trimmer caps

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From: Jeff Herman (jherman@uhunix.uhcc.Hawaii.Edu)
Date: Mon Feb 14 1994 - 21:41:49 EST


It was mentioned that these 90-420pf trimmers might be too high for HF
work. Here's how I'm using them:
I posted an 80M xmtr on here a couple weeks ago and I'm currently collecting
the parts for it. Three variable caps are called for, but since this xmtr is
being built around a 3579khz xtal I'm using these trimmers - I'll set them
once and that will be it (variable are too hard to come by in my `dumpster
diving', and these trimmers are cheap).
In the tank portion of the xtal osc, a 250pf cap in parallel with a
7.9uh inductor. The circuit calls for a fixed 250pf but I'll want to
tweek to perfection so I'll use a trimmer [don't sneeze while operating].
Next, a 75pf variable cap is called for to couple the xtal osc to the PA
xsistors - here I'll use one of the trimmers IN SERIES with a 45pf cap
so that the mid range of the trimmer will land at 75 pf.
Finally, the output stage calls for 2 caps in parallel: 250pf fixed
and 100pf variable (total 350pf), with these paralleled caps in series
with a 5.65uf inductor. Here I'll use just a trimmer in place of the two
paralleled caps [don't cough - you'll change the temp.]

Some will be unhappy to hear me using these trimmers in this manner,
rather than using variables, but remember - this is xtal controlled
and that xtal is fixed, and the temp here in Hawaii remains rather
stable...

72,
Jeff NH6IL


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