chuck@csn.org (Chuck Luciano) (03/19/91)
There is no newsgroup that specifically suits itself to this flavor of question, so if you feel this question is out if place, please hit the 'n' key so that only one of us wastes bandwidth in an inappropriate place. Now then, if you are still reading I have a question about microwave ovens. 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. I have serviced a lot of electronic equipment, amplifiers, tuners, tv's, etc. but never a microwave. 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? The microwave is a sharp convection/microwave it is about two to three years old, I could provide an exact model if needed (I don't know it off hand). 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. Any info on this is greatly appreciated, reply by mail and I'll post a summary if there is sufficient interest, and somebody can suggest the most appropriate newsgroup for this line of discussion. Chuck Luciano 303-421-9113 chuck@csn.org Disclainer: Don't try this at home kids, we're professionals here. -- Wow a .sig, I never had a .sig before.
FC138001@ysub.ysu.edu (Phil Munro) (03/19/91)
My recomendation is *DON'T MESS WITH YOUR MICROWAVE* unless you are interested in finding out what cataracts are like. Stray microwave energy is very dangerous to your eyes!!!! And from the questions asked in this post it looks like the writer does not understand this! If any reader doesn't know what cataracts are, do some research! It is blindness, or partial blindness, due to a change in the clear lens of the eye. In other words, microwave can cook your eys, just like the white of an egg turns from clear to opague!!
jgd@Dixie.Com (John G. DeArmond) (03/19/91)
chuck@csn.org (Chuck Luciano) writes: I've never actually "fixed" an oven but I HAVE dismantled a number for their RF components. Can you say QRO at 2 gig? I thought you could :--) >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. The magnetron is the oscillator. Normally a microwave is run with a grounded anode and the filament (the 2 wires) excited at a high negative voltage (2- 4 kvp typically). This is convenient so that you don't have to stand on a rubber mat while operating the oven :-), since the waveguide is typically attached to the anode. >Does anybody know if the magnetron is the most likely point of failure, Yep. >or is the supply? Generally not. The px is typically a half wave rectifier with a power factor capacitor in series with the anode to ground. Little to fail here. >Can I use a volt meter to measure the voltage from the >power supply? Yep. Won't mean much since the output is half-wave rectified but you can get an idea of whether juice is there or not. You'll need a high voltage probe because the voltages involved are as serious as death. >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? No and no. When the oven turns on, you should see a high voltage that decreases as the filament warms up in a second or 2. If you see this characteristic but with no heat or if you see the voltage come up and not decrease much at all, then the tube is shot. You can buy replacement tubes for under $70 at your local appliance repair parts shop. Take the tube with you for comparison. Note that the tube has some fairly powerful magnets so take care around magnetics. Last caution, the capacitor is typically a microfarad at 4000 volts. It holds a charge for a LONG time. This is the voice of experience speaking :-) It will make you hurt yourself. John -- John De Armond, WD4OQC | "Purveyors of speed to the Trade" (tm) Rapid Deployment System, Inc. | Home of the Nidgets (tm) Marietta, Ga | {emory,uunet}!rsiatl!jgd |"Politically InCorrect.. And damn proud of it
steveq@syd.dms.CSIRO.AU (Stephen Quigg) (03/19/91)
In article <1991Mar18.165140.24974@csn.org> chuck@csn.org (Chuck Luciano) writes <stuff deleted> >I have serviced a lot of electronic equipment, amplifiers, tuners, tv's, >I have a hard time believing that some sensible precautions, wouldn't >reduce this to a fairly safe activity, but maybe you know otherwise. ***************WARNING****************** Microwave ovens are very simple, just a power supply (usually a doubler) and the magnetron. The thing is, the supply is LETHAL. It supplies 3-4 Kv at several hundred mA. Get across that and you've had it. Even experienced service people (here in Oz) have been killed by these things. If you are going to work on one of these things hot, then be VERY VERY careful. You won't get a second chance. Being aware of the risks, and having a healthy respect for the thing will go a long way to staying alive. <stuff deleted> Steve Quigg. D
mcovingt@athena.cs.uga.edu (Michael A. Covington) (03/19/91)
In article <91077.142209FC138001@ysub.ysu.edu> FC138001@ysub.ysu.edu (Phil Munro) writes: > > My recomendation is *DON'T MESS WITH YOUR MICROWAVE* unless you are >interested in finding out what cataracts are like. Stray microwave >energy is very dangerous to your eyes!!!! And from the questions >asked in this post it looks like the writer does not understand this! > But why would a routine electrical repair have any effect on this? He's talking about replacing a major component, such as the magnetron. This is a standard kind of repair and has no effect on the seals, nor does it expose the technician to any hazard for the simple reason that the microwave oven is never powered on during repair. -- ------------------------------------------------------- Michael A. Covington | Artificial Intelligence Programs The University of Georgia | Athens, GA 30602 U.S.A. -------------------------------------------------------
fmgst@unix.cis.pitt.edu (Filip Gieszczykiewicz) (03/19/91)
NOTE: I posted this reply and then read several more posts - and I decided
to cancel and modify it before reposting. Why? Well, for one thing,
I don't want someone who hasn't seen what 5KV @ 200mA can do to a
human being trying to do what I listed below! I assume that you know
what you are doing. Now, replacing a magnetron is not a very
involved procedure BUT MEASURING HIGH VOLTAGE IS! Also, microwave
ovens are NOT FORGIVING! They will kill you - it's a test - you pass
by living or your fail with a funeral. If you are a novice, try fixing
a radio first - you won't get a second chance with microwave!
