"Lynn" wrote in message
...

"Richard Knoppow" wrote in
message
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.

Anyone ever bother to check resistor values? During
overhaul and repair of lots of tube era marine
electronics, many composition (and films sometimes)
resistors had changed value considerably. Especially those
used in voltage dropping circuits. High value resistors
( half megohm or more) seemed to be pretty wild too.
Of course checking frequently meant disconnecting from
associated circuitry to check is a real
pain in the you know what, but the end result, (including
the condenser replacements) resulted in amazing results!
Anybody find similar resistor drift?

Old Chief Lynn
I do check resistor values. Its typical for carbon
composition resistors (aka mud resistors) to go up in value
when heated and with age. For the most part these were not
precision resistors although some were sold as 5% resistors.
Most of those found in equipment built before about 1946
were +/- 20%. After that 10% tollerance was more usual. They
can be quite far off but seldom fail catastrophically. Also,
composition resistors are noisy and were always noisy even
when new. IMO Ohmite were the best of a bad lot.
Older carbon film resistors often had staked end caps.
The caps can develop poor contact or even corrosion at which
point the resistor goes open. Many early metal film
resistors used a similar method of attaching leads. If done
right end capped units can be quite reliable but many were
not. Another method of attaching leads was used primarily on
precision resistors and high reliability parts. This was the
use of conductive epoxy resin to glue the ends of the leads
to holes in the ceramic core. When made correctly the break
stength of the connection is greater than the wire lead.
Metal film resistors can be made to have a very low or even
zero temperature co-efficient and, in general, are the
quietest of resistors. Both deposited carbon and metal
resistors can go open if sufficiently overloaded. Military
parts are tested for a seven times overload in manufacture.
The bad ones sound like Chinese fireworks going off.


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Richard Knoppow
Los Angeles
WB6KBL