[Collins] 75S-3 S-meter acts oddly; also STBY/OPR no difference

[Dr. Gerald N. Johnson, electrical engineer]

When interconnected with a transmitter, the receiver has to be in STBY
to let the transmitter mute it. OPR overrides the transmitter muting. If
not connected to a transmitter, then not muting in STBY can be a hint
that there's a short in the mute connector or circuit.

The S-meter pegging is probably normal at power up, especially if the RF
gain is backed way off. The RF gain control injects a lot of negative
voltage into the AGC line and the S-meter is just a simple one tube VTVM
(giving a second use to an IF tube) showing AGC voltage. No S-meter
reading on signals can mean no AGC and should lead to distorted signals
from too much signal at the detector or last IF stage. No AGC can come
from a bad tube (detector) or excessively leaky black beauty AGC bypass
and time constant capacitors or from the AGC being turned off manually.
At least some of the S-line had provisions for turning AGC off.

Be sure the S-meter is not deflecting negatively. That is a sign of an
IF tube (or more than one) driving the AGC line positive from grid
emission. Cured by tube replacement.

73, Jerry, K0CQ, Technical Advisor to the CRA
-- 
Entire content copyright Dr. Gerald N. Johnson, electrical engineer.
Reproduction by permission only.