[Collins] 75A-2 question

[Dr. Gerald N. Johnson]

On Thu, 2007-08-23 at 11:18 -0700, telegrapher at att.net wrote:
> I notice that when i turn the RF gain control up on my A-2 that the AGC 
> attack time drops way down.  I normally run it to where the S-meter just 
> moves on audio which is more than adequate.  However if I'm listening to 
> a weak station  and have to turn the RF gain up higher the meter is very 
> sensitive and jumps around a lot.  It also seems to distort the Audio a 
> bit.  I guess the main thing I'm wondering about is should it affect the 
> AGC reaction time.  Higher gain setting, faster attack time and VS. 
> Doesn't seem right to me but then what do i know.
> 
> Any thoughts on this.
> 
> Larry
> W0OGH
> 
It could be important to know what modifications have been made to the
AVC. I presume you are listening to SSB with the mode switch in the CW
position. Looking at the schematic, there is no AVC voltage coupled from
the detector to the AVC line in CW mode, its all fixed by the RF gain
control derived from the -50 volt line. That would lead me to suspect
that C94, a 50 mfd electrolytic is dried out. Its the last filter for
the -50 volt line. The negative voltage is derived from the total power
supply current lifted off ground by some resistors, mostly R71, R72,
R36, R37, and R38. I also see on the schematic of the original receiver
there is a small electrolytic on the cathode of the AVC amplifier stage
(not in use on CW) C97 that I'd suspect in other modes.

Have the molded oil filled capacitors been replaced? Those performing
AVC bypass and time constant can cause the AVC to be less effective
leading to distortion, but here I think its audio varying the -50 and so
varying the AVC voltage.


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