[R-390] Caps

[Mark Johnson]

Charles..

I'm  always hesitant to comment on these "replace the caps" email threads; between the different Radio Reflectors there are hundreds of them, that would have a person "shotgun" cap changes because it is a widespread practice and I feel I'm going against the generally accepted view point. 

Your last paragraph is bang on at least with me. I think there is a lot of unnecessary cap replacements that take place mostly out of fear and because a lot of people use this as a trouble shooting approach vs developing a knowledge of how the radios work and learning good troubleshooting skills. In other words, if you can't find the bug, replace all the caps. 

To your point, I think the risks of creating further problems are huge. You could replace a cap incorrectly, cause a solder bridge, damage other components, etc. And of course, my favorite, the tendancey to "tack" solder in the replacement. 

My R390 is still running along fine just the way she came from Collins, exception being the old rectifier that provides DC for the stdby circuit. 

Personally, when I buy a boat anchor radio and the seller tells me he's re-capped the radio, I tend to offer less money as the job is usually not done correctly and I have a lot of work to do to back out the ugly work. 

Just my thoughts. Thanks for your comments Charles. 

Mark 
VE3LU/VE3DJU

> Date: Thu, 25 Apr 2013 16:11:35 -0400
> To: R-390 at mailman.qth.net
> From: charles_steinmetz at lavabit.com
> Subject: Re: [R-390] Caps
> 
> John wrote:
> 
> >I had planned to replace most of the caps in my '54 Motorola R-390A 
> >with 400WVDC Panasonic PP metalized film caps.  Although 
> >conservatively I was going to use a 600WVDC PP film cap "Sprague 
> >OD"  for the filter coupling cap.  Has anyone experienced failures 
> >with caps such as these used for replacement purposes?
> 
> Yes.  I've tested several types of Panasonic PP caps and found 
> unacceptable self-healing events.  If you insist on using metallized 
> film caps, the Panasonic ECWH(A) series of 800 volt parts is about as 
> good as you can do.  But why, when it is just as easy and only a 
> little more expensive to use film-and-foils?
> 
> And again, for the vast majority of caps in any boatanchor -- power 
> supply bypass capacitors -- why would one use film caps at all?  1 kV 
> disk ceramic caps perform better in this application and are 
> extremely reliable, besides being small and therefore easy to install.
> 
> If one is going to invest all the time and effort to replace lots of 
> caps, it seems only sensible to use the best parts for the job unless 
> they are prohibitively expensive.  And neither film-and-foils nor 
> disk ceramics are.
> 
> Finally, why shotgun the caps in a 390A in the first place?  There is 
> no pattern of general failures in 390As (unlike Hammarlund SP-600s 
> and other BAs that are notorious for cap failures).  Remember, by far 
> the greatest number of lifetime failures of capacitors (as well as 
> most other electronic components) happen in the first few hundred 
> hours of operation.  You are at least as likely to install a cap that 
> fails sooner than the one you removed as you are to extend the time 
> to failure.  Of course, if the particular radio has a history of cap 
> failures, it could indicate that the manufacturer was using a bad 
> batch of caps when the radio was built, or that the radio has 
> experienced environmental stress in its lifetime, in which case 
> wholesale capacitor replacement may be indicated.
> 
> Best regards,
> 
> Charles
> 
> 
> 
> 
> 
> 
> 
> 
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