"K3HVG" wrote in message
...
Well, let's see... I've done four of them, to date. Maybe
I've been lucky or something, but I haven't had too much
of a problem aligning the dials. Perhaps my spec is not
tight enough? At any rate, two of the four had tubular,
vice disc, capacitors that had to be changed out. The
worst problem, but doable, are those on each of the L/C
elements in the turret and those under the RF box.
Replacing the caps on the L/C elements for the LO may be
of great help in dial alignment.


My JX-17 had bad caps on the 7.5mc to 14.5mc oscillator
assembly. The problem was extreme instability. I replaced
the bad guy. Since this was a 1% cap of specified value I
built one of the right value from two dipped silver micas in
parallel, measuring it on a bridge. The result was just
about perfect calibration on that band and very good
stability. I may have to change others.
I am aware of he problem with the Sangamo "Black Beauty"
caps. they were all replaced with disc ceramics in my JX-17
but only in the RF deck of the JX-21, or perhaps its a
JX-26. I've replace a number of them that were obviously bad
but there nearly 60 in the receiver and not all are bad.
BTW, these ubicutous caps were originaly marketed by Sangamo
as deluxe quality replacements for wax dipped paper caps.
They are paper impregnated with Mylar in molded resin cases.
I suspect the problem is that the casing is not stable and,
as it drys out, disturbs the connection of the capacitor to
the leads and also lets moisture in causing very high
leakage. Black Beauties were used in a variety of high
quality electronic equipment c.1950s and 1960s. Sangamo also
sold dipped epoxy encapsulated versions of these sold as
"Chocklate Drops". AFAIK, these have proven to be a lot more
reliable.
But back to the calibration problem: I discovred that
molded silvered mica capacitors, once considered to be most
reliable of all, evidently suffer from a similar problem,
i.e., they become unstable with age. One symptom is a rapid
variation of capacitance sometimes called "scintillation"
and that they can change value radically. For instance, the
51uuf caps used to resonate L-34, one of the 4 mhz IF coils,
will change as the receiver heats up throwing off the tuning
of this stage. I changed this cap in both of my receivers. I
found other bad guys in the JX-17 in various places, all
molded silver mica types.
The main tuning capacitor of the SP-600 has stators which
are fastened to the frame with three screws. The stators can
move up and down and also sideways to some degree. The
should be exactly centered but the up and down movement was
evidently used in manufacture to adust the exact value of
each section to the nominal value. I am about to try this on
one of the receivers where I need to remove the RF deck
anyway. The outside plates of the rotor sections are split
but evidently plate bending was NEVER done and should not be
done to calibrate the things. Somewhere, someone probably
has the factory procedures but they do not appear to be on
the web anywhere.
I have a lot of other unanswered questions about these
receivers. So far, even after just a couple of hours I've
found at least some of what I want in this news group, for
which I thank everyone.
I should add that my JX-21 (or maybe JX-26) was bought as
junk at C&H Surplus some 30 years ago. It was very cheap
because it had wires sticking out of it. Someone had
evidently begun to modify it to a JX-17. I removed the
partial modifications and restored the original circuits. I
then did a very large number of small repairs. The RX was
missing the top cover for the capacitor compartment (I've
just recently aquired one) the shield for the crystal switch
(I made one but would like an original) and a couple of
knobs, the RF Gain knob and one of the small Xtal control
knobs. I found knobs enough like the originals to work.
Recently the main filter capacitor gave out and I decided
to really bring the thing to life along with replacing the
cap. I couldn't find an exact replacement but did find one
of the correct physical size but slightly greater
capacitance (not a problem :-) ) but discovered some
additional problem in the coarse of doing this.
The JX-17 was aquired in nearly mint condition many years
ago but was in storage for a long time. That one has come
back to life very well.
I get very frustrated by my lack of time to work on these
guys and the difficulty of obtaining parts even here in the
big city. I have gotten a liberal education about sources in
the last few months. Perhaps I've just gotten old but I am a
bit shocked at the ignorance of younger people about
component level electronics. Oh, well, enough grousing.


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Richard Knoppow
Los Angeles, CA, USA