Rewiinding a MOT

Ceramic transmitting tubes and semiconductors also use BeO. Those warning l abels don't always follow equipment through production and testing. A test tech on the GRC106 ended up in the hospital when one of the Eimac finals wa s damaged during shipping from Mexico to Cincinnati. They were just tossing the paperwork since they couldn't read English. Luckily for that tech, the man at the next bench saw him with the damaged tube. He wrapped a plastic bag over the tech's hands and the tube. He had someone call for an a.mbulsn ce as he hurried him running to a sink to wet down the dust. He saved his c oworker's life. After that, the paperwork was wired to each subassembly tha t contained BeO. YOLO.

I have to admit I was parroting what I have read on the Internet. The text stated that roughly half of the magnetrons in microwave ovens contain a toxic substance. I have never read the datasheet of any magnetron.

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RoRo

You knocked out the shunts, the transformer is probably going into saturation. Does it overheat real fast with no load? If you can measure power factor, you can can probably measure the no load wattage too.

Microwaves are high production items. They've cut every possible corner possible. If your original microwave said a line voltage of +/- 10% of 120 volts was OK, you can rest assure it won't be happy 1% out of that range. Removing the shunts will surely botch up the least amount of copper and iron design they used.

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** In defense of the much maligned MOT - when operating as intended in an oven they do a brilliant job.

A small, low cost and efficient power supply that delivers a whopping DC voltage with a low PF to boot.

Believe it or not, the load current is almost sine wave - allowing ovens of over 1kW to run on 120VAC.

... Phil

The datasheets I have read don't say either way what it is made of. When disassembling dead magnetrons I choose not to break up the ceramic parts, just in case I am very wrong about it being harmless.

The filament quite likely is thoriated tungsten too, and if it has burned up due to loss of vacuum, I would avoid inhaling that dust. I've probably breathed enough of that already from grinding TIG welding electrodes.

And I agree strongly with your comments regarding the extreme danger of powering a microwave oven transformer on the bench whilst it still has its secondary winding present.

I can believe that BeO must be pretty nasty because they used to give warnings about it, even back in the time when warnings were reserved for actually very dangerous things. Nowadays due to every harmless thing being plastered with warnings, they have become rather devalued and it is much harder to identify what really is dangerous.

Agreed. The business part of a microwave oven is dead simple- 1 transfomer, 3 windings, 1 diode, 1 capacitor and the magnetron. More parts are required to run the piezo buzzer.

MO transformers are some of the first I've seen with the plasma welded laminations.

I still don't understand the shunted transformers in mercury arc lamp ballasts or ferroresonant power supplies, although they appear to be related.

That's quite some effort you're putting into this. Can you get a dead computer UPS from a dump or business? They have the type of tranformer you may be able to use as is for getting around 12 volt battery charging voltage.

I once had the great idea of replacing internal batteries of a UPS with some large external ones. During the runtime test the UPS basically exploded. They lack proper cooling to operate past the expected runtime of the internal battery without overheating. It's another great example of designing out all extra unneeded costs of a product while just meeting specs.

Thanks that's a useful story. Sometimes in addition to limiting runtime the battery also adds thermal inertia to a device.

yup. Starbuck Coffee in CA post the prop 65 warnings about dangerous chemicals (yes, coffee) in their stores. They have lots of free time in CA it seems.

They're almost always a pink ceramic, and extremely hard to break up, even with a big hammer.

I can't find a SDS on a magnetron either, but came across this interesting one. Looks like what's in your kitchen, but needs liquid and fan cooling:

output power is 10kW. What a beast. Looks like you run the filament hot, then drop the voltage once it's running. Maybe the current to the anode is so high it just keeps hot on it's own. Reminds me of "press to start" flourescent light fixtures.

put electrode in a drill and walk up to the grinding wheel is what I was shown for sharpening those.

The capacitor is usually what gets people. Guess those bleed resistors fail open.

I drilled holes through the captive part of the winding. that loosened the packing and allowed me to chisel out the remains

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  Jasen.

I believe I read somewhere that you should not do that. With an ordinary welder, you will be putting enough heat into the core to risk damage to the insulation between the layers of steel in the core. The factory welding technique is extremely fast, so that much less heat is required.

Firmly clamping them together was the recommendation.

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RoRo

It seems they are not all the same. I have done two. One was easy, while the other one was terrible.

--
RoRo

Ok, so trying to make sense of this. It's basically a SCR/Triac preregulated power supply like Sorenson or a HP bench supply, but with a transformer that's undersized and would burn up at full line voltage?

Sounds like a lot of effort to make a transformer smaller than it should be.

Here's what I have the Carver AV-705

The description mentions "magnified current amplification" and "This allows the output section to deliver high voltage when necessary, or high current. The older designs used "Magnetic Field" technology, which allowed only high voltage. Both designs are class H, in that the rail voltage is variable, making the amplifier very efficient and cool-running. "

Maybe there's a TRAIC light dimmer in there too? I once had a service guide for it, but that vanished as well as the copy I bought years later. I don't recall them going past block diagrams though. Considering there 5 identical amplifier modules, it seems like less a direct lighting strike, you'd be able to fix any problem it has by checking any of 4 other identical parts.

Looks good.

It's supposed to be a plasma welder, so like a plasma cutter with less gas flow, somehow.

The oxide coating should be pretty tough- it's applied with heat in the first place.

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** Basically - yes.

** The Caver Cube ( M-400t ) was a 6.6 inch cube that weighed only 12 pounds. Rated output was 201wpc 8 ohms. It was very cheap to make and sold like hot cakes.

** Doubt it - that thing weighs 42 pounds.

Hmm, what kind of warning would CA put on a magnetron coupled to a waveguide full of pressurized SF6? :-) That was pumping 5MW pulses to a big dish...