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 identifiedthe 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 know 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 Orangedrops.
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.
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