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

[David Vest]

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