[Milsurplus] Rethinking Re-Capping

[David Stinson]

Rethinking Re-capping.

Concerning the restoration of
historically-significant radios: my views and
goals have evolved over the years.  My goal-
personal fulfillment from answering the challenge
of these sets aside- is to give the radios the
best chance to be preserved beyond my time as
their temporary care-taker.  To do so, I believe
the radios must play, and should do so with the
minimum disturbance of the original assembly
possible.

There was a time when I would say "replace very
electrolytic and every paper and every Micamold"
as a matter of course.  I no longer do that.  Been
working on a set of three RAX receivers that have
been awaiting their turn in my barn and storage,
for at least 12-15 years.  All have been playing
12-24 hours now without failure.  One needed no
cap replacements at all.  The second needed one
and the third needed two.  I changed no resistors.
Did repair some bad trimmer caps in two receivers,
but not counting them for this post; different
issue.  To find those that actually need te be
changed requires a reliable schematic, a good
meter, a variable B+ supply, patience and a finger
tip.

In a tube type receiver such as the TCS, SCR-274N
or RAX, there are several paths that B+ travels to
ground.  Most of them go through tubes.  When the
tubes have no filament voltage, the tube plate or
screen pin is a "dead end;" an open circuit.  No
current should flow through the tube's plate or
screen circuits.
But there are usually other paths- like
screen-voltage dividers- that provide a path for
some current flow.

If you spend some time with the schematic, you can
trace all the B+ distribution busses, looking for
paths to ground.  For instance- in the -274N
receiver, two 7000-Ohm resistors are connected
between B+ and ground as a screen-voltage divider.
With the tube filaments off, the receiver out of
the rack and with no local control box installed,
the only path for B+ to ground is through those
two resistors, unless there are leaky bypass caps.
So, with no filament voltage, if we connect our
variable V+ supply and increase to 100V, we should
observe no more than  100/14,000= 7 milliamps of
current flowing.  If we have more, we have leaky
capacitors somewhere.

So how do we find the leaky cap(s)?  Did this test
on one of the RAX receivers.  After allowing
"reforming" time for the electrolytic caps (and no
filament voltage), at 100V on the buss, it was
drawing 20mA.  100V was dropping somewhere and at
20mA, the leak was turning that current into 2
Watts of heat.  That doesn't sound like much, but
it builds-up quickly.  After a few minutes, a
finger-tip on one of the .05 uFd screen bypass
caps felt the heat and the ZM-11 proved it was the
villain.  After replacing that cap, the current on
the B+ buss dropped to 6 mA; just 2 mA above that
expected and close enough to operate.  Left the
100V on the buss for four hours and no increase in
the current.  That's not a promise no other cap
will fail- they are 70 years old.  But if the goal
is to do the least possible and given I'll be
running these at reduced B+, the risk of damage is
small and acceptable.  If one can't "feel the
heat," a study of the B+ distribution on the
schematic will reveal places one can lift a wire
or part to isolate the branch with the bad actor.

In many radios (not in RAX but in others), there
are coupling caps that go to grids which can be
leaky.  Apply B+ with no filament voltage as
before.  Measure the grid of the tube fed by the
capacitor coupling.  If you have a positive
voltage on that grid with no filament voltage,
change the coupling cap.  NOTE: Use a VOM for
this, not a VTVM.  The very-high input impedance
of the VTVM may be "spoofed" by an electro-static
charge on the grid side of the capacitor plate.  A
good VOM will load it just enough to give you a
"real" reading.  Exception:  Leaky cathode
bypasses, especially the one at the Audio PA, need
to be checked individually, since there's no
current path to find them without filament
voltage.  Quick check:  Calculate the current that
should flow through the cathode resistor if you
put +10V on the tube cathode.  i.e. if the cathode
resistor is 330 Ohms and you connect +10V to
ground to the cathode, you should see 30 mA of
current flow.  If (after a reasonable "reform"
time) it's pulling more than 30 mA, your cathode
bypass is leaking.

But what about OPEN bypass or coupling caps?  With
the danger of frying things from bad bypasses and
coupling caps past, one can power the set and
check performance.  If you have open bypasses or
coupling caps, *the circuits will tell you.*
Oscillation in IF amps or Converters usually
equals open bypasses.  A screwdriver touched at
tube bases will often change the "motorboating"
and tell you which stage is oscillating, or use a
scope.  If the oscillation changes with tuning,
the Mixer/Converter stage is oscillating (this
happened with one of the RAX receivers- open
cathode bypass on the Converter).  If the audio is
distorted, look for open bypasses in the 1st Audio
and Audio PA.  If you have no gain through a
stage, check any coupling caps that might be
involved.   But we shouldn't assume in these cases
that capacitors are the culprit; resistance and
voltage chart checks are your friends.  For
instance- I've revived three TCS receivers lately
and I find wonky resistors to be an even bigger
problem than capacitors.

With these techniques, I have three RAX receivers
on the bench playing nicely for hours.  I've
changed three capacitors, no resistors and one
tube.  Did also repair some trimmers as noted
before.

GL OM ES 73 DE Dave AB5S