[R-390] IF Trimmer notes

[Cecil Acuff]

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....