Re: Type 503 Oscilloscope Issues

Dave Casey

Evan -

With this test, you've shown that most of the supply functions properly.
Yes, it's not dead on for voltages, but they are all reasonably close
enough to what they're supposed to be. T620 is probably also fine, as it's
still making over 3kV at the terminals for V692 without causing any
problems in the other supplies due to flashing over.
What we haven't yet ruled out is T601, which is a likely failure. If you
re-install V692 (doesn't matter which orientation as long as the two wire
end is toward the transformer), and then disconnect wires 6 and 7 from
T601, you should see V692 with only the faint orange glow of the filament
and no arcing/noise. For this test, you can tie the 6 and 7 wires together
under a plastic wire nut (like for house wiring) after you've disconnected
them from T601. Position them away from anything grounded and yourself. You
can also then measure the -3kV if you have the means to safely do so. If
V692 glows orange and quiet, then T601 is where your problem lies, and
there's an easy fix by adding another small transformer to your scope. If
the light show in V692 continues with the -3kV disconnected from T601, then
the problem is somewhere else such as C692A or B, or perhaps even V692

Proceed safely. You've nearly got this one figured out!

Dave Casey

On Wed, Apr 19, 2017 at 12:12 AM, enchanter464@... [TekScopes] <
TekScopes@...> wrote:

Hello everyone,

Thanks for the instructions and tips on what to do next. I only now got
time to work on the scope, after a busy weekend.

First, regarding my observations on the scope with everything plugged in
and turned on, the tube plate in V620 did become orange/red, especially
when viewing it from the top. The previously mentioned blue glow along the
sides (excess electrons) was also present, but it did seem to get more
orange each time it was run. However, even with the lights off, I was
unable to see any arcing to any part of the oscilloscope. V692 is not as
bright as it was before, with a blue ring at its center that occasionally
lights up orange in the tube, but only when it sputters (causing the arcing
sound). V659 does not light up at all, and nearby V634 comes and goes with
lighting up.

As for the further tests, I was not able to locate pin 12 on T601 (max
number is 11), and only one of the -3000 V wires (the orange stripe ones)
links to it at pin 7. However, I ended up just desoldering V692 (easier to
do from my view). With the scope powered on and no V692 present, there was
also no sound of arcing, and the tubes all illuminated normally (with no
flickering). There were no blue electron beams on V620 (and it was not
glowing red). However, V659 was a constant orange glow (whereas it was
previously dark). However, the area around V620 was still warm (but not as
hot as before), and something still smelled like it was burning slightly,
but it could just be due to old plastics and the heat from the general

Voltage measurements were all constant, and the values reported below are
from the analog voltmeter (but confirmed by the digital multimeter to be
valid, however some values differed greatly):
-100 V bus measured at ~85 VDC (94.2 V)
+250 V bus measured just above 300 VDC (336 V)
+100 V bus measured at ~120 VDC (125 V)
+12.6 V bus measured at 15 VDC (17.1 V)
+500 V bus measured at ~495 VDC (538 V)
+85 V bus measured at ~85 VDC (99.5 V)

Below is a link to photos of the scope when running with V692 removed (in
case it helps):

Overall, all of the voltages were slightly higher than the expected value.
However, since the values were constant, does the problem seem to be coming
from the removed V692, or the HV transformer (T620)? If it is V692, I can
order a new 5642 tube. If the values are escalated due to a probably now
fried V620, I can also order a new 6DQ6B tube.

Thanks for all the input.

- Evan

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