[Lowfer] A bit of a problem with trimmer caps

[Bill Ashlock]

Peter,

>>Last night I installed the 'loop killer' solid state relay in the >>final 
>>to the 3/4" test loop.

>That's tubing size, right?

No that's my latest diameter..... the pocket LF loop, you know. It takes 
650,000 Amps to be equivalent to the std #12 50/50 loop. Figure I'll use a 
ferrite core with 650,000 turns @ 1 amp on the primary. ....... Or was that 
10 turns on the primary and 1/65,000 of a turn on the secondary?  Where is 
that loop-lover from Scarborough, UK, when I really need him?  :)

>>Hmmm - the only likely component that could cause this is the
>>compression mica trimmer capacitor.

>Interesting as I plan on ordering some high Cap mica trimmers to use
>out-of-doors from Dan's.

These must be the 3/4" sq Arco jobs? I think mine are the 3000pf one's. My 
total capacitor bank on the test loop has 0.022uf so it takes some trimmer 
capacity to have enough effect on the tuning. This is the first time I've 
had to go to this much tuning capacity as the normal loops require about 
7000pf total. These trimmers appear to be well built but probably are not 
intended to be used in a damp environment.

>But can a check of capacity be made relative that commenced with?

It's a bit difficult to disconnect the trimmer to make a measurement, once 
installed, and tuned up. Lot of heavy-duty conductors there because of the 
high current (4A on this one). I have more faith in the loop inductance, and 
other components not changing values - so the resonant frequency of the 
combination is probably the best capacity indicator.

>My instincts would lead me to now substitute the cap. Further, if the >new 
>cap works ok, could a measure of cap. be taken so if the problem >repeats a 
>combination of fixed micas could replace it?

Yah, the fixed capacitor approach is the best one, but certainly not the 
most versatile one. Any change to the loop that modifies its size in any way 
requires a re-adjustment (assuming I'm always at 185.30000). On the big loop 
I have an extra terminal on the outside of the box that allows me to clip on 
fixed capacitors between the terminal and one of the loop connectors. These 
silver micas appear to be water proof so I often leave them on the outside 
for weeks.

>I'll stay attuned on the outcome.

Will let you know what comes out of this.

Bill

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