Author Topic: Microwave bad magnetron?  (Read 3574 times)

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Offline canadaboy25

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Microwave bad magnetron?
« on: November 11, 2018, 03:56:05 am »
Hey guys I have a Panasonic microwave with an inverter that isn't heating.  The microwave runs for about 20 seconds, stops, and displays an error code.  I followed the service manual steps to check the inverter and seem to have come to the conclusion that the inverter was ok.  However, I wasn't convinced so I unplugged the high voltage wires from the magnetron and jumped them with a very thin strand of wire from a multistranded cable.  Immediately after starting the microwave it vaporized the wire.  This is a repeatable result so it seems the inverter is in fact good.

This would suggest that the fault is in the magnetron.  However, the filament in the magnetron tests good at 0.2 ohms so it seems strange to me that it would be causing the problem.

Just wondering if anyone here knows anything about microwave repair and could let me know if replacing the magnetron is the right call or of there is something else that could be the problem.

Thanks
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Offline amyk

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Re: Microwave bad magnetron?
« Reply #1 on: November 11, 2018, 04:06:58 am »
The fact that the inverter can output for a brief period of time doesn't mean it can hold its output continuously. What's the exact error code?

You can check if the magnetron has a short between the electrodes too; checking if it's lost vacuum is much harder, however, but in my experience the magnetrons are reliable and the inverter is usually the problem.
 

Offline xavier60

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Re: Microwave bad magnetron?
« Reply #2 on: November 11, 2018, 04:48:58 am »
Hey guys I have a Panasonic microwave with an inverter that isn't heating.  The microwave runs for about 20 seconds, stops, and displays an error code.  I followed the service manual steps to check the inverter and seem to have come to the conclusion that the inverter was ok.  However, I wasn't convinced so I unplugged the high voltage wires from the magnetron and jumped them with a very thin strand of wire from a multistranded cable.  Immediately after starting the microwave it vaporized the wire.  This is a repeatable result so it seems the inverter is in fact good.

This would suggest that the fault is in the magnetron.  However, the filament in the magnetron tests good at 0.2 ohms so it seems strange to me that it would be causing the problem.

Just wondering if anyone here knows anything about microwave repair and could let me know if replacing the magnetron is the right call or of there is something else that could be the problem.

Thanks
In case of someone not knowing. Placing a thin strand of wire between the ends of the 2 thick red wires is only loading the low voltage filament supply. The actual EHT supply of over -4000 volts is present common mode between the red wires and chassis.
Btw, when magnetrons test short from the pins to frame, the short is likely to be in the socket base. There are feed through capacitors built into the socket base that can break down.
« Last Edit: November 11, 2018, 04:50:36 am by xavier60 »
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Offline canadaboy25

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Re: Microwave bad magnetron?
« Reply #3 on: November 11, 2018, 05:37:54 am »
I had forgot to test between the filament and the shell.  Just tested it and it shows 180 ohms of resistance between the pins and the housing.

It is showing error h98
canadaboy25

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Online james_s

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Re: Microwave bad magnetron?
« Reply #4 on: November 11, 2018, 06:03:22 am »
That doesn't seem right, IIRC the filament and shell should not show any continuity.

You could try borrowing a magnetron out of another microwave, I've swapped them a few times, using magnetrons from junk ovens to fix others, generally if it's close enough it will work.
 

Offline xavier60

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Re: Microwave bad magnetron?
« Reply #5 on: November 11, 2018, 06:20:50 am »
I had forgot to test between the filament and the shell.  Just tested it and it shows 180 ohms of resistance between the pins and the housing.

It is showing error h98
It is possible to check further by removing the cap and cutting the choke wires away from the socket base's internal pins.
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Offline canadaboy25

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Re: Microwave bad magnetron?
« Reply #6 on: November 11, 2018, 06:57:49 am »
I had forgot to test between the filament and the shell.  Just tested it and it shows 180 ohms of resistance between the pins and the housing.

