[Collins] 75S-1 Problem (update)

[Dr.Gerald Johnson]

Tube elements are known as cathode, control grid, screen grid, suppressor 
grid and plate in a pentode going from cathode to plate (anode) in that 
order. There's no mica on the screen grid, its on the control grid.

There's not much effect on the tuned circuit from that grid coupling 
capacitor because at the extremes its just connecting and disconnecting 
the few pf of the tube grid. Be sure its not the tube changing.

There were two types of postage stamp micas, and they date from half a 
century before dipped silver micas. If their case is tan or brown they are 
made from sheets of mica and sheets of foil. They depend on the pressure of 
the molded case for stability. If the case is red, they are silver mica where the 
electrodes are painted on the mica sheets which means virtually no 
uncontrolled air gaps between the electrode and the insulator which 
generally means better stability. The can still depend on the pressure of the 
molding to make contact between stacks of silvered mica.

The more recent construction of dipped silver mica depends on brass clips to 
hold the stack together and to make contact for the wire leads (and those 
brass clips are what make the ends a little fatter than the middle). Then the 
assembly is dipped in the coating, without applied pressure. Sometimes the 
dipping process is too rapid and air bubbles end up inside and that make for 
capacitors with jumps in incremental value.

Often an intermittent can be forced by alternated heat and cooling though 
it doesn't begin to act like its temperature sensitive. Intermittent connections 
can be sensitive to vibration or to minute length changes of components 
and that's where heat and cooling can force those minute dimension 
changes. Its a lot easier on a circuit to run a few heat and cold cycles than 
to unsolder and replace parts.

My 9th edition 75S-1 manual shows 1K for the IF Gain potentiometer. It shows 
some parts changes in the PTO on an addendum sheet.

My Rockwell 32S-3 manual shows that 200 pf to be a dipped mica. A 
DM15E201F-300WV 1% tolerance.

There is no ALC in a receiver. That stands for Automatic Load Control in a 
transmitter. The receiver has Automatic Volume Control or Automatic Gain 
Control.

Watch out for that audio coupling capacitor to the output tube grid. When 
it leaks (and if its still a black beauty, IT LEAKS) it counteracts the fixed bias 
applied to the output but and makes it run even hotter.

I suspect one of those IF tubes wasn't a 6BA6.

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