last 15 years
> the microwaves haven't come with a notice to let you know this.
> Running the microwave empty allows the rf to bounce back into the magnitron
> and kill it. 'Bev Compfer' inventer of the magnatron
>
> After 70 minutes the power supply might have burnt up.
>
> I haven't seen a magnitron that can run with less then 1200 volts.
>
>
I also managed to contact a microwave service
--
Wow a .sig, I never had a .sig before.bwhite@oucsace.cs.OHIOU.EDU (Bill White) (03/25/91)
In article <1991Mar19.170413.24498@csn.org> chuck@csn.org (Chuck Luciano) writes: >This is not a 9 volt transistor radio, there are DEADLY voltages, and REAL >RISKS of exposure to HARMFUL RADIATION involved in working on one of these >beasts. You could think everything went fine and a year from now wonder why >you have cancer. Doubtful. Check out most of the experiments run on microwave radiation. A friend did a run with X-linked lethal mutations on fruit flies and got zero results over several generations with various exposures; I checked up and saw most of the evidence *for* a cancer link was shaky at best and poor experimentation at worst. ELF fields are another story. In any case, as long as you get the case back on you shouldn't have a problem; if you're paranoid, use a microwave sniffer. As for everything else, just give it the same respect you'd give a TV set and you'll be fine (not that I'm downgrading the risks, keep in mind you still have a fairly beefy cap or two there holding a good killing charge. But if you discharge it -- CORRECTLY I might add -- you'll be fine). -- | Bill White Internet: bwhite@oucsace.cs.ohiou.edu | | VIQUE'S LAW: | | A man without religion is like a fish | | without a bicycle. |