[R-390] R390 Re-cap kit

[Charles Steinmetz]

Bob wrote:

>We will have to wait and see if he can do this kit, and if he will.

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