[R-390] capacitors and suppliers

[rbethman]

Consider the issues that we went through.

Bob - N0DGN


Charles wrote: (3/2/2014)

If someone is going to the trouble to put together a re-cap kit, PLEASE 
get 1kV ceramics (X7R or X7U) for all bypass capacitors, and 600v 
film-and-foil (NOT metallized film) caps for the few coupling caps (for 
example, genuine SBE/Vishay/Sprague Orange Drop P225 Series -- but watch 
out for metallized film "orange dip" imposters sold by some vendors). 
Also use film-and-foil caps for the AGC capacitors, but you don't need 
600v parts there -- 100v is fine.

The ceramics do the job much better than any plastic cap in bypass 
applications, they are much easier to squeeze into the tight spaces in a 
390A, and they are much less expensive as well.  Some of the money saved 
can then be invested in getting high-quality film-and-foil caps for the 
non-bypass applications without the total cost getting out of hand.  And 
the radios so equipped will have the best parts available for each spot.

A kit should also come with 10 feet of teflon (and only teflon) 
spaghetti tubing to fit the capacitor leads.

All that said, I am generally NOT a fan of replacing capacitors 
shotgun-style in a 390A.  I have looked after several hundred of them in 
my day, and am still in touch with the owners of many of them.  With a 
few rare exceptions (notably, radios that had been underwater for a time 
or were otherwise badly abused, and a very few that seem to have been 
built with a bad batch of caps), the 390A does not seem to suffer from 
serial capacitor failure.  So, I believe the best approach is to replace 
the few known troublesome caps and then just run the radio.  If you have 
two or three cap failures within a few years, then you might conclude 
that you have one of the rare, failure-prone exceptions and consider 
wholesale replacement.

The above does not apply to the electrolytic filter caps, IMO -- at the 
first sign of trouble, I'd replace all of them with brand new, 
high-reliability, high-temperature aluminum electrolytics.  At this 
point (2014), the best commonly available (in the US) high-voltage 
aluminum electrolytic caps seem to be the United Chemi-Con "KJX" series. 
  For low-voltage applications (<= 50v), the United Chemi-Con "EKZM" 
series seem to be the best.  Mouser has both, as most of the major parts 
distributors probably also do.

In brief response to those who may ask, "Why not shotgun the caps -- 
there's nothing wrong with improving the radio, is there?":  To change 
the caps in a 390A, you are working in very tight quarters, and some of 
the caps are attached to standoff terminals that are quite fragile and 
hard to obtain these days.  It is almost certain that a person with 
average electronic construction/repair skills will burn a bunch of wires 
with the soldering iron and break a few standoffs in the process of 
replacing all of the caps in a 390A, as well as knacker a few other 
things along the way.  "If it ain't broke, don't fix it" is a valuable 
piece of advice taught by those with lots of experience.

Best regards,

Charles