"Scott Dorsey" wrote in message
...
wrote:
I=92m just about finished refurbishing a Heathkit SB-401
amateur
transmitter that has, according to the manual=92s parts
list, five 0.2uF
=93resin=94 capacitors. What are these things and how
reliable are they?
Does conventional wisdom dictate that such caps should be
replaced?

They are called "resin-dipped" because of the outer
casing. You don't really
care about the outer casing, you care about what is
inside. If the dielectric
is plastic, keep it. If it's paper, replace it.

I=92ve searched the web for information on this, but only
get hits
concerning Black Beauty caps, which most always need to be
replaced.

Where are the caps and what will happen if they fail? In
a receiver IF,
a bad cap can take out an IF transformer, but if it's just
driving another
tube stage, the consequences of failure are minimal.
--scott
--

I'm not so sure paper dielectric caps are automatically
so bad. The key is to measure the cap, see if the capacity
is near what its supposed to be and if its lossy.
The casing can have an effect. For instance, the
notorious Black Beauty caps are often found with cracks or
even bits of the case cracked off. These have exactly the
same dielectric and construction as the Orange Drop caps,
namely paper impregnated with a plastic, the difference is
that the BBs are molded resin, not sure of what it is, and
the ODs are dipped Epoxy. OD's are pretty reliable where at
least early BBs are trouble. I think the problem with the
BBs has to do with moisture leaking into the body of the cap
but also the winding in the ones I've dissected were
distorted, perhaps by shrinkage. Curiously, BBs were sold
originally as deluxe capacitors for long life and operation
over an extended temperature range.
Old waxed paper caps should be checked but I've found
plenty which were still good. In general, where a paper cap
is used well under its maximum voltage rating it will have
long life.
The effect of a decoupling cap going bad may be to cause
feedback or to short the power to the element. They are
almost always used with a resistor to form an RC filter. I
don't see how a bad decoupling cap could take out an IF
transformer because its ahead of the transformer and will
not draw current through it. A bad coupling cap can put bias
onto the following tube grid (or transistor) causing high
distortion or other problems. Where an audio amp has
unusually high distortion coupling caps are always at the
top of the list of suspects. Coupling caps should have the
lowest leakage obtainable.
I would check the suspect caps with a bridge, or even
just with an ohm meter, to see if they should be replaced.
If they are not bad just leave them alone.


--
---
Richard Knoppow
Los Angeles, CA, USA