[R-390] IF Trimmer notes

[Barry]

Cecil,

Good analysis.  I'd assume that with any of the ceramic trimmer types, if
you don't get two peaks (or nulls as the case may be), the cap isn't of
sufficient value due to whatever due to the mechanical nature of their
construction.

Barry - N4BUQ

----- Original Message -----
From: "Cecil Acuff" <chacuff at cableone.net>
To: "R390list" <r-390 at mailman.qth.net>
Sent: Tuesday, January 31, 2006 5:45 PM
Subject: [R-390] IF Trimmer notes


> Hi Folks,
>
>         Discovered something in the last few days that might be of
interest.
> I've not seen it talked about here in the past.  While doing the alignment
> on my R-390A project (mostly done R-390's) I have gone through the
> adjustment of the trimmers across the mechanical filters.  The gain seemed
a
> bit low on the 4 kc position so I figured it was the dreaded worn out
filter
> scenario.  While adjusting the trimmers for max gain I noticed a couple
only
> had one peak.  I would expect on the trimmers that will rotate 360 degrees
> to have two peaks.  Some had two peaks and that was on the more sensitive
> positions.
>
> The ones with only one peak would lead one to believe he had found the
> correct position for the trimmer which isn't so.  What is in fact the
> case...the fixed cap is not the correct value to properly resonate that
end
> of the filter.  I substituted various values in place of the factory cap
and
> found one that would allow adjustment through two peaks and the gain came
> way up.  After getting all the top end tuned I pulled the deck loose and
> stood it up in the radio to adjust the lower trimmers as my side panel
does
> not have the holes to allow adjustment while the IF deck is in place.  I
> found three of the four bottom trimmers to have only one peak.  I will
have
> to pick up some more caps to continue the tuning of the lower end of the
> mechanical filters but I expect the final outcome to be a much better
> performing radio.  It should allow me to reduce the overall IF gain and
> improve overall sensitivity beyond what I already had achieved by
improving
> the efficiency of the coupling of the signal through the mechanical
filters.
>
> Bottom line is if you only get one peak on the trimmers, improvements in
> performance can be achieved by getting the correct value cap in the
circuit
> to allow proper resonance setting.  Can't go by the values in the book as
> each filter will probably be different.  Two of mine had been changed from
> the Collins to the later Whitewater filters....that probably accounts for
> two positions being out of resonance. The caps I pulled were 82 pf and
> tested OK.
>
> I also had an intermittent gain problem in the 4 Kc position which turned
> out to be the lead on the original fixed cap had rotated in the body of
the
> cap causing a change in capacitance anytime the cap was pushed on....or
when
> the temp. in the radio changed.
>
> This radio is getting close to .1uv a little at a time with most bands
> running .13 to .15 @ 8Kc.  Not bad for one that came out of the SJC pile!
>
> Cecil....
>
>
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