[Collins] 75A4 AGC distortion

[David L. Thompson]

The Collins 75A4 had a fairly good product detector but lacked the proper
set up for AGC/AVC that became incorporated in later Collins and especially
the Drake R4 series.  Bob Stankus W2VCZ and later K4NV (now SK) had mods to
bring the A4 more current in AVC action.  I do suspect that Collins made
improvements in the later A4's to save riding the Rf gain control.

I have a number of older receivers that have the same Rf gain problem.  The
HQ 170 did have several settings for AVC but still the pumping could get to
you after several hours of operating. The RME 6900 grounded the AVC on CW so
you had to ride the Rf gain.  The mode switch had AVc settings for SSB and
AM (slow for SSB) but loud SSB signals do blast at times.  When I got a
Drake R4A in 1965 SSB tuned in just like AM on the A4....sure was nice on
the ears especially when operating long hours on SSB or CW.  I never did own
a 75S series receiver but I have borrowed a 75S1 and its close to the R4.

The A4 was a definite improvement over the 75A2 with product detector.  A
local ham had the A2 with collins approved product detector and I finally
had to run the Rf gain at half open
so my headphones would not pop off when operating CW.  He used it to call
the Army MARS net just below 80 meters and I also noted it was noisy
something that had been improved in the A4.

73 Dave K4JRB