Collins 75A-4 Receiver 7 Deadly Caps – Ripple
The 75A-4’s can be plagued with problems associated with low gain and audio popping or distortion. The source of these problems have been pin pointed to those interstage coupling capacitors which couple the plate of one stage to the grid of another.
In my trials to finish off the A-4 on which I have been toiling, I did a little research on the “classic” cap problem and identified the trouble areas. They are:
C-34 100pf (V-3 Pin 3)
C-52 4pf (V-5 Pin 7)
C-68 470pf (V-7 Pin 2)
C-75 470pf (V-8 Pin 1)
C-81 470pf (V-9 Pin 1)
C-101 .01uf (V-22 Pin 1)
C-104 470pf (V-21 Pin 1)
I later spoke with Butch, K0BS who added the following 3 capacitors:
C-71 1000pf V-7 Pin 6 C-71 is a known high failure item
C-95 .01uf V-11 Pin 6
C-96 .01uf V-12 Pin 2
The capacitors in the picofarad range should be replaced with dipped mica’s with 500 volt ratings. The .01uf capacitors are best replaced with 600V Sprague Orange-drops.
I also replace all the “black beauty” .1mfd capacitors in the A-4 with .1 mfd, 400V Sprague Orange drops. I believe there are 13 or 14 of them.
9 October 1997