Microwave problem.....again

Ok, so I got a new microwave diode for the microwave I posted about a few days ago (Samsung MW8490W). I should have checked the new one before installing it, but I was excited and tried it anyway. Turn microwave on....same exact thing. No smoke, popping noises, nothing odd. I take the diode out and hope it's at least still good. The original was shorted. Now this one has become open! I start checking other parts.

There's one, very low resistance xformer winding that's connected across the magnetron terminals (cathode?). This makes sense. Then another winding that's higher (~70ohms) that's connected in series with the capacitor and diode. I tested the capacitor with a Sencore Zmeter and it says it's perfect. However, I can only check leakage to 600V. Still, no leakage detected. No shorts to the capacitor casing, either. The magetron is open between the terminals and the casing, but the resistance between the terminals is low as it should be. I start drawing out the connections to see how this thing works. This doesn't look like the voltage doublers I've seen, but whatever. I can't understand how it blew the new diode. Apparently something is happening only at high voltage. Any ideas?

I'm ready to buy a new microwave anyway....one that mounts above the range and uses what I remember being called "inverter technology" or something like that. Instead of cycling at lower power levels, it just produces less power.

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I just realized that my diode is probably not destroyed. If this thing has a forward voltage drop of ~10V, my meter won't show this. I'll test it later using another method. (I just assumed it has a drop of about 0.7V as most other diodes do!)

This still doesn't explain why the darned microwave is still buzzing and vibrating loudly. This is the same symptom as when the original diode was shorted. (See post "Microwave buzzing a vibrating" from Feb

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What does it do if the HV diode is not installed? If the same thing, then either the magnetron is shorting at full voltage or there is a problem in the transformer that isn't severe enough to blow the main fuse but enough to result in no heating. That wouldn't explain why the HV diode blew originally though.

Also, of course, double check you put it in the correct way around!

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Sam Goldwasser

It Seems you need new Magnetron for your Microwave,{ Assuming your inside of microwave is Clean including waveguide Mica window}Check this web site for more info,

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With Best Wishes Cyrus®

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Before you start any of this please discharge the capacitor . Yes make sure you put the diode in the right way . Try unplugging the magnatron wires and run the oven . If it sounds normal its probably a bad mag .

Those inverter microwaves are a bit touch & go for lasting .. The first ones out were a disaster . The later ones are better . If you buy a ``mount in`` one make sure you want to be taking it out if it breaks . I have seen bad magnatrons in more newer microwaves than ever before . some of them are still brand new only used a few times not enough to even get it dirty . I have also been seeing samsung branded magnatrons in a few other popular brand ovens .. so much for quality

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Ken G.

Well, it seems it's the magnetron. The diode is still good. In summary:

Sounds normal with magnetron filament connector removed. Also sounds normal with magnetron filament connected, BUT everything else removed. Buzzes like crazy with everything connected.

And, if I run it with the filament disconnected, then unplug the AC cord and go to discharge the cap, it actually sparked, for the first time. So it seems the HV is shorted through the mag. A new mag is $57+shipping....hardly worth it.

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I have a Sharp Carousel microwave which I bought new in November

1999. Magnetron and everything else are OK. I don't know what brand magnetron is in mine, but it must be a good make if it still works after more than six years.

I agree with the advice regarding over-the-range microwaves. I've seen at least one of these (a friend of mine has one in his kitchen) ; I hate to think what a job it may be to remove it for repairs.

I would think off-brand microwaves (and those ultra-cheap ones with spring-wound dial timers) which can be had for $39 or less in department stores, etc. would be a lot more prone to very early (read premature) magnetron failures. The micros with digital displays and touch-pad controls should last much longer assuming they are made by a reputable manufacturer, though these days it isn't uncommon to see appliances with well-known names on them that have actually been manufactured by some offshore company no one ever heard of.

As to magnetrons only a few months old (or even newer [!]) going bad: This can happen if the user tries to operate the microwave empty; this will ruin any magnetron in almost no time flat. Also, the use of metal pans, utensils, even aluminum foil in a microwave will destroy the tube in very short order. Power surges on the AC line the microwave is plugged into can also wreak havoc on the internal circuitry (such as the microprocessor which controls the operation of the magnetron and everything else in all recent microwaves), but I wouldn't expect the magnetron to short or open from a line spike. This is not to say the latter cannot occur, but it is possible.

Jeff, WB8NHV (email addy not shown to deter spammers) Fairport Harbor, Ohio USA

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Jeff, WB8NHV

You have to carry out proceedure for discharging the h.v circuit then unclip the filement leads.Attach meter to leads then switch on,the reading will be between 3 to 4v a.c. Switch off,discharge Cappie,re-attach fill.leads then clip meter on to fill.terminals,switch on.Reading should be the same,if not,Mag is suspect.

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Have been reading your microwave problem, and sounded a bit familiar...

I just recently got 2 tossed out ovens, one had an arcing from the antenna...seems someone washed the mica RF window and carbon'd it up...which caused the antenna to start arcing over to the waveguide....(I googled about oven magnatrons and learned how they work)

I used to work on Navy Ship Radar's so figured this can't be too hard...took off the mica cover...then took out the magnatron...cleaned it all up...but the tip had badly arced over...so put large shrink tubing on it to protect it from further arcing...(what the heck it was going in the landfill anyway)... Blew out lots of dead bugs while inside...measured the resistance's..all seemed ok..been working great for months now...no mica window..was going to try to find some plastic won't heat under RF, but seems ok without it, just I make sure no moisture gets up in the guide area...and keep it clean/dry.....

2nd Oven sounds more like your problem...buzzing like heavy load on the transformer....so checked diode, cap...and resistances...seemed ok?? So figured better look further, took the mag out, and pried open the shielding lid and could see the RF filter coils inside...but strange the plastic where the filament leads come through....was loose...so pulled it back further and could see it had cracked and also on one lead was signs of a carbon arc path...funny didn't show up as high resistance?? Maybe a High Pot meter would detect it? Anyway, cut the lead where the RF filter coil (leaving some copper on the steel lead) then used a hot iron to melt the potting epoxy so I could pull the male lead and all out with pliers... Then on inspection, seems there was a factory void in the potting epoxy or it just broke down from someone running the oven with nothing in it? Had to carefully drill out the charred epoxy and any remaining carbon, used acetone to flush...dry...then took the filament pin connector and put many layers of shrink tubing over it, all the way from the tip to the connector (could aways razor trim later).... Then used some GE clear silicone glue to get the inner part to glue and pushed the inner plastic back onto the pin through holes, then let sit dry overnight.

Next day, soldered on the pin to the RF choke, and then mixed up some clear high temp epoxy, got some at ACE 2 mixes in one tube...used a long chinese stick and slowely pour drops into the connection side of the filament connectors (filled it up) and put some dabs over my solder connection too....let stand another day....cleaned it all up, been working great ever since!!!

So I saved somemore things from the landfill... :-))

Figured about 1 hour honest labor....and some fun learning too...

Now I wish I could re-program the presets, as it boils my coffee...and maybe get the timing so the coffee cup handle always returns to the front?? heh heh....

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