It is showing error h98
It is possible to check further by removing the cap and cutting the choke wires away from the socket base's internal pins.

So you're suggesting it could be shorting here?
canadaboy25

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Offline xavier60

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Re: Microwave bad magnetron?
« Reply #7 on: November 11, 2018, 07:00:57 am »

So you're suggesting it could be shorting here?

Yes, I have seen a few. Cut the wires close to the socket pins.
Hioki AS100D vom, HP 54645A dso, Fluke 87V dmm, AN8008 dmm, Agilent U8002A psu,  FY6600 function gen, New! Brymen BM857S-(With Battery)
 

Offline canadaboy25

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Re: Microwave bad magnetron?
« Reply #8 on: November 11, 2018, 08:16:47 am »

So you're suggesting it could be shorting here?

Yes, I have seen a few. Cut the wires close to the socket pins.

Ok, so with the wires cut, there is no continuity between the filament and the case.  One of the pins passing through the housing has multiple megaohms of resistance to the case but the other pin was the one causing the issue like you thought because it has around 190 ohms of resistance to the housing.

This would case a no heating fault?
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Offline canadaboy25

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Re: Microwave bad magnetron?
« Reply #9 on: November 11, 2018, 08:38:32 am »
Well i jumpered the wires from the magnetron to the inverter and the microwave heated just fine but now when I try to repeat the results I just get a small zip sound from the inverter area and the microwave stops...

However after a small cooldown it will work again.  While running there is what sounds like vary faint arcing-ish sounds coming from the same area.
« Last Edit: November 11, 2018, 08:42:24 am by canadaboy25 »
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Offline xavier60

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Re: Microwave bad magnetron?
« Reply #10 on: November 11, 2018, 08:44:09 am »
That 190 ohms would drop to something even less when the inverter tries to apply -4000 volts to it. That is definitely the cause of the oven not working.
« Last Edit: November 11, 2018, 11:48:59 pm by xavier60 »
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Offline xavier60

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Re: Microwave bad magnetron?
« Reply #11 on: November 11, 2018, 08:45:47 am »
Well i jumpered the wires from the magnetron to the inverter and the microwave heated just fine but now when I try to repeat the results I just get a small zip sound from the inverter area and the microwave stops...

However after a small cooldown it will work again.  While running there is what sounds like vary faint arcing-ish sounds coming from the same area.
Don't do that. There is likely to be microwave leakage.
Hioki AS100D vom, HP 54645A dso, Fluke 87V dmm, AN8008 dmm, Agilent U8002A psu,  FY6600 function gen, New! Brymen BM857S-(With Battery)
 

Offline canadaboy25

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Re: Microwave bad magnetron?
« Reply #12 on: November 11, 2018, 08:53:58 am »
What do you suggest for a repair besides a new magnitron if it isn't possible to jumper it?
canadaboy25

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Offline xavier60

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Re: Microwave bad magnetron?
« Reply #13 on: November 11, 2018, 08:57:28 am »
What do you suggest for a repair besides a new magnitron if it isn't possible to jumper it?
I'm getting worried now. I didn't expect you to jumper it or what you are likely to do next.
Best to replace the magnetron.
Hioki AS100D vom, HP 54645A dso, Fluke 87V dmm, AN8008 dmm, Agilent U8002A psu,  FY6600 function gen, New! Brymen BM857S-(With Battery)
 

Offline canadaboy25

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Re: Microwave bad magnetron?
« Reply #14 on: November 11, 2018, 09:02:46 am »
What do you suggest for a repair besides a new magnitron if it isn't possible to jumper it?
I'm getting worried now. I didn't expect you to jumper it or what you are likely to do next.
Best to replace the magnetron.

No offense intended but if there is no way to repair it why does it matter if it was an internal fault in the first place?  I may not have experience with microwaves but I have experience with much higher voltage systems and know how to be out of harm's way before trying something potentially dangerous.

Since you suggested that it was only an external fault on the magnitron you obviously have repaired some in the past and I'd greatly appreciate if you'd share your experience with me.

