[Collins] 75S-1 Problem

[Dr.Gerald Johnson]

Lets look for other symptoms. A Collins S-line is always STINGY on the S-
meter reading. Needs a KW next door to get to full scale.

Aligning the first IF without the swamping network (two whole parts and 
two microgator clips aren't that hard to have!) can have ended up with 
gobs of gain at mid band and little at the band edges.

Ordinarily the IF stages are running full tilt to get the recommended gain, 
I've never seen a great need to decrease the gain. Running lots of gain 
means lots of current and the 6BA6 lifetimes are on the short side. They 
often fail from grid emission. Most checkers don't check for grid emission. 
It shows up as a drifting negative s-meter zero.

The S-meter circuit uses the gain controlled stages as a DC VTVM for 
AVC voltage. Any wild resistor drifts or changes in those stages could 
upset that bridge. Not controlling all the stages (loosing all but the stage 
with the meter in the screen) could cause a lot of AVC voltage for a 
given signal trying to get some AVC control. In a receiver with a switch 
to shut off AVC, the detector AVC voltage runs wild too.

AVC in the S-line is generally very good, very little variation in audio from 
the weakest copyable signal to the neighbor next door. An AVC that 
goes wild and pegs the meter hints that there should be a poorer quality 
AVC and probably some audible distortion on strong signals because 
the AVC can't do it all with just one stage.

Time to begin checking the less obvious. First check voltages around the 
S-meter circuit stages with a VTVM. Look for darkened resistors. Then 
check each resistor with an ohmmeter.

Could the meter movement have been changed for one with more 
sensitivity like 500 or 100 microamps? Certainly movements with those 
sensitivities have been made that would interchange with the original 1 
ma movement. Unless you have a digital VTVM that uses very low 
current in the ohmmeter circuit, checking a meter movement can be 
fatal to the movement from excess current out of an ohmmeter circuit.

Check the bias supply voltage, especially in the circuit around the RF 
gain control. Does the s-meter go past full scale when you back off on 
the RF gain? That might be another hint of a wrong meter movement.

Is the selenium bias rectifier still in use? That could make the bias voltage 
low as forward drop increases a great deal with age.

The signal generator set up wants a SERIES 50 ohm resistor according to 
the 75S-1 manual.

73, Jerry, K0CQ, Technical Advisor to the CRA

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