Now that I got the point across, let me repost the article.
In article <1991Mar18.165140.24974@csn.org> chuck@csn.org (Chuck Luciano) writes
>
>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.
Greetings. I do not think that running a micro[wave] with or
without anything makes any difference. Notice, for example,
that some people (like my grandma) like to raise a large
bowl of water to a boil but running the micro for an hour or
so. (by mistake, of course. Our micro has survived dozens
of these "mistakes").
>
>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.
I've fixed (or attempted to...) about 20 or so micro's. All of
them have: fuse, diode(s), relay(s), HVcap(s), timer/computer,
_big_ transformer, magnetron, and some hoopla - like broiling
coils etc.
>
>Does anybody know if the magnetron is the most likely point of failure,
These things are quite sturdy.
>or is the supply? Can I use a volt meter to measure the voltage from the
I doubt that the transformer burned out
>power supply? Should I disconnect the supply from the magnetron before
If you want sparks to fly all over and most likely burn out
something else - like you - then don't!
>measuring the supply? If I disconnect the supply, should I provide a dummy
>load? Any other suggestions?
Well, from the 2 dozen I've seen, this is the breakdown of the
components that fail:
relay - tell me why they put a 16A relay for a 15A load?!?
I had one micro where I replaced the burned relay 3 times
before I put in a 25A model and it works to this day!
So, unless anyone flames me in a big way, when replacing
the relay - put in one that is rated for 5A more. (As in
original is 15A - put in a 20A, etc.)
fuse - DO NOT DO TO THE FUSE WHAT YOU CAN DO TO THE RELAY!!!
In other words, don't just put in a fuse with a bigger
rating. Just replace it with the same as specified.
I've seen perfectly good micro's blow a fuse not because
there was something wrong but, perhaps, because it was
on the same line as, say, a compressor. The drop in line
voltage (when the compressor kicked in) was enough to
blow the fuse. (or maybe if was the spike - anyone? ;-)
switch - I'm talking about the micro-switch that they use to
detect when the door is opened. Some poor designs put
the whole load on a 15A-rated switch. Most use the
switch to drive the relay - I have yet to see one switch
that fails when used in this way... your milage may vary.
keypad - On the "cheapo" designs, the keypad for the computer is
not sealed - ie. in a few months (depending on use) the
grease makes it's way inside and nothing seems to work.
Replace - best way. Clean - cheap but _will_ fail again
in the future. Again, your milage will vary.
downhill- from this point on, all the other components failed with
no apparent order. Things to check:
fan motor - some people use this 24h/day
transformer - not the _main_ one. The one that is used
to supply voltage to both the relays and
the electronics.
switches - most micro's have 3 or more. Check them
all.
magetron - I guess I'm lucky - I have yet to see
a microwave with a bad one :-)
Xformer - _main_ one. Same as above.
cap - check the capacitor (the HV one). I've had
a micro where the cap "burst" from heat.
This was in one of those micro-broiler
"combo" units. The one that has a electric
heating coil.
Well, that's about it. By the way, if you "fiddle" with micro's
in a big way - ie. more than 1/year :-) - get one of those
leakage meters - just to make sure your didn't mess up.. :-)
Also, I have a spare magnetron and Xformer. No promise that they
work but, looking above, there is no reason why the shouldn't.
The model number is a SHARP #2M172J (magnetron) and 120V 60Hz
RTRN 0139WREO TAH-001H (Xformer). By the way, the electonics were
fried in this one because the small Xformer got shorted and let
120V through a 5V path :-) How much? you pay shipping and maybe
$20 for both :-)
>
>The microwave is a sharp convection/microwave it is about two to three
>years old, I could provide an exact model if needed (I don't know it off
>hand).
Hey! That's what the above parts are out of... ;-)
>
>Does sams provide photofact folders on microwaves? Does the manufacturer
>put service manuals in the hands of mere mortals.
Most micro's have a full schematic somewhere inside... unless
someone else got to it already... (I don't have one for the above)
>
>Any info on this is greatly appreciated, reply by mail and I'll post a summary
>if there is sufficient interest, and somebody can suggest the most appropriate
>newsgroup for this line of discussion.
