[Lowfer] A bit of a problem with trimmer caps

[Bill Ashlock]

Last night I installed the 'loop killer' solid state relay in the final to 
the 3/4" test loop. When I re-installed the final on the loop the current 
was down about 3db and some detuning was noted. DARN! ....figured the relay, 
in the OFF state, lowered the Q of the loop. After overcoming the 
disappointment and mulling over the 'possibilities' I returned the final to 
the bench and disconnected the relay from the resonant capacitor bank. Then 
I checked the current on with a loop simulator test coil and found it was 
still low. Hmmm - the only likely component that could cause this is the 
compression mica trimmer capacitor. Bringing the final inside, likely, 
caused some condensation due to the large temperature differential and it 
could have caused some leakage (which I used to experience on the other 
final before adding the vent holes at the bottom of the enclosure). That was 
as far as I got last night.

After thinking about this today, here at work, I'm not confident that I know 
the mechanism for the trimmer leakage. The visual evidence that I used to 
see on leaky trimmers was a white powder, likely Al oxide, but I never could 
understand how it could cause the capacitor Q to be low, because I couldn't 
measure any leakage with an ohmmeter. One thought I just came up with is 
that the powder could be, in effect, connected to the plates of the trimmer, 
and has capacity to the adjacent plates. Because the oxide is not a good 
conductor, this resistance appears in series with the capacity of the oxide, 
thus lowering the Q of the capacitor (and also adding some capacity, IE: the 
detuning I noted above).

Mike S came up with a fix for a similar problem he had with the same type of 
trimmer that's in my final. He simply immersed the trimmer in a solvent that 
dissolved the Al oxide. What was the solvent you used, Mike?

All this is academic since the real problem: using a component vulnerable to 
condensation, has not been addressed. Any suggestions out there? I'm 
thinking that I could even disassemble the trimmer and spray the aluminum 
plates with some kind of sealer so that no oxide will form but this might 
effect the capacity stability. Was also thinking about using a polyvaricon 
(portable radio tuning cap) as the trimmer since the insides can be sealed 
by using a little RTV and the Al plates are separated by large teflon 
spacers that limit the oxide formation. I'm really looking for a small 
trimmer with at least 500pf max, so that rules out vacuum variables.

Thanks in advance for any thoughts on this.

Bill A








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