The AM Forum

While inventorying some of my parts yesterday, I checked in on my vacuum variables and saw something depressing. One of my Jennings caps - a 1000pf version earmarked for the load cap on an 813 rig - is showing what appears to be oxidation on the bellows. Now, I'll admit that my memory could be faulty, but I'm pretty certain that it was bright and shiny when I bought it. Note in the picture (the one with the blue is the good one, for comparison) that the rest of it, the actual plates, look okay; it's just the bellows that show the oxidation, if that's what it is. I'm suspecting that it's lost its vacuum, but here's my question, for someone who knows more about the internal construction of these things than I do: with the adjusting screw tension at minimum, (turned counter-clockwise until the plates are fully engaged) is the force that holds the plates engaged and the bellows fully extended provided entirely by the internal vacuum, or is the bellows acting like a spring? I ask this because the force required to disengage the plates and compress the bellows seems exactly the same as with the "good" capacitor.

had the same sort of thing happen a few years back with a crack at the gas evacuation seal off.
wondered if it cud still be used - remembered that load caps on most rf amps are multi stage air variables and that most gaps are such that 900Vrms of rf doesn't arc.  maybe try a hi-pot of it and see just much it can stand.  whattadaya gotta lose?

Yeah, I sort of figure that a vac-variable isn't really required for the load cap; I just like their compactness.
And yes, I do wish I had a hi-pot tester! I've passed on a few at hamfests over the years 'cause I didn't think I'd use it often enough to justify their bulk, even though the price was nice.
Retired last summer; if I was still working I'd bring it to my lab room and test it there.

Did you notice if the mechanical action changed once the vacuum was gone?

I'm not sure but it seems likely but in a different direction due to bellows pressure diff'll.  the worst case for arcing would be a partial pressure rather than atmospheric or vacuum

Never having messed around with a completely leaked-down one, my best guess is that the bellows probably goes to a roughly neutral position - about half travel - once the pressure differential is no longer pulling it inward toward full capacitance.
I'm going to keep an eye on this one, to see if the oxidation continues to spread as the vacuum bleeds down, and I will observe how - or if - the mechanical action changes over time, just for the learning experience.
Someone on eBay, a while back, was selling a broken one - glass shattered - for $129.99. "For parts."  ::)

They will take some voltage once the vac is gone but wont work as a load cap for a five band amp with 3500V on the plate....don't ask me how I know.  I would still try it...