The AM Forum

Hola,

I'm building up a new 6M amplifier and just got a nice little
vacuum variable from Big Al, K1JCL. It ranges from about
5-50pf and is 15KV, just right.  

Question # 1) The problem is that I notice the inside copper all the plates
are of a darker color, not shiny like my other good vac caps.
The dark brown looks like what happens to copper when it is
exposed to the weather outside.  I see no signs of pitting on the
plates from arcing. It checks good in the cap checker, but have
no hi-pot tester yet.

Does this color mean there is a vacuum leak and it has air inside?  
I notice the pull is moderate, so maybe it still has SOME vacuum,
but it is a small cap and might be normal.  I always thought in a
vacuum everything stayed inert.

There was once someone who posted about putting a vac cap in
the refrig and then taking it out to look for condensation on the
inside if it's leaky? How does that test work again?


Question # 2)  I was testing a vac cap the other day. I threaded it for
minimum capacitance to measure. All of a sudden the threads
popped out of the socket and the moving "rotor" rammed back
to the rear. It blew the glass back out like an explosion and
scattered glass all over the room. I was yellified since it was a
beautiful, working vacuum cap still in service...

In the past, when the threads come out, the front hub just falls off
and the insides are OK.  What in the whirl happened here?

Tnx.

73,
Tom, K1JJ

A glass usially has high internal tensions because surfaces are cooled down faster than a glass in the volume because of low thermal conductivity, so a small crack may cause a big bang.
Also, a pressure of outside air added more tension.

Tom,
I would suspect a cap with air inside would measure higher C.
How about hanging across mr ugly power supply with a series
fuse from a strand of RG8 shield.
test at fully meashed plates.

I might try that, Franz.. good idea.

Though, I stuck it in the refrig/freezer last night and when
I pulled it out I cud see some condensation form on the inside.
Not good, I guess. Only a trace, but not sure what is too much.

Also the plates inside look brown from air contamination and the
pull for vacuum seems scrotless.

So, doesn't too good. Your test will tell the tale. Though I
think it's rated at 15KV, but I only have 7KV supply, no load.

T

Treat it like an M-80 if you are going to test it with high Voltage.

You mean light the fuse and throw it in the toilet?  8)

--Thom
Kilovolt Arc One Zorching Gassy Capacitor

Thom, in a vacuum there is no moisture to form condensation, plus copper requires air to oxidize. If you saw it, then the vacuum is gone, and replaced with good ole air. I would retest and make sure I saw condensation on the inside. And I would do that before the Hi-v test fer sure....

Honolable Thom Vu,
                            Have you tested the cap with a digital capacitance meter? Does the cap read what it's supposed to?

Hi Joe,

I'm not sure what it is supposed to be, capacitance-wise.
No markings on it.

But the cap has all the signs of being shot, so I gave it back
and found a new one to use.  Water vapor inside, copper
inside all corroded and poor pull on the housing indicating little
vacuum.

All set now, Tnx for axing.

73,
T

Tom, I might as well weigh in here. You can always check it out with a megger or HI-POT tester. The vacuum is for the higher voltage rating correct?  If it HI-POTs at and above the rated voltage at full mesh (piston insertion) then it should be fine. By doing that, you erase any doubt it being good or bad.

Hi Mike,

Yes, I shud build up a high pot tester.

But the cap is gone, so no need for now...

73,
T
(http://www.supershadows.com/starwars/pictures/3/leia_2a.jpg)