[Collins] 75A-4 S Meter Pot Mod Hardware

[Dr.Gerald Johnson]

S-meter working down scale and going negative sounds to me as if the 
IF tube that is also the DC VTVM (meter in the screen circuit) for AVC 
voltage is developing grid emission. That puts a positive bias on its part 
of the AVC line because that impedance is a few megohms. Switching 
the AVC off discharges the large time constant capacitor that has 
been charged by that grid current from the grid emission. The cure for 
grid emission is a new tube. It comes from cathode material having 
splattered to the grid and then the grid getting hot from radiant heat 
of the cathode. It can be other tubes with AVC control also but when 
its the meter tube that voltage change can be on the grid end of the 
isolation resistor to not quite move the AVC line, just to let that tube 
grid go positive.

Though if the AVC line voltage doesn't change when checked with a 
VTVM, the problem might be drifting resistors in that same stage, 
though turning AVC off shouldn't cure them.

Most meter checks of capacitors are not critical enough for AVC 
capacitors, you need leakage up in the range of 100 megohms or 
better and most meters don't go that high.

Mouser has RV-4 molded pots that are good. I have no idea what the 
bracket should look like but do have sheet metal tools to make some if 
badly needed.

73, Jerry, K0CQ, Technical Advisor to the CRA

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