Microwave Oven - Magnetron End Cap Replacement

[jim hardy]

Question of you radar guys..

My 14 year old Panasonic Microwave Oven suffered a magnetron end cap failure
it commenced arcing and melted off a corner.

I noticed it's not sealed, brand new ones have a hole in the end ...
so I'm wondering just what does the endcap do ?
I find plenty of descriptions of cavity resonance and the path of whirling electrons

but only brief mention of the endcap ..

Is the endcap part of the tuned antenna assembly , with dimensions that are critical?
image courtesy http://www.radartutorial.eu/08.transmitters/Magnetron.en.html

or might i get away with soldering a copper pipe cap over the end ? With a hole of about same area as in the old endcap?

How hot does that part get ?
What is its purpose - just to short the end of the coaxial coupling ?

Thanks for a reality check,
link to instructive material appreciated.

old jim

 

[OCR]

Question of you radar guys..

Well, I'm not a radar guy, but...

...or might i get away with soldering a copper pipe cap over the end ?

Take a look here... don't know if it will help or not. .

.

 

[jim hardy]

Thanks OCR

i found a "magnetrons for dummies" style Youtube that's sort of qualitative. Here's a snip I've annotated

extrapolating it to the point I'm on thin ice,
i surmise

since the endcap is about 1/7th wavelength

I'm going to guess its length isn't critical and try adding a ½ inch copper pipe cap sweat fitting over it.
That will keep me entertained while the replacement magnetron is en route.
It'll either work great or make a shower of sparks ,

Meantime I've learned about the mica 'window' that covers the waveguide at oven wall . I think that was my trouble - it had pizza spatter on it that got turned to charcoal , probably reflecting energy back into the waveguide. Replacement mica sheet is about five bucks on Ebay or Amazon.

Ahhh i love how it's the small things of the Earth that confound the mighty ! So much to learn so little time ...

old jim

 

[OCR]

it had pizza spatter on it that got turned to charcoal

Told you so, see this!. ↓

I think that was my trouble

Well...? ! . . .

Lol, you're welcome... .

Thanks OCR

.

 

[jim hardy]

Take a look here... don't know if it will help or not. .

Thanks -
one of those videos at your link features a burnt mica window

mine looks very similar

new magnetron is due to arrive Monday.

Tell Mrs OCR i admire her troubleshooting skill !

old jim

 

[anorlunda]

Microwaves are so inexpensive nowadays. What's the chance Jim that the new window and new magnetron cost as much as a new oven?

 

[jim hardy]

Microwaves are so inexpensive nowadays. What's the chance Jim that the new window and new magnetron cost as much as a new oven?

Part of it is "Beating the system". I've brought this one back to life so many times now i know my way around it .
And it's a pretty nice one with all stainless interior and exterior.

Magnetrons on Ebay cost from around 15 to 165 bucks , so i ordered a $15 one.
The mica window is about five bucks for enough material to make half a dozen of them.
If that doesn't fix it, well, Fair Anne gets to pick out a new one.

old jim

 

[jim hardy]

Thanks OCR

Well i just had to tinker some more.
The metal antenna cap resisted all my soldering tricks, even copper plating with sulfate.
So i decided to investigate by experiment whether my arcing was due to the charred mica waveguide window or to the melted corner on antenna cap .

I filed the jagged molten edges off the cap and reassembled the oven sans mica waveguide window.
To my considerable surprise it works just fine. No arcing and heats water as before the trouble.

I tried a piece of thin cardboard for a waveguide window, no go it's smoking and charring within twenty seconds.

So it looks like i'll have a spare magnetron Monday. Ordering some proper mica window material as soon as sign off here.

Thanks @OCR ,
Mrs. OCR's comment was pivotal.

now i know how important is that little translucent mica waveguide window. (Windowpane?)

.

 

[dlgoff]

The metal antenna cap resisted all my soldering tricks, even copper plating with sulfate.

I recently had the same thing happen to mine. Mine was made out of aluminum. But instead of trying to repair it, I did what @anorlunda suggested; bought a new one.

 

[jim hardy]

Well, it's still going after almost a week

sorry for blurry image, my first attempt at taking a picture ever with Fair Anne's flip phone.

It doesn't seem to mind the burnt end.

I think it's time to cut and fit the new waveguide window.

old jim

 

[kdobrev]

I have troubles with my microwave - the high voltage fuse keeps burning, but all electrical tests I performed showed no problems. So i think it is the charred antenna and antenna cap of the magnetron. I pulled them apart and tried to clean them with steel wool,but this is as far as i could get. Di you think i can clean them with sand paper and then polish them with 1000 grid? Would it work or they have special coating that is now or would be gone after sanding?

 

[berkeman]

Thread closed for Moderation...

 

[berkeman]

I have troubles with my microwave - the high voltage fuse keeps burning, but all electrical tests I performed showed no problems. So i think it is the charred antenna and antenna cap of the magnetron. I pulled them apart and tried to clean them with steel wool,but this is as far as i could get. Di you think i can clean them with sand paper and then polish them with 1000 grid? Would it work or they have special coating that is now or would be gone after sanding?

Thread reopened provisionally.

Welcome to PhysicsForums. What is your background in EE or Electrician work? What appliances have you worked on in the past? We take member safety pretty seriously here. Thanks.

 

[Tom.G]

Hmm... I haven't seen a fuse in the high voltage circuit in the few I've taken apart. Can you post a photo of it with an indication of its dimensions... without getting too close to the parts than can kill you?

 

[kdobrev]

I have a systems engineer bachelor degree in telecommunications. I work as a developer, though. I have repaired this oven once before, and have repaired a lot of (all things i have at home and had broken at some point in the past) This is my first magnetron, but I did my extensive search on safety precautions and the supposed berilium oxide that is in the ceramic. I will take a picture , but it's specs are 0.8A 5000V.

 

[kdobrev]

Here is the burning fuse. It is about 40mm long but that seems irrelevant.

 

[kdobrev]

OK, while waiting I grabbed a pair of pliers, wrapped a fabric around the cap and started pulling and jiggling around. It came off almost not damaged. With just a tiny dent. Then I wet sanded with 400, 1000, 2000 grit sand paper. The internal part if the cap had some really nasty charred parts that I removed with a dremel and a stone.
The cap then was loose on the antenna, but with the help of a hex rock, three pins and pliers I managed to squeeze it back to a tight fit.

This is the result. I will post an update after the test.

 

[kdobrev]

I tested and the results confuse me more. There is a strange sound coming from the oven, the fuse is not burning out, but it is not heating the glass. Everything points to a diode, but it seems fine. I will buy a replacement tomorrow and test.

 

[kdobrev]

Diode was changed, but still not heating, so I ended up putting a spare magnetron, that is not for this oven but managed to put it in there securely. It works now.

 

[Keith_McClary]

spare magnetron

Always keep a spare magnetron. I have not needed it, but I used the main fuse (the fuse on the newer MW blew (just because)). Also used some of the cable to repair my dryer.