[Collins] 75A-4 BFO results

[wabate]

Hi again,

I was able to get some bench time in today before the grandchildren 
arrive.  I am attempting to align the BFO/Passband tuning so that the 
audio shift is in the order of a few hertz, per the manual.  Well, after 
a few hours of adjusting I have to admit dismal failure! Here is my 
technique.  Let me know if I really messed up somewhere.

First I put the PTO vertical and peaked the cal signal with the 3 KHZ 
filter in AM.  I then reset the passband knob to zero on the shaft.  I 
checked with the 500 HZ filter and I touched it up a bit.  Maybe Collins 
figured some people would not have a 500 filter.

I then went to CW mode and used C118 to set zero beat.  Now this is my 
creation so let me know how this sounds.  To me the passband should be 
able to be shifted without changing the beat note.  Again, within a few 
hertz.  HA!  I set the KC dial on the low side of frequency on 160M to 
produce a 300 HZ tone with my frequency counter.  Usually got within 15 
HZ.  Since I don't have a 6 KHZ filter I used the 3 KHZ but limited the 
passband excursions to 1.5 on the passband settings instead of 3.  I 
always tuned the BFO on the 'Lower' range.  At .5 I had a tone of about 
420 HZ.  At 1.0 I had a tone of 500 HZ.  At 1.5 I had a 660 HZ tone 
(these are all approximate frequencies).  Not too good!

I will fast forward a few hours.  After walking L31 gradually around the 
clock, 90 degrees each way, the best I could do was 400, 500 and 625 for 
the three positions of the passband tuning knob.  So I did not improve 
things very much.  The passband is centered on the IF as a 1 KHZ 
excursion either way produces a 1 KHZ tone, as I would expect.  I even 
changed dial frequency by 500 KHZ and got repeatable results.  The dial 
error 100 KHZ each side of calibrate was less than 100 HZ.  The red dot 
on the BFO shaft is still there and pointing up.

I don't know what else to check.  I feel I'm chasing my tail.  Could one 
of the fixed caps in the BFO be bad or changed that much in value to 
throw calibration out the window?  Any ideas?

Thanks and 73,

Bill, K3PGB