Thanks
canadaboy25

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Offline xavier60

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Re: Microwave bad magnetron?
« Reply #15 on: November 11, 2018, 09:12:41 am »
Ok, I feel better about this now.
I have replaced the faulty sockets with ones pulled from other discarded magnetrons where the socket is still good.
I leave about 10mm of wire attached so that a good mechanical connection can be made to the choke wires by either twisting them together or binding them together with a strand of wire then soldering. The enamel has to be scraped of first.
The flange part needs to be firmly attached to the cage with screws.

Hioki AS100D vom, HP 54645A dso, Fluke 87V dmm, AN8008 dmm, Agilent U8002A psu,  FY6600 function gen, New! Brymen BM857S-(With Battery)
 


Online james_s

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Re: Microwave bad magnetron?
« Reply #17 on: November 11, 2018, 06:14:09 pm »
I bet you could take the socket out and replace it with some crimp spade connectors and squirt a blob of silicone around to hold them. Ugly but functional, and if it's already defective there isn't much to lose.
 

Offline shakalnokturn

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Re: Microwave bad magnetron?
« Reply #18 on: November 11, 2018, 07:12:03 pm »
I've repaired a few dozens of microwave ovens including ones with bad magnetron's, I have never been curious enough to break the filament connector though.
I'm guessing it could contain low value high voltage capacitors, forming a LPF with the 2 chokes to keep the RF safely in the shielding.
Can anyone confirm?
 

Offline xavier60

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Re: Microwave bad magnetron?
« Reply #19 on: November 11, 2018, 09:08:47 pm »
I measured the capacitance of some disassembled  magnetrons and got readings of about 300pf per pin.
The big unknown is how much microwave leakage this capacitance prevents and if the cheap replacement sockets contain capacitors.
I have ordered some very cheap ones on ebay for testing.
Hioki AS100D vom, HP 54645A dso, Fluke 87V dmm, AN8008 dmm, Agilent U8002A psu,  FY6600 function gen, New! Brymen BM857S-(With Battery)
 

Offline hermit

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Re: Microwave bad magnetron?
« Reply #20 on: November 11, 2018, 10:31:20 pm »
Magnetrons rarely go bad via open filament.  The most likely thing is actually mechanical deformation.  I worked on them for years and diagnosis was more of a sound thing that you pick up with experience.
 

Online james_s

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Re: Microwave bad magnetron?
« Reply #21 on: November 12, 2018, 02:11:19 am »
I wouldn't worry too much about microwave leakage. Obviously it should be minimized but the whole thing is inside a metal cabinet so even if you left the filter out entirely it wouldn't be dangerous although you might knock out WiFi reception in your house.
 

Offline xavier60

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Re: Microwave bad magnetron?
« Reply #22 on: November 12, 2018, 02:18:59 am »
I wouldn't worry too much about microwave leakage. Obviously it should be minimized but the whole thing is inside a metal cabinet so even if you left the filter out entirely it wouldn't be dangerous although you might knock out WiFi reception in your house.
I don't think there will be harmful leakage. There could be enough to interfere with the workings of the electronics.
Keeping in mind that it didn't function properly with the socket removed.
Hioki AS100D vom, HP 54645A dso, Fluke 87V dmm, AN8008 dmm, Agilent U8002A psu,  FY6600 function gen, New! Brymen BM857S-(With Battery)
 

Offline hermit

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Re: Microwave bad magnetron?
« Reply #23 on: November 13, 2018, 04:24:33 pm »
Microwaves can't leak out from the screen mesh in the door because the holes are too small.  I've heard they are 'way to small'.  So, you can use that as a rough guide.
 

Offline tehmagicsmoke

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Re: Microwave bad magnetron?
« Reply #24 on: November 14, 2018, 06:54:04 pm »
Start with the basics. Most microwaves have 3 door switches. I've repaired a couple microwaves, and that was the cause. Then I'd suspect caps.
 


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