My reply bounced - for some reason....
>
>
>Disclainer: Don't try this at home kids, we're professionals here.
^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
ditto
Take care.
P.S. No warranty expressed or implied...
--
_______________________________________________________________________________
"The Force will be with you, always." It _is_ with me and has been for 11 years
Filip Gieszczykiewicz "... a Jedi does it with a mind trick... " ;-)
FMGST@PITTVMS or fmgst@unix.cis.pitt.edu "My ideas. ALL MINE!!"terryb.bbs@shark.cs.fau.edu (terry bohning) (03/19/91)
I don't know if this has been posted yet, but there is at leas 1 book on Microwave Oven Repair: "Practical Microwave Oven Repair", Homer L. Davidson, 1984, TAB Books, Blue Ridge Summit, PA.
feg@moss.ATT.COM (Forrest Gehrke,2C-119,7239,ATTBL) (03/20/91)
In article <91077.142209FC138001@ysub.ysu.edu>, FC138001@ysub.ysu.edu (Phil Munro) writes: > > My recomendation is *DON'T MESS WITH YOUR MICROWAVE* unless you are > interested in finding out what cataracts are like. Stray microwave > energy is very dangerous to your eyes!!!! And from the questions > asked in this post it looks like the writer does not understand this! > > If any reader doesn't know what cataracts are, do some research! It > is blindness, or partial blindness, due to a change in the clear lens > of the eye. In other words, microwave can cook your eys, just like the > white of an egg turns from clear to opague!! I wonder why you posted this article since it is obvious that you never tried to fix a microwave oven. Just because the outside cover of the oven is removed is no indication that microwave energy is now going to be spewed around even while operating. The oven door controls are still in effect, and there is no need to override them and stick your head into the oven as there is nothing in that compartment to fix anyhow. The only possible item that might involve microwave radiation into the environment is the propeller paddle that many ovens use to prevent fixed nodes in the oven. If this item needs repair or replacement one would do this without the oven operating. It would be difficult to do otherwise as some of the electronics must be disabled to get at that mechanism driving the paddle. Incidentally, if the oven needs to be operated in order to observe or measure anything, do put something into the oven to work on to provide a load, like a pot of water. One more item for anyone attempting to repair the electronics: watch out for that capacitor connected to the magnetron. It is holding a lot of voltage and since the capacitor is about a microfarad it can hand you quite a jolt. So with power off, discharge that capacitor first thing. And BTW, that capacitor can hold that voltage for many minutes as I discovered after about 15 minutes. Forrest Gehrke k2bt feg@dodger.att.com
young@eg.ecn.purdue.edu (Mike Young) (03/20/91)
Yeah, I fixed a microwave once. It was a Sears, and the power transformer had gone short. Sears gave me a *very* hard time when I tried to buy a replacement! I had to sign a release that I wouldn't hold them responsible if I got hurt while repairing the thing. Very strange. Anyway, I swapped in the transformer and fired it up, everything was peachy. My conclusion is that if you know how to work on 2KW linears and have the proper respect for HV and RF, working on microwave ovens shouldn't be a challenge. Having said that, beware that if the containment (door seal, cooking chamber, etc.) is in any way involved with the repair, find/borrow/buy a survey meter and use it after you're finished, to make sure things don't leak. If this happens to be an oven that others might use besides you, I'd call this mandatory. Could be a sticky liability point later on. 73s, and good luck | Mike Young KA9HZE | young@ecn.purdue.edu | | Purdue University EE Dept. | ...!pur-ee!young | | W. Lafayette, IN 47907 | | _____________________________________________________________________ -- | Mike Young KA9HZE | young@ecn.purdue.edu | | Purdue University EE Dept. | ...!pur-ee!young | | W. Lafayette, IN 47907 | | _____________________________________________________________________
murray@sun13.scri.fsu.edu (John Murray) (03/20/91)
In article <4P02y1w163w@shark.cs.fau.edu> terryb.bbs@shark.cs.fau.edu (terry bohning) writes: > >I don't know if this has been posted yet, but there is at leas 1 book >on Microwave Oven Repair: "Practical Microwave Oven Repair", >Homer L. Davidson, 1984, TAB Books, Blue Ridge Summit, PA. Er, I've seen a few TAB books that were just rife with errors. In one basic electronics book I happened to pick up, they couldn't even get the formulas for series and parallel capacitances correct... ...Not sure I'd trust them to tell me how to mess around with HV and high-power RF... -- *Standard Disclaimers Apply*| ---Get Out Of HELL Free!--- John R. Murray |The bearer of this card is entitled to forgive murray@vsjrm.scri.fsu.edu |Himself of all Sins, Errors and Transgressions. Supercomputer Research Inst.| -- D. Owen Rowley