I have been rebuilding a JX-17 for some time which I
have nearly finished. A couple of weeks ago it developed a
rather mysterious problem, namely an intermittent high noise
level. The noise would sometimes come up and stay for a time
and sometimes just vary around fairly rapidly. I discovered
this was actually a variation in the RF gain. My first
thought was that it was in the front end somewhere but I did
some more thinking and made some measurements. It turned out
to be the filter capacitor for the -10 volt bias supplied to
the last IF stage, also called the IF driver (V-11). This is
a fixed gain stage driving the diode detector. The cap is
C-128, a bathtub type electrolytic. This cap was causing the
bias to be low and to vary considerably. It was also pulling
down the -60V supply for the manual bias (also affects the
AVC) so that bias at the RF tubes was reduced from -41V (at
minimum gain) to about -38 volts. Doesn't sound like much
but its enough to throw off the gain structure.
I am passing this along in case someone else has a
similar problem, perhaps I can save some struggling. This is
the second of these 10uf, 100V bathtub units in the bias
circuit to go bad in this receiver. I think this circuit is
identical in all versions of the SP-600, with perhaps the
exception of the VLF RX.
This particular RX was in storage for some time. What
ever happened to it resulted in a number of the silver mica
caps also going bad. Perhaps this is just typical.
I will be doing a bit more work on this guy, and
another SP-600, I am pretty sure a JX-26 and will report
here about anything interesting I find.
BTW, it appears that the voltages specified in the
charts are very close to the actual voltages to be expected.
Note that most were measured with an old fashioned VOM with
1000 ohms/volt DC ranges. I also found that the chart in the
on-line JX-17 book is missing the asterisks indicating the
voltages to be measured with a VTVM. These are in other
SP-600 handbooks and should be checked and the corrections
entered.
I have had one of these RX for some 30 years and the
other for perhaps 15. This has turned out to be a lifetime
endeavor:-)
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Richard Knoppow
Los Angeles, CA, USA
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