[Collins] 75A4 RF Gain Action

[Dr. Gerald N. Johnson]

On 5/26/2013 2:58 PM, Wally Klinger wrote:
> Hi All:
>
> Working on a 75A4 that seems to be low in sensitivity and no
> indication on the S Meter.

An improper RF or IF tube can upset the gain distribution and the 
resulting AGC voltage. The S-meter circuit essentially duplexes an IF 
tube to act as a DC VTVM while acting as a gain controlled stage. In 
most Collins receivers the RF gain control diode gates a negative 
voltage to the AGC line.

A higher gain RF tube will control the gain faster than the IF tubes and 
prevent developing rated AGC voltage from a signal and so limit the 
s-meter reading. A tube with less remote cutoff than the 6DC6 will do 
that too.
>
> Looking at V8, trying to capture voltages on the pins.  When I lower
> the RF gain control, the meter does not deflect like my other A4
> does.  I can adjust the RF gain to deliver -5.7 volts on the meter,
> but it calls for a S9 meter reading which I can not generate.

The z-meter zero and adjust pots have been know to fail open and those 
would prevent the meter from indicating.
>
> Aside from the tube, any other parts here at this stage I need to
> troubleshoot, such as C75 the coupling cap, or C77 on the cathode?

Capacitors can upset the circuit, I presume you have replaced all the 
black beauties to save on troubleshooting?

AGC controlled tubes can develope grid current and counteract the AGC 
voltsge from the detector and RF gain control.
>
> Or any other ideas?
>
> 73--Wally W9BEA
>
73, Jerry, K0CQ, Technical Adviser to the Collins Radio Association.