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:-)


--
---
Richard Knoppow
Los Angeles, CA, USA






--
Posted via a free Usenet account from http://www.teranews.com

I'll back up what you just said. I had to replace all of the
bathtub caps in my two SP-600's (JX-26's). All
were leaky.
Steve

"Steve" wrote in message
...
I'll back up what you just said. I had to replace all of
the
bathtub caps in my two SP-600's (JX-26's). All
were leaky.
Steve

One must remember that these guys are 50+ years not a
bad lifetime for these caps. One problem I've had is getting
new electrolytics with really low leakage. I have a few
Sprague Atom caps, which must be getting on for 20 years
old, that have lower dissipation than many brand new ones.
Any suggestions about sources? You would think that in a
city the size of Los Angeles, plus one that is a center for
the electronics industry, that one could get anything even
in small quantities. Not so, its very frustrating.
I also found that the failure rate of the silver mica
caps in my JX-17 is high. Not so in the JX-26. I think the
difference may be that the latter has been in almost
continuous use for the last 30 years where the 17 was in
storage for a long time.


--
---
Richard Knoppow
Los Angeles, CA, USA




--
Posted via a free Usenet account from http://www.teranews.com

Query... regarding the bathtubes, were your SP-600's the ones with paper
caps or disc caps? So far my late JX-26 (with disk caps) has not had
any capacitor issues... I think..?

"K3HVG" wrote in message
...
Query... regarding the bathtubes, were your SP-600's the
ones with paper caps or disc caps? So far my late JX-26
(with disk caps) has not had any capacitor issues... I
think..?

The JX-17 has all ceramic caps. Evidently it was modified
by a military contractor c.1957. It also has the IF gain
mod. I have not had any problems with the ceramic caps but
there have been a number of other cap failures.
The JX-26 has the original Black Beauties in it. I have
not yet replaced them all but will do so. A number of these
were cracked and a couple so badly that part of the casing
had fallen off. I've checked some that were whole and find
the leakage rather higher than I like. Ceramics have very
low leakage. The other caps in this RX have been reasonably
trouble free. The original filter cap failed and I replaced
it. I also replaced the obviously bad BB's and a couple of
unstable silver micas. I intend to rebuild this one and will
replace all the BBs. Both RX appear to be about the same
age. Internal inspection stamps suggest both were built
about the end of 1954. At this point both are still works in
progress.


--
---
Richard Knoppow
Los Angeles, CA, USA




--
Posted via a free Usenet account from http://www.teranews.com

Richard, If you lose any of the bandswitch turret module clips while
you're rebuilding them (cap repl.), let me know. I have a small
quantity of spares. Also, good fortune with replacing the caps up in the
RF box. Its a necessary chore and can certainly be done. Jeep/K3HVG

"K3HVG" wrote in message
...
Richard, If you lose any of the bandswitch turret module
clips while you're rebuilding them (cap repl.), let me
know. I have a small quantity of spares. Also, good
fortune with replacing the caps up in the RF box. Its a
necessary chore and can certainly be done. Jeep/K3HVG

Thanks:-) I accidently deleted your e-mail from my spam
blocker, please send it again.
My JX-17 was completely recapped at some time in its
life. It has a sticker from a service agency, probably a
military contract, who also installed the IF gain mod. The
JX-26 is a curiosity, the RF deck has been recapped but not
the rest of the RX. It may be that the RF deck is not the
original but there is no way of knowing. The RF modules on
the turret still have the BB's on them. I am careful with
small parts but if you don't have spares things tend to fly
off into the forth dimention so your offer is welcome.
What I am looking for are the spring contacts for the RF
deck. The ones in the JX-16 were damaged at some point.
While I have been able to repair them sufficiently to make
the RX work I would like to replace them. Lots of soldering
but it doesn't look at all impossible. The assemblys are
held onto the deck by solder and appear to be self aligning.
I am also interested in power supply voltages in
receivers. The books specify the voltage at the cathode of
the rectifier to be 305V (max RF gain) to 320V (minimum RF
gain). Both my RX measure considerably lower despite having
good rectifier tubes and new filter caps. Both RX meet specs
for noise and sensitivity. Am I fighting a ghost here?


--
---
Richard Knoppow
Los Angeles, CA, USA





--
Posted via a free Usenet account from http://www.teranews.com

As I recall, I also have a few of the complete clip/contact assemblies.
I'll have to locate them, though. Don't let me forget...hi!!

de K3HVG