Microwave troubleshooting

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rcuitry: transformer, diode, cap, magnetron....not an inverter type or anyt hing fancy like that. One morning it seemed to mostly work, but the output rapidly dropped over a few minutes. No bang, no smoke. Just stopped heating .

stance measurements are "good". I know there's about 10A to the filament wi th indirect testing, and I know the capacitor has near 2kV on it right afte r running the unit and unplugging it. This was determined by knowing the ca p has an internal bleeder and using the RC time constant to allow the volta ge to drop to a safe value to measure.

ut magnetron troubleshooting leads me to believe there's nothing wrong with it, but then again the rest of the circuit also seems to be functioning co rrectly.

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** Might not have a good vacuum any more ?

Happens with power output tubes quite regularly.

..... Phil

Would that not burn out the magnetron heater if it was exposed to the atmosphere?

RapirFAQ may be of use to the OP:

I bought out a distributor of electronic gear for Radio/TV/VCR/etc. repair shops and they had a couple of boxes of Microwave parts. So if the OP figures out what he needs and can't find it online I may be able to help...

John :-#)#

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John Robertson wrote: ====================

** Why ? Don't happen with regular tubes.

When they lose vacuum, the " getter" absorbs the oxygen ( goes white ) and leaves mainly nitrogen. Nitrogen is what incandescent light bulbs are full of.

So it is like a light bulb with a very low filament temp.

..... Phil

I've seen plenty of regular tubes whose filaments burn open shortly after the tube body is cracked. The getter can only absorb X amount of oxygen after all.

Of course the microwave couldn't work if the tube looses its vacuum in any case - it was just that the OP mentioned he checked the filament and it seemed OK and I thought it should have burned open if not in a vacuum any more...

I have no experience with magnatrons other than heating up my food, so it is purely guesswork based on what I've seen happen with evacuated tubes after they loose integrity.

One is always learning!

John :-#)#

Oh I've read the repairfaq. Good information, but it seems nearly all (or a ll?) magnetron failure modes involve something more dramatic than what I'm getting. The filament could be shorted to itself, but it claims that's unli kely. The other is low output resulting from decreased cathode emission, du e to wearout. Mine failed over a few minutes though. There's other issues l ike incorrect oscillation frequency, but water is still supposed to heat.

I'm really leaning towards bad magnetron because everything else *seems* go od and how it didn't fail instantly.

** The case here is likely to be a tiny, slow leak of air. ** Takes time and a lot of oxygen rich air.

..... Phil

rcuitry: transformer, diode, cap, magnetron....not an inverter type or anyt hing fancy like that. One morning it seemed to mostly work, but the output rapidly dropped over a few minutes. No bang, no smoke. Just stopped heating .

I repaired a similar case and it was the faston type plugs did not make goo d electrical connection to the magnetron. I cleaned surfaces and it came ba ck to life.

In case anyone wonders, I replaced the magnetron and it's functioning again. I partially disassembled the old one and there's nothing obviously wrong with it. I did not cut the tube apart however. I mainly wanted the magnets.

Good job! Glad that the unit didn't go to the scrap yard in favor of a new one.

Many times the replacement Megatron can be close to the cost of a new microwave. Especially those under $ 150 or so.

snipped-for-privacy@yahoo.com wrote: ======================

** Bet my loss of vacuum theory was accurate.

..... Phil

Same as a refrigerator compressor. I like to see things repaired, personally. Especially well built older things. Don't let the modern industry's planned obsolescence win! I hope that "right to repair" catches on here like it is in Europe.

Exactly. I hate throwing things out.

Nah, this cost me just under $53USD shipped for an OEM part. As it's an over-the-range model, it's more expensive than the budget countertops.

So.....magnetron seems to have failed again, not quite 1.5 years later. Microwave was running, then stopped heating. No bang, no smoke. I put my wattmeter on it, tried cooking again, saw around 1.2kW usage for a few seconds and thought it was ok, but then dropped to around 300W. I haven't taken it apart yet but my bet is a bad mag.

This was a genuine OEM replacement part from a reputable supplier. So either 1. the part was defective and prone to early failure from the start or 2. something is making it fail early. But what? As I think about the other parts, the transformer, diode and capacitor, I can't come up with a way for those parts to cause early mag failure. Transformers don't fail in a way that causes increased output voltage (not that I've seen), and a bad diode or cap doesn't seem to be capable of destroying the mag.

The original mag lasted 14 years and the usage has always been the same.

Or, another part has failed. Such as a relay, the contacts within the relay, something else....

Peter Wieck Melrose Park, PA