[R-390] Capacitor Replacements Vitamin Q

[Tisha Hayes]

Even the best capacitors of the day in 1960 were quite a bit different than
some of the high rel component choices we have now, unfortunately most
capacitors today are not rated for 600 volts.

The "Orange Drop" type capacitor has developed a near legendary status as a
replacement into BA gear. There is some justification for this as it is a
fine capacitor for some applications.

Some of the downsides of the Orange Drop;
1) it is big, this is not your father's ceramic disk
2) it can be expensive

Newer is not necessarily better. Look at how many computer and radio vendors
got bit by the surface mount caps in everything from PC's to a Motorola
Spectra. They die a slow death by suicide yet they had really nice looking
temperature and MTBF specs from the original manufacturer. I consider the
problems with those capacitors to be in the same boat as the "Black Beauties
(sometimes brown) tubular caps of the 40's and 50's.

Electrolytic capacitors have their own issues, they begin to age the minute
they roll off of the manufacturing line. You can store them in a lead lined
vault, you can reform them (sometimes) but eventually the electrolytic (or
oils) inside of the capacitor gradually dries out or evaporates. Nowadays
finding a octal base multi-section can capacitor that is "new" may be
impossible for some values. Some of us end up restuffing the old capacitor
cans or making other accommodations to get a 20-30 or 40 uFd capacitor into
that space.

Entropy kicks in and even begins to get to the power transformer and chokes.
What may be a buzzy or hot transformer today may be an open or a short when
eventually that mechanical motion pulls off a lead or wears down the varnish
on the windings.  Look at the f aux pas with the Collins mechanical filters
and their magical foam that turns into a black goo? RF slugs that get stuck
in the coil forms or the way overloaded on/off microswitch.

If there was a modern redesign of the R-390A do you think that it would use
those unobtanium min-BNC connectors or even the 125 ohm twinax balanced
antenna input?

Trying to stay at least somewhat true to the original design is what makes
it an R-390 or R-390A. That is not to say that there are not freakish
variants out there with 8 ohm audio outputs, some substitution for the PTO
or a different filter setup. Someday the last 6DC6 will wear out and end up
being replaced with a FET.

The suggestions on not making a wholesale replacement of any component type
is based upon the ugly lessons other folks have learned by going down that
road.
-- 
Ms. Tisha Hayes/ AA4HA
----------------
"I'll be a diode, cathode, electrode
Overload, generator, oscillator
Make a circuit with me." -- The Polecats