[Collins] 75S-1 Problem

[Dr.Gerald Johnson]

My 75S-1 manual says nothing about setting S-9 at 100 microvolts. It does 
demand a 50 ohm resistor in series with the signal generator. That will 
reduce the signal to the receiver. Then it says to apply 1.5 microvolts and 
set the IF gain to show a just detectable increase in AGC voltage when 
monitored using a VTVM.

I don't have a great trust in the output calibration of the URM-25. I have 
one. It has many black beauties.

The receiver could have high gain if there was something besides the 
original tubes in RF or IF stages, like 6GM6 in the RF tube socket or if the 
AGC bypass or time constant capacitors were leaky. IF tubes with grid 
emission also counteract AGC voltage, but they tend to make the S-
meter zero setting drift negative as the tubes heat up. Receiver gain 
might be high and the RF gain control relatively ineffective if the bias 
rectifier (CR-3 a selenium device) or its filter capacitor are bad.

73, Jerry, K0CQ

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Entire content copyright Dr. Gerald N. Johnson, electrical engineer.
Reproduction by permission only.