[Collins] 75A-4 Recap

[Bonddaleena at aol.com]

In a message dated 5/21/2006 10:23:36 PM Eastern  Daylight Time, 
geraldj at ispwest.com writes:
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

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Dr. Jerry has good advise, as usual. I use his high voltage methodology for  
testing caps and it works easier and better than my considerable collection of 
 cap testers. I also found several bad mica caps in mine.
It is always interesting to see a Black Beauty read perfectly correct (for  
it's value) on my NIST traceable cap bridge, but leak 350 volts of 400 volts  
applied to it! I don't even test old caps any more, I just change them to 
Orange  Drops.
The old girl performs excellently, now.....
 
 
ron
 
N4UE