[Collins] Re: {Collins} 75S3......odd discovery in s meter circuit......solved

[w9hak]

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 negative 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.

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.

Smith Bradford
W9HAK

Ok here is the scoop....

Number one item, the non Collins 27 ohm resistor is attached to the 
negative side of the s meter, not the positive side...
As /Dr. Gerald N. Johnson suggests it may have been installed to 
reposition the wiper of R13. So that control has been cleaned and seems ok.

A systematic inspection found R57, (rf gain pot), open at the very end 
the resistive element. For the time being I have reversed the leads, and 
not using that end of the resistive element.

I am seriously thinking of rewiring the 75S3 to duplicate the wiring of 
the 75S3 B/C in the s meter circuit. It would eliminate one pot and a 
few small parts. In addition there would be no gain change in the second 
i.f. amplifier when setting the s meter zero adjust, small as it might be.

Smith Bradford
W9HAK


/