[R-390] Caps

Thu Apr 25 18:20:26 EDT 2013

There are definitely two camps on this...and that's ok..because I only have to look after the radios I own and not everyone else's.  I will say that I am from the "replace all the paper caps" camp.  But to be fair I have to state that it is not as a troubleshooting method...neither is my replacing resistors that are out of spec. But it does in many cases result in improved performance.  I can honestly say that I have never found a paper cap that I removed from an R-390A that didn't fail a leakage test at rated voltage on my trusty Sencore cap analyzer.  Now before you reach for that enter key... I know many are used in circuits that don't subject the caps to rated voltage...but I have to believe that if the cap has degraded to the point that it fails at rated voltage how long will it continue to function properly before it degrades to the point that it fails at operating voltage.  It is a progressive disease.  They are slowly changing from caps to resistors.  

On the subject of resistors...those are often overlooked and are every bit as important to the proper function of the radio as the caps...and maybe more so.

And just for the record...all my radios didn't come out of the St. Julian's Creek pile...this effects all radios of this vintage that use the same quality of parts..

The notable exception to my "change all paper caps" rule is the R-390/URR.  It used top quality parts and rarely needs its paper caps replaced...I never do wholesale change outs in those..it's truly not proven to be necessary....that's not to say they don't fail...just not often.

Cecil
K5DL
Sent from my iPad

On Apr 25, 2013, at 3:57 PM, Mark Johnson <mvjohn at sympatico.ca> wrote:

> 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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