[Collins] 75s-3 RF gain and muting problem

[Gerald]

On Wed, 2005-11-30 at 12:41 -0500, David Vest wrote:
> I have run into a strange problem with my 75S-3, really two but I believe
> they are related.  The RX will not mute, and my first thought that there
> must be a short to ground in the mute line, what I found were 12.5 k ohms to
> ground which makes taking a look at the schematic.  I can make the receive
> mute by applying bias to pin 9 of the V3 (1st mixer), yes the rx is in STBY.
> Check the resistor values in the circuit and all appear to be within
> tolerance.  What it appears to be is the bias is never getting to V3, and
> checked the wiring and thinking about pulling one side R58 (330 ohm to
> ground where the mute jack and bias voltage meet up) wondering if this
> stands to reason and if anyone else has seen this type of issue in the
> 75s-3.
> 
>  
> 
> On the other problem if I turn the RF gain control (front panel) back about
> 25% the receiver goes into cutoff and the s-meter goes to the right hand
> peg, makes no difference if in STBY or OPR as well the AGC setting has no
> affect on either issue.
> 
>  
> 
> Any thoughts from the group?
> 
>  
> 
> Thanks,
> 
> Dave
> 
> _

Receiver bias supply. The RF gain and the muting comes from a negative
bias supply. Some vintages of receiver probably used a selenium
rectifier for that bias rectifier (a little rectangular block from
Sarkes Tarzian) and it goes bad from age. My S3B schematic says the bias
voltage across the filter capacitor should be -65 volts.


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