[Collins] 75A2

[Dr.Gerald Johnson]

In general, receivers use an IF stage screen circuit to run the S-meter. They are 
using that IF stage as a simple DC VTVM of the AVC voltage.

When the S-meter fails to work that says that particular IF stage isn't seeing AVC 
voltage. So its necessary to trace the AVC from the detector through the filter 
capacitors and time constant capacitors and AVC bypass capacitors to the 
grid off the controlled stages. 

Leaky capacitors, open resistors, an AVC OFF switch, and tubes with grid 
emission can remove AVC from some or all stages depending on where they are 
in the circuit. With no AVC showing on the stage with the S-meter, there should 
be lots of AVC developed at the detector. Then it can be a matter of circuit 
tracing.

Tubes with grid emission tend to drive the S-meter zero negative relatively slowly 
as the tube warms up. Grid emission comes from cathode material having 
sputtered onto the control grid and as the cathode heat warms the control grid 
that emitting material starts to act like a cathode. A few tube checkers notice 
this as leakage, most ignore it. There are some specialized tube grid circuit tube 
checkers and I've know tube type TV servicemen to rely on those for finding 
more faults faster than the fancier tube testers and to fix TV sets faster. The grid 
of a tube with grid emission will tend to go positive while connected to ground or 
to a negative AVC circuit through a large value resistor, typical of AVC circuits.

If the AVC isn't working, there should be distortion of the detector output as the 
last IF stages get driven to clipping.

73, Jerry, K0CQ, Technical Advisor to the CRA

-- 
Entire content copyright Dr. Gerald N. Johnson, electrical engineer.
Reproduction by permission only.