Gary,
Your message came to me as an email but I didn't see it posted so I'm
copying your message and my reply to the list.
WB6OGD said:
>>> My R-4A (13 tube s/n 3740) had an almost exact same problem. Took me
almost 10 years
to find it. Mine did the signal level change with the STBY recovery, only
difference was mine
would do it for the first hour then it would be ok. It was tricky to find
because it was so
intermittant and because it was somehow on the AVC line it affected
voltage readings
everywhere!!
>>> I finally found it was C126, a 0.1uF paper(?) decoupling cap for the
passband tuner circuit.
It was a big black cap on one of the small "circuit boards" and was also
mechanically
unstable in that you could rock the board and the problem would happen.
>>> Gary WB6OGD
I'll put the good news first and then talk about the analysis. I did
change that capacitor tonight, replacing it with a Sprague "Orange Drop" 0.1
ufd/600v, and the receiver ran flawlessly for well over two hours. I checked
the capacitor on the DVM and it shows infinite resistance so that doesn't
really tell me anything, and I don't have a capacitor checker.
That capacitor and associated resistors on the PCB were "in my sights"
already. It's a funny coincidence that you mentioned that capacitor since
I've been wondering about it since yesterday, when I hit it with a quick
blast of "Zero Mist" circuit cooler and the problem disappeared. If the
problem was mechanical in nature (as in, a faulty mechanical connection inside
the capacitor) the probable source with mine (since the unit had to heat
up for nearly an hour for the problem to appear) was probably a gradual
heating of the wire from pin 7 of V4 that goes to that capacitor, causing the
mechanical interface inside the capacitor to become intermittent. Wiggling
that wire would also cause the problem to come and go as you noted but I
initially thought it was a tube socket issue. Re-soldering connections to
the little PCB didn't help.
Checks of other circuitry using a 'scope and DVM confirmed that the
PTO, crystal oscillator, and 9 vdc supply all remained perfectly stable when
the problem occurred. B+ voltages were pretty constant with the only
change being a slight (maybe a volt or two) increase in the 160 vdc level when
the problem appeared, which is consistent with the tubes in the later
stages drawing slightly less current because of the lower signal level present.
Conversely, a 'scope probe on the grid of V5 (pin 1) showed the input
signal to that tube varying in level as the problem appeared and
disappeared (I ran the crystal calibrator as a test signal). I also noticed
that
manipulating the PBT had a momentary effect on the problem, so that also
pointed to a possible problem in the circuitry or components around V4 and the
PBT.
From the benefit of decades of professional troubleshooting experience
I can tell you that a flaky non-electrolytic capacitor can be among the
hardest things to nail down. This is one reason why, IMO, it isn't a bad
move to just replace not only the electrolytics in an old radio, but also the
paper caps. Granted that 1960s-vintage paper-style caps were certainly
better-made and more robust than their 1930s ancestors, but nevertheless they
can go weird from time to time and if there aren't too many of them in the
radio it's not a bad practice to change them all out (although I did an old
SX-42 Hallicrafters and it took a whole bag of Orange Drops to completely
re-cap it).
We'll see how 5757G holds up over the next couple of days. ;-)
73/arf
Paul, K4MSG
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