Junkbox vacuum cap:

[KC9LKE]

A fellow HAM gave me a Jennings vacuum variable out of a box of T-368 spares. Its, IIRC, rated at 10 Kv and tested on the cap checker at 400 pf in max position. He just grabbed one and handed it to me without knowing is condition. I need one for a plate tune, 2Kv B+, plate modulated amp.

Anyway.

When I operate it toward minimum, so the meshing cylinders just unmesh, I noticed that there are 3 or 4 “spots” on the EDGES of the cylinder, but NOT the sides. I look’s like the cap was at maximum capacitance with the edges to close to the bottom of the pockets and it flashed over.

I plan to kludge up a HIPOT tester out of a variac, neon sign transformer, a series resistor, and test it to 8 Kv.

Opinions appreciated. Make a lamp out of it  , or seen it before and if it passes try it , etc.

Thanks!
 
Best regards

Ted / KC9LKE

[WA1GFZ]

It may be OK. how much force does it take to pull the bellows. If there is good force the cap may be good. As long as the flash marks didn't distorte the position of the plates you will be fine.

[KC9LKE]

“ how much force does it take to pull the bellows.”

Right now I’m using a machine screw to pull it apart by hand. It resists pretty well, cant keep it for more than 5 or 6 seconds before having to returning it back to max cap.           (gently). I definitely believe it has vacuum.


“As long as the flash marks didn't distorte the position of the plates you will be fine.”

Nope they are not distorted. The “spots” look to be about a 1/16” and on the edge, kind of like a microscopic hacksaw blade. I wonder how many used vac caps are like this (unknowingly), and are pressed into service successfully. 
 

Wanted to add:

Right now I’m using a machine screw to pull it apart by hand. I believe these caps use a square thread?? If so I need to find one. I think its ¼ 28 and not easy to find in square.
Using a soft machine bolt will give limited life and will probably cause a “crash”.

Frank:
Thanks again for the help. BTW what’s the picture in your avatar? Been wondering for years.

Best regards

Ted / KC9LKE

[Mike/W8BAC]

I have three Russian vacuum variables purchased new. I think they are rated at 10KVA. All three have flash over marks that aren't hard at all to find. The person that purchased them new was told the flash marks are from quality testing at the factory. Seems like an odd way to test something like this. I didn't pay much for them and if they have problems I will chalk it up to experience. Anyone seen this before?

Mike

[K1JJ]

Just a heads-up:

I don't know what type of vacuum variable mechanical assembly is prone to this problem, but it happened to me one time....

I turned the screw all the way out until the moving plates of the capacitor disconnected from the main screw and it took off like a slingshot inside. It smashed against the back and broke the glass. Ruined.

Other vac caps disconnect and stay put inside with no damage. I'm still not sure why this happened, but these days I never back a cap all the way out, just to be sure.

T

[WQ9E]

Sounds like maybe the Russian testers weren't using a proper current limited Hipot tester  

I love the company name of my HiPot tester,  Slaughter.   It makes me wonder if originally they built electric chairs but found out they weren't sufficiently destructive.

JJ, I have had the mechanism unscrew on caps but thanks for the heads up in pointing out that some self-destruct.

[WA1GFZ]

Ted, that is my 22 FET push pull parallel class e 80 meter final.
check Nov. 2005 QEX

[KM1H]

Some of those flash marks could be a deliberate part of the debarnacling process.

[flintstone mop]

Just a heads-up:

I don't know what type of vacuum variable mechanical assembly is prone to this problem, but it happened to me one time....

I turned the screw all the way out until the moving plates of the capacitor disconnected from the main screw and it took off like a slingshot inside. It smashed against the back and broke the glass. Ruined.

Other vac caps disconnect and stay put inside with no damage. I'm still not sure why this happened, but these days I never back a cap all the way out, just to be sure.

T

Happened to me too. There were so many cuss words for days........it was a beautiful 5-1000PF 30kv vacuum variable. I turned it too far and SNAP!!!
I cannot afford to replace that cap now.
I thought if it is glass and the copper was not tarnished, you could assume it was still under vacuum.
Try to hi-pot test your unit.

[KC9LKE]

Update:

Well I applied 5.5 Kvdc, the best the HV transformer would do, with the plates fully meshed and no flashovers or leakage occurred so I’m going to give it a try. 
Thanks for the advice!

After building the HIPOT I purposely tried some old silver mica’s. As I slowly increased the voltage you could see quick jumps of the 400uA meter, like the “whiskers” were being zapped away. Kind of cool. I pitched them.
Not a bad exercise and now I have a HIPOT tester on top of it.

Best regards

Ted / KC9LKE

[Detroit47]

I have three Russian vacuum variables purchased new. I think they are rated at 10KVA. All three have flash over marks that aren't hard at all to find. The person that purchased them new was told the flash marks are from quality testing at the factory. Seems like an odd way to test something like this. I didn't pay much for them and if they have problems I will chalk it up to experience. Anyone seen this before?

Mike

Mike
I have bought a lot of that Ruskie stuff. They almost all had those marks. I haven't had any problems with them either.

John N8QPC