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

[Dr. Gerald N. Johnson]

On Fri, 2007-05-11 at 12:45 -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 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.

Reducing the current in the variable helps it last longer and makes its
adjustment less sensitive, so long as it actually makes it to zero.
> 
> 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.

Note that the S-meter bridge is dependent on the screen current of the
first IF stage controlled by AVC and that RF-gain adjustment so the RF-
gain adjustment has to be made before zeroing the S-meter.
> 
> 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
> 
So the S-3/3A circuit sounds like it is the same as the S-1/2?

A beauty of the later circuit is that only the meter current flows in
the wiper which adds to its longevity. It does take a different value
which allows replacement with a plastic or ceramic element to give
better longevity.

Watch out for the meter zero drifting negative. That comes from grid
emission from AVC controlled tubes, most often (in my experience) from
the 6BA6 IF tubes that are run hard to achieve adequate receiver gain
though it can come from any tube getting grid emission that is on the
AVC circuit.
-- 
73, Jerry, K0CQ, Technical Advisor to the CRA
All content copyright Dr. Gerald N. Johnson, electrical engineer