[Collins] Collins 75S-1 Receiver Question.

[Dr. Gerald N. Johnson]

On Wed, 2006-11-15 at 19:55 -0700, DW Holtmnan wrote:
> Hello,
> 
> I'm doing a little maintenance/troubleshooting and have the following 
> question that hope fully someone can help me with.
> 
> I've removed the Metal cans that cover the RF switches, S2, S3 and S4 
> and the associated circuits. The front section,  (the Oscillator) in the 
> 21 MHZ switch position shows two caps on the schematic, C120 (15PF Mica) 
> in parallel with  C75 (8 to 50PF Variable). My radio only has C75, C120 
> is not there. Is it left out from the factory? Should I put one in. I 
> have looked at the SIL's and there is no mention of this cap. Apparently 
> it aligns with the cap missing, but the variable cap is close to it's 
> max position, but not maxed out. I know that it will work either way, 
> just wondering if other radios have this cap installed, of if there is 
> some error in the schematic. My manual is the 9th edition, 15 June 1961.
> 
> Thank you in advance for any help.
> 
> Best,
> DW Holtman
> WB7SSN
> 
The schematic in my 1959 vintage yellow book shows only the C75 variable
in that position. My 9th edition 75S-1 manual shows C120. It clearly was
an addition because of the high number of the C120. Note that most all
the other paralleled variable and fixed capacitors have adjacent
numbers. 

If it aligns without maxing out the variable, that's all that's
necessary. Likely if you started out with a slightly different set of
presets on 10m that moved the oscillator slug up a bit, it might not
align without C120.

If you do put in C120, be sure to use an NPO, not some general purpose
drifty tiny cheapest ceramic capacitor. A dipped silver mica would work
well there too.

Digging a bit, ASAB1004 dated 8-3-60 says C120 was added to prevent
misalignment of the oscillator on 15 meters. Probably without the fixed
capacitor one could get to the third harmonic instead of the second of
the crystal and would have poor receiver performance. At last that's my
guess. That ASAB is available on the CCA web site, "Amateur Service
Agency Bulletin."

-- 
73, Jerry, K0CQ, Technical Advisor to the CRA
All content copyright Dr. Gerald N. Johnson, electrical engineer