[R-390] IF deck question

[David Wise]

> From: rbethman@comcast.net [mailto:rbethman@comcast.net]
>=20
>     I have an older R-390A, a 1952 Collins contract, S/N 252.=20
>  The IF deck
> is S/N 35.  It was only brought up to "MOD 1".  It has no variable
> capacitors to tweak the filters into line.
>=20
>    2)  Or should I try and scare up eight of the buggers and modify it =
so
> that I can bring them right in on the money?

If I saw reasonable gain out of the deck, I wouldn't bother.

Unless you're a stickler for historical authenticity
(in that case, you wouldn't put them in in the first place),
you can use any trimmer that covers the range and fits in
the available space.  It won't be just a trimmer; the
capacitance needed is too large.  Instead, it will be a
fixed cap paralleled by a trimmer.  I don't remember the
values, but they're in the manual.

The manual doesn't say what the trimmer TC is; probably NPO.
I would probably compare an NPO to an N750 and pick the one
that stayed on the best.  Mouser sells a variety of trimmers
in various TCs.  I used a couple to redo my PTO's compensation;
one NPO, one N1500, and turn one up and the other down until
the drift was minimized.  By the way, changing the total
capacitance modifies the gross linearity, bowing the
curve up or down in the middle.  Some of you might be
able to avoid a linearity adjustment with that trick.

73,
Dave Wise