No explanation, Morris, but I'd like to remind all that I have
a bag 6.8nF/4kV and 10nF/4kV polyester film caps that make
fine replacements for the HV BB's. I'm only asking $0.50 each;
that's a break-even price. I have more than I will ever use.
Dave Wise
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-----Original Message-----
From: TekScopes@...
[mailto:TekScopes@...] On Behalf Of Morris Odell
Sent: Friday, October 12, 2012 3:30 AM
To: TekScopes@...
Subject: [TekScopes] Interesting fault in 535A
Hi all,
I recently fixed a 535A (actually a RM35A) with some interesting and
deceptive symptoms. The scope powered up OK after 10 years of
storage but
apart from a green flash when the relay pulled in there was no
display. All
the LV supplies were in spec and the beam finder neons worked
properly. The
HV rectifier filaments lit and the oscillator screen was at
+91V (normally
+85). Checking with a HV probe revealed that the cathode of
the CRT was at
-1150 V (should be -1350) and the grid at -1450 V thus
cutting off the CRT.
When I jumpered the grid and cathode the CRT lit up nicely
with -1150 on
both electrodes. Definitely something fishy in the HV section.
The grid supply diode V822 looked like it had been replaced
and the cathode
supply diode V862 looked pretty grubby and I wondered if it was weak.
Bridging it with a suitable Si diode restored function
alright, but both
supplies were low at -1150 V, not what you would expect if
V862 really was
weak. The grid supply filter cap C820 had been replaced with a
modern HV
disk ceramic and it was only after I looked a bit more carefully that I
noticed the two cathode supply filter caps, C842 and C831
tucked away under
the rectifier diodes. They were still the original Sprague
black beauties!
Fortunately I had a couple of modern HV ceramics in the parts
drawer and
after a few minutes with the soldering iron and silver solder,
in they went.
That was all it took to restore perfect operation. The cathode
supply test
point was running a little high but it adjusted to spec nicely. What a
beautiful scope it is, with a razor sharp display!
What was interesting was the behaviour with the Si diode in place. That
would have increased the current available to the cathode
circuit but didn't
have the effect of raising the cathode supply voltage because
the caps were
just too leaky to sustain a higher voltage than -1150.
Instead, the grid
supply fell to meet it. Why? I can only explain it by the
leaky cathode caps
pulling so much current when fed with the Si diode that they
loaded down the
oscillator and reduced the power available to the grid circuit.
I'd be interested to see any alternative explanations.
Morris
