OT Microwave repair?

These things are fairly simple, a high voltage transformer, a combination rectifier/filter to turn the High Voltage AC into DC, a cavity magnatron vacuum tube to turn the HV DC electricity into microwaves & a timer/power control circuit board that simply switches the power on/off to the transformer.

To me it sounds like the magnetron has gone dead, but it might be the rect./filter. To figure out which you would need a (really) high voltage voltmeter & the knowledge how to use it safely (or we WILL be reading your obituary :( ).

If it is the magnetron the bad news is that it's probably cheaper just to buy a new oven, the good news is there's lots of neat stuff you can use the old transformer for- do a web search :).

-Howard.

Try this:

Best Regards, Keith Marshall snipped-for-privacy@progressivelogic.com

"Even if you are on the right track, you'll get run over if you just sit there." - Will Rogers (1879-1935).

a neon bulb will tell you if it's emitting any microwaves. trace out the schematic and see what's going on - I found on mine that a thermal fuse had blown due to overcurrent in the magnetron - I bypassed the protection momentarily, there was a spark from the magnetron as whatever was causing the problem vaporized, and it's worked properly for a decade since.

What exactly do you mean by "a neon bulb will tell you"?

Stick a neon bulb, little light tester..that sort of thing, in a microwave and it will glow and pulsate under the microwaves. Let you know if its putting out juice or not.

Gunner

That rifle hanging on the wall of the working-class flat or labourer's cottage is the symbol of democracy. It is our job to see that it stays there. - George Orwell

I fix a lot of microwave ovens that I find at the dump and give them to students and others in need of them.

The major problem I find is, after heating too many pizzas, grease builds up over the magentron exit area. This will cause a nice fireworks display and the homeowner throws the oven away in a panic. A thorough cleaning of the inside is enough to get them running again.

If the oven seems to be drawing energy from the powerline, I would suspect a shorted diode or shorted transformer winding. That energy is going somewhere. The fuse is good, otherwise you wouldn't get anything.

If these simple checks don't help, scrap the unit, take the magnetron apart for the magnets, save the interlock switches and trash the rest. There are lots more at the dump for the taking.

Earle Rich Mont Vernon, NH

I use an ordinary 40 or 60 watt incandescent light bulb for the same purpose. In addition to showing you if any microwave energy is present in the oven, its brightness and how hot it gets in 1-second will tell you far more about the health of the oven than will a neon bulb.

IMPORTANT CAUTION -- If the microwave oven is operating properly, the light bulb will become so hot in 1-second that you cannot touch it without getting your fingers burned. Also, never leave the oven on for more than a second or two because you may cause the glass envelope of the bulb to melt and burst!

This trick even works with a burned-out bulb (which comes as a surprise to many people). The intense heating of the bulb's surface evidently comes as a result of the microwaves being absorbed by the argon or xenon gas fill that is present in your average incandescent bulb.

If the bulb doesn't immediately light and become very hot, it's time for a new microwave oven, since these ovens are generally cheaper to replace than repair.

Harry C.

If you mount it on a stick - and don't have any of the wires shorted - you can close the door and sweep around the closure when it is running to check a general way of protection.

It might be better to do at night with the lights out.

Mart> >

What about the normal caveat of not putting metal into a microwave? I'm thinking of the metal socket of the bulb.

Yup, I've got that bug, too, but it's really hard to justify fixing one when they're being sold so cheap. One with twice the power and a third again the cavity size of the one I've got runs about $45 at the discount stores around here. I can't even get the replacement magnetron for mine for that with S&H included. You've got to figure your time is worth something, too. If it's over 10 years old, figure it's been amortized and go get a new one with more power and jazzier features(and for a lot less money). The magnets off the old magnetron make dandy swarf-snatchers.

The HV supply inside isn't like the ones in TVs, it can kill you stone dead. It won't be just a tingle. There's a big capacitor that can carry quite a charge, too, you've got to make sure that it gets discharged before you ever stick a mitt in there to twiddle with stuff. Same drill as with all HV equipment, keep one hand in your pocket. The local library may be one place you can get some repair info, ours carries a lot of appliance repair books. I've got all the tools(and the HV voltmeter) to do this stuff from my TV repair days, but I figure it isn't worth my time unless it was a really top of the line unit or had some combo feature like a convection oven.

Stan

has something about microwave oven repair.

If the magnatron tube is bad, it'll probably cost at least $30-40 from an electronic parts supplier like EPS (1-800-227-0104) or

but some are much more expensive. Generally you can substitute any magnatron if the mechanical fit is the same and the power range is close. When I change the magnatron I also like to change the high voltage capacitor, high voltage diode, and the high temperature cut-off switch that mounts on the magnatron. The transformer rarely goes bad, but you can check it for high voltage by mounting a neon bulb on a plastic or dry wooden stick (cut off the bulb's leads) and running it along the transformer's output wires (the ones connected to the diode and capacitor) in a dim room. Do NOT try to measure its voltage directly because it's about 1,000VAC and can easily kill you.

The high voltage diode is supposed to have a built-in bleeder resistor that drains off all the voltage when the oven is turned off (always unplug the power cord before working on the oven), but bleeders are known to crack with age, so always discharge the capacitor manually with a plastic-handled screwdriver across the capacitor leads before you touch or detach any of the wiring. If it's not possible to short both terminals that way, wrap some bare wire around the metal shaft of the screwdriver, connect the other end of the wire to some bare metal on the oven chassis (like the capacitor's clamp), and then short each terminal individually. Remember, the wire must be clamped to the chassis BEFORE the screwdriver touches either terminal.

The diode may test bad with a meter because it's several diodes in series and may need more voltage applied to it than the meter can provide. An old-style analog meter on a higher range may work, or you can use a 6-12V DC source with 10,000 ohms in series and connect it across this diode. In one direction the voltage across the diode should then measure exactly the same as the source, but in the other direction you should measure only a few volts.

The magnatron attaches with bolts or nuts and is sealed against microwave leakage by a ring of brass braid. Be sure to tighten all those nuts or bolts and reinstall the oven's metal cover before applying power -- don't even think of taking shortcuts or you could get cataracts or skin burns.

You can measure the output power of a microwave oven by heating up a measured amount of water in a glass or plastic container. For the metric version, fill the container with exactly 1 litre of water, stir a Celcius thermometer and note the reading. Heat at full power for exactly 60 seconds, and stir the thermometer again and note the temperature. The power is:

watts = 70 X (final temp - starting temp)

For the old English version, substitute a Fahrenheit thermometer and 1 quart of water, and use:

watts = 37 X (final temp - starting temp)

MCM Electronics has a web site, and will send you a catalog. They sell magnetrons. The magnetron is not hard to change, I have done it twice successfully. Prices vary with the magnetron, but you will quickly figure out whether you want to go down this road.