[Collins] 75A-4 Recap

[Dr. Gerald N. Johnson]

On Sun, 2006-05-21 at 16:03 -0700, Kenneth Lopez wrote:
> Howdy folks,
> 
> Well, I've just acquired another 75A-4, just after selling mine.   
> These things seem to follow me home somehow!  I guess I'll have to  
> keep this one...
> 
> Serial # just over 1000, nice physical shape, some functional problems.
> 
> First, where would one mount the blocking cap to take the B+ off the  
> filters? It is noted as C 144 on a later schematic.  This A4 is early  
> enough not to have it installed.  What type of cap should I use here,  
> disc ceramic?

Disc ceramic or dipped silver mica would work well. I don't know the
details of where to mount the capacitor.
> 
> Next:  I've seen the list of the "7 deadly caps" and note also the  
> need to replace the Black Beauties.  Which others should I change?

All the black beauties for sure.
> 
> What is the preferred type of cap?  Sprague Orange Drops OK for the  
> black beauties?

Sprague Orange Drops are super for replacing the black beauties.
> 
> It is working, but has pops, hum, etc.

Pops can come from black beauties. Hum can come from electrolytics or
from tubes. If 120 Hz saw tooth, its from electrolytics, if 60 Hz
sinewave its heater to cathode leakage in tubes.

>   Probably need to replace the  
> HV mutli-section can.  I see one at Antique Electronics.  Expensive,  
> but the only ones I could find.

Good electrolytics are those just recently made or formed. NOS
electrolytics can be poorer than those in the radio.
> 
> As I dig further I'll report nasty symptoms.

After you replace the leaky paper and mica capacitors and the aged
electrolytics, you should have few problems.
> 
> Lastly, I am open to suggestions and guidance to bring this rig back  
> to spec.
> 
> 
> Cheers,
> 
> Ken N6TZV

-- 
73, Jerry, K0CQ, Technical Advisor to the CRA
All content copyright Dr. Gerald N. Johnson, electrical engineer