[Collins] 75A-4 Recap

Eugene Hertz ehertz at tcaf.org
Mon May 22 21:35:59 EDT 2006


On the subject of recapping and testing. I have a basic question.  I have a cap tester that will show leakage in uA for caps tested up to 600v (sencore lc53).  I have heard that ESR is what one should be looking for, somehow its a more accurate or telling measurement.

My question is, can't leakage and ESR be equated? For example R=V/I. Couldnt ESR be somehow derived from the leakage? Could ESR=600v/uA ? 

Anyone shed some light on this subject?
thanks
Eugene




>-----Original Message-----
>From: Bonddaleena at aol.com [mailto:Bonddaleena at aol.com]
>Sent: Monday, May 22, 2006 02:39 PM
>To: geraldj at ispwest.com, collins at mailman.qth.net
>Subject: Re: [Collins] 75A-4 Recap
>
>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 
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