[Collins] 75S3......odd discovery in s merer circuit

[Dr. Gerald N. Johnson]

On Thu, 2007-05-10 at 11:31 -0500, w9hak wrote:
> I have owned this receiver for about a year now. I've completed the, 
> "Dennis Brothers", recommended modifications and touched up the 
> alignment. Works great.
> 
> I did find a non Collins resistor, (27 ohm), from the positive side of 
> the s meter to ground. Apparently someone in the past history of the 
> receiver installed this resistor. I am guessing they  were dealing with 
> an s meter reading issue. The resistor would lower the  plus voltage at 
> the positive terminal of the meter. This  lower plus volts would cause 
> the meter go towards zero. Additionally, it would lower any s meter 
> reading with a given input microvolt setting.

I don't have the schematic of the 75S-3/3A, but I do have books on the
75S-1/2 and 75S-3B/C. Those are different in the circuit arrangement
which parts are variable. The S-meter uses a resistive bridge to sample
cathode current of an AGC controlled IF stage. Essentially its using the
IF tube in a single tube DC VTVM circuit. So there's a zero adjust pot
that in the 1/2 affected the IF gain which was not the case in the 3B/C.
The S-meter zero doesn't change the tube operating conditions in the
later receiver. So to get the meter to zero, the circuit has to put the
same voltage on both ends of the meter as the meter is the bridge
circuit indicator. My guess is that the S-meter zero pot is very noisy
or bad at its original setting and the 27 ohm resistor moved the zero
setting to a good part of the pot, though its quite possible some other
resistors in the bridge have drifted in value, particularly those with
screen or plate voltage across them and dissipating power over half
their power rating.
> 
> Just wondering if this is a systemic issue with the s meter, or another 
> case of original component values changing? And then treating the 
> symptom instead of effecting a repair.

I don't think its systematic, likely that worn pot or resistors drifting
a lot in value and yes the added shunt low value resistor is treating
the symptom, not doing a repair.
> 
> Smith Bradford
> W9HAK

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