I am in the process of restoring a Hallicrafters S-40A
and have posted a bit about this already. First of all, I
replaced all the paper caps. This is generally recommended
when rebuilding old electronic equipment. Most of these were
the original wax filled types. I measured these for
capacitance and dissipation factor. D is the inverse of Q
and is a way of measuring ESR but has a constant value. The
result was:
One cap a dead short.
One cap open
The rest had C much higher than the marked value and rather
high D, anywhere from about 0.2 to about 0.5, I think one
was even higher. There were a couple of molded plastic caps,
one a blue colored Cornell-Dubilier the other an Astron.
These were probably replacements. Both were close to the
marked C value with D of around 0.1, probably still good.
The paper caps were made by Industrial Condenser Co, of
Chicago and Sprague.
For comparison, new film caps measure right on the nose
for C and have D too low to measure on this bridge.
The measurements were made on a General Radio Type
650A, an oldie but goodie, which has been calibrated with
precision resistors and caps. The capacitance measurements
are made at 1.0Khz. If I get curious enough I may check the
RF impedance of a couple of these caps on a Q meter but I
suspect they are not very good.
While the high C value may not seem to be a problem I
suspect it may indicate some deformation of the capacitor
elements or some other problem. These were not remotely
precision caps when made but the values are so far off the
marked values that I suspect a change in the internals
rather than manufacturing tollerance.
So, the advice to shotgun paper caps seems to be right
on.

Another note, probably should be a separate post.
I mentioned before that the bandspread cap went the
opposite direction of any other I've seen in a Hallicrafters
RX. I got a couple of responses that others also had similar
S-40's. I found a couple of other oddities in my receiver
(for instance someone had connected the output of the BFO
directly to the detecto rather than by means of the wire
"gimmic" cap, it won't work this way). I am now pretty much
convinced that the band spread cap was modified to make it
"set" at 100 rather than the usual zero used in H receivers.
I was able to make it go in the right direction by crossing
the dial drive cable but I don't like makeshifts. Also,
despite the nearly symmetrical construction of the band
spread cap I suspect the minimum capacitance is slightly
different when its running in the correct direction. That
would affect the "normal" dial calibration. The RX
calibrates OK but I am still going to have a go at removing
the stop peg. I think I will have to drill it out but will
try something less "invasive" first.
BTW, I used to consider Hallicrafters stuff junk but
after rebuilding this set and an S-20R have changed my
mind. Actually both are well made and well designed.
Hallicrafters definitely built their equipment to fit
certain price ranges, mostly economy. I think they did quite
well in delivering good performance value at the prices they
aimed at. One proof is that the stuff is still working after
60 or 70 or more years.
The above really applies to the components too. While
paper caps have a bad reputation keep in mind that most of
the ones we deal with are very old and have worked well for
a long time. Plastic film was not available when these guys
were made and other types of caps, mostly tin-can oil-filled
types, were very expensive in comparison to the common wax
filled types. I am not sure they lasted any longer. One
exception is the oil-filled molded plastic Sprague Black
Beauty cap. These were made and sold as extra-high quality,
long life caps but Sangamo evidenty had manufacturing
problems, one of which was the way the oil filling tube was
sealed. These began failing within a couple of years of
manufacture where the "less reliable" wax filled paper caps
went along for many decades.


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Richard Knoppow
Los Angeles
WB6KBL