Microwave troubleshooting

[hon...@yahoo.com]

I have a microwave from the mid 2000s that failed on me. It uses basic circuitry: transformer, diode, cap, magnetron....not an inverter type or anything 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.

Everything tests ok, the transformer, diode, cap, even the magnetron resistance measurements are "good". I know there's about 10A to the filament with indirect testing, and I know the capacitor has near 2kV on it right after running the unit and unplugging it. This was determined by knowing the cap has an internal bleeder and using the RC time constant to allow the voltage to drop to a safe value to measure.

Would this be indicative of a bad magnetron? Almost everything I read about 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 correctly.

[Phil Allison]

hon...@yahoo.com wrote:

======================

> I have a microwave from the mid 2000s that failed on me. It uses basic circuitry: transformer, diode, cap, magnetron....not an inverter type or anything 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.
>
> Everything tests ok, the transformer, diode, cap, even the magnetron resistance measurements are "good". I know there's about 10A to the filament with indirect testing, and I know the capacitor has near 2kV on it right after running the unit and unplugging it. This was determined by knowing the cap has an internal bleeder and using the RC time constant to allow the voltage to drop to a safe value to measure.
>
> Would this be indicative of a bad magnetron? Almost everything I read about 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 correctly.

--------------------------------------------------------

** Might not have a good vacuum any more ?

Happens with power output tubes quite regularly.



..... Phil

[John Robertson]

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

RapirFAQ may be of use to the OP:

https://www.repairfaq.org/sam/micfaq.htm

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 :-#)#


--
(Please post followups or tech inquiries to the USENET newsgroup)
John's Jukes Ltd.
MOVED to #7 - 3979 Marine Way, Burnaby, BC, Canada V5J 5E3
(604)872-5757 (Pinballs, Jukes, Video Games)
www.flippers.com
"Old pinballers never die, they just flip out."

[Phil Allison]

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

>
> > ** Might not have a good vacuum any more ?
> >
> > Happens with power output tubes quite regularly.
> >
> >

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

** 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

[John Robertson]

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 :-#)#

[hon...@yahoo.com]

On Saturday, July 17, 2021 at 6:10:52 PM UTC-5, John Robertson wrote:

> Would that not burn out the magnetron heater if it was exposed to the
> atmosphere?
>
> RapirFAQ may be of use to the OP:
>
> https://www.repairfaq.org/sam/micfaq.htm
>

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

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

[Phil Allison]

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

> On 2021/07/17 4:30 p.m., Phil Allison wrote:
> > John Robertson wrote:
> > ====================
> >>
> >>> ** Might not have a good vacuum any more ?
> >>>
> >>> Happens with power output tubes quite regularly.
> >>>
> >>>
> >> Would that not burn out the magnetron heater if it was exposed to the
> >> atmosphere?
> >
> > ** 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.
> >

> >

> I've seen plenty of regular tubes whose filaments burn open shortly
> after the tube body is cracked.

** The case here is likely to be a tiny, slow leak of air.

> 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...

** Takes time and a lot of oxygen rich air.


..... Phil

[Jeroni Paul]

> I have a microwave from the mid 2000s that failed on me. It uses basic circuitry: transformer, diode, cap, magnetron....not an inverter type or anything 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 good electrical connection to the magnetron. I cleaned surfaces and it came back to life.

[hon...@yahoo.com]

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.

[Michael Trew]

On 7/26/2021 10:39 AM, hon...@yahoo.com wrote:
> 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.

[Ralph Mowery]

In article <sdn46g$2ne$3...@dont-email.me>, mt99...@ymail.com says...

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

[Phil Allison]

hon...@yahoo.com wrote:
======================

> 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.

** Bet my loss of vacuum theory was accurate.


..... Phil

[Michael Trew]

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.

[hon...@yahoo.com]

Exactly. I hate throwing things out.

[hon...@yahoo.com]

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.

[hon...@yahoo.com]

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.

[Peter W.]

On Sunday, December 25, 2022 at 2:42:54 PM UTC-5, hon...@yahoo.com wrote:
> On Saturday, July 17, 2021 at 3:57:08 PM UTC-5, hon...@yahoo.com wrote:
> > I have a microwave from the mid 2000s that failed on me. It uses basic circuitry: transformer, diode, cap, magnetron....not an inverter type or anything 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.

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

Peter Wieck
Melrose Park, PA