[Collins] 75A3 Problem

[Dr. Gerald N. Johnson, electrical engineer]

All receivers of the A3 era (e.g. made with diode detectors only) tend
to not copy SSB well with the RF gain full up. That's probably because
the AGC has too slow an attack and too little hang. The standard
operating technique always has been to reduce the RF gain.

Your receiver's AGC may be a little weak, leading to the IF overdriving
the product detector, because of leaky AGC bypass and time constant
capacitors. If they are originals and are oiled paper (molded papers
tend to be the worst offenders) the ARE leaky for sure and need to be
replaced with Orange Drops to cure their leakage. Leakage that won't
show on an ordinary ohmmeter is more than enough to upset the AGC.
That's also true of the audio coupling capacitors through the audio
stages. They can easily up set the output stage bias with a leakage of
50 megohms that will only show that low a value with 200 volts DC
applied.

The fact that your receiver motorboats at high audio gain settings shows
that the electrolytic filter capacitors are on their way to open. They
have high series resistance from drying out. A normal mode of failure
for electrolytics, which don't age well without applied voltage. The
filter capacitor closest to the audio output tubes is the main culprit
when a receiver motorboats. Replace them all to save later embarrassment
of them failing violently and filling the chassis with shredded foil,
conductive crepe paper, and conductive electrolyte.

73, Jerry, K0CQ
-- 
Entire content copyright Dr. Gerald N. Johnson. Reproduction by
permission only.