[R-390] Caps

[Charles P. Steinmetz]

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