[R-390] Recapping question

[Tim Shoppa]

N4BUQ wrote:
> Tim wrote:
>> Barry, am I too far off if I guess that the
>> resistors which drifted worst were the 3.9K's?

> Yes, the 3.9k plate resistors were both high by nearly the exact
> same amount (don't remember exactly how much).  They aren't
> charred nor are they 50% or  100% high, but are well out of spec.

I don't have a lot of statistics to back this up (having only a couple
of 390A's and a bunch of other mil-spec and consumer stuff from
the era) but to overgeneralize:

1: Triode stages seem to be a lot more likely to char and burn plate
and cathode resistors than pentode stages. In R-390A terms this means
the ones around 6C4's and 12AU7's.

2. The carbon comps that drift up the most tend to be in the low K-ohm
to 10's of K-ohm range. The others are not immune but the problem
is not as endemic there.

Finally:

3. Even though I have a vengance against parts that are 50% or 100%
out of spec, I cannot say that replacing them has often repaired
anything.
Most oscillators were still oscillating, most mixers were still mixing,
most
amplifiers were still amplifying before and after!

Same goes for most leaky wax/paper/electrolytic caps that were
measurably
or visibly bad: fixing the cap rarely fixes a problem that stopped the
radio from
working. The only exception is leaky coupling capacitors (some of
which were responsible for charring the plate/cathode resistor of the
next stage!)
Now getting rid of those crackling filter lytics probably was a good
idea anyway
but despite massive leakage they usually didn't actually cause things to
not
work (although hum and crackling of course improve after replacement.)

By far most repairs are effected by cleaning loose connections, fixing
cold
solder joints, and cleaning dirty switch contacts.

In transmitters things are often more clear-cut, because at least some
bad
parts (not necessarily the one for the fault!) char, explode, etc.
making
it more obvious, and usually plate currents in the power stages are
metered
and will blow a fuse if they go too high.

Tim.