Re: Type 503 Oscilloscope Issues

Dave Casey

Evan -

First of all, Dan means T620, not V620 when he says "unobtanium". It's easy
to get them mixed up.

It certainly sounds to me like V620 is dying, and it is probably T620 or
V692 that is killing it.

I would recommend removing V692 (de-soldering it) so that the 3kV winding
of T620 doesn't have any load on it. You could more easily disconnect the
3kV wires that come from the transformer, but then you have to secure them
so they don't arc to the chassis, etc. With V692 removed, you can see about
getting the other supplies working and stable. If you still have arcing
sounds with V692 removed, then there may very well be a problem with the
3kV winding in T620.

You may also find that V620 is now too far gone to run the other supplies,
in which case you will need to replace it.

So in summary:
1) Remove V692.
2) Measure power supply voltages; look/listen for arcs.
3) If voltages are bad but all is quiet, try a new V620.
4) If voltages are good and all is quiet, try a new V692.
5) If all is not quiet, perform further testing on T620 unless you have
obvious arcing somewhere else.

Remember also that if the 3kV winding has a problem, that voltage could be
arcing to other wires. So be careful as there may be 3kV in other places
that you're measuring where you're not expecting it to be.

Dave Casey

On Thu, Apr 13, 2017 at 7:25 PM, Daniel Koller kaboomdk@...
[TekScopes] <TekScopes@...> wrote:

Hi Evan, Question: When the scope is on, and starting to act up, what is
the color of the plate of V620? Is it glowing red, or orange? It should
not be! V620 does run hot (very hot to the touch) but the plate should be
grey and not glowing. If it it, it's probably pointing to a vailure of
T620. The -100 V is not correct, so the regulator circuit is driving V620
harder to compensate and it's at its limit. Don't run it for very long if
it's overheating like that.
There is no point measuring the -3KV if the other voltages are not
correct. It won't be; It's all the same transformer, so the voltages are
all proportional. I am concerned that the arcing you hear is not coming
from V692, but from inside V620, in which case it's going to be a hard
repair, as V620 is "unobtanium". If you should be so lucky and can find an
external source of HV arcing, then you may be in luck. Perhaps the ceramic
terminal strips have some muck on them and they are shorting the HV to
ground? One thing you can do is look at the HV circuit with the lights
out and see if you can spot the arcing.
I don't know what to make of what you describe around V620. +560V seems
too high. On the other hand, I vaguely remember one of my scopes running a
bit high too. My latest measurements on a good RM503 show the +500 bus was
running at 513V - plenty within range of power line fluctuations. 560V is
only 12% above nominal. Instead of 117VAC you have a 131VAC line? That
+500V should be the value under load of course, and something is wrong with
the power supply, so that may be expected.
It seems unlikely that the 500V supply would arc. It seems equally
unlikely that the -3000V would short to the primary side of T620 and then
arc to ground through V620, but maybe??? It's ~25KHz AC after all, so the
path to ground may not be entirely obvious. I would take V620 out and
clean off the base carefully, maybe gentrly with steel wook and a bit of
alcohol too. Try to get any carbon tracks off of it if there are signs of
arcing. DON'T clean around the base of V692 unless you actually see
evidence of arcing to ground, or your cleaning operation may increase the
chance of arcing. Also be extraordinarily careful around the leads of V692
- they break if you look at them wrong.
See where these steps get you and hope that you are not arcing from the
-3000V winding. If it turns out to be a bad T620, I can provide you with
coil resistance measurements to confirm it, but unfortunately, there is not
a whole lot you can do about it except look for a parts donor or learn to
wind your own coils.
Good luck and be careful.

On Thursday, April 13, 2017, 7:31:57 PM EDT, enchanter464@...
[TekScopes] <TekScopes@...> wrote:
Ok. So I have a lot of measurements to report:

1. For the 500 V bus test, with V620 and V634 removed, I first checked the
two tubes. V620 (6DQ6B) gave a heater resistance (pins 2 and 7) of 0.9 Ohm
(very low), while V634 (6DJ8) gave a heater resistance of 2.1 Ohm, same as
the other 6DJ8 tubes. Also of note, on V620, there was visible
scorching/blackness at the base of the metal components.

For the voltage testing with the new fuse in place, the 500 V bus
registered at ~560 V on the analog meter, and +566-567 V on my digital
multimeter, which was constant over 10 minutes of being turned on (with the
two tubes removed). There was no noise, burning smell, or any activity from
the tubes. Stability-wise, the voltage only increased by +1V over the 10
minute period, and no fuse blown.

2. With the tubes back in, using the analog volt meter:

a.) At the -100 V bus, it only gave a voltage from -25 V to -50 V
(fluctuating back and forth around -55 V, and steadily decreasing until
around -25 V). There was a buzzing sound and flickering again from V692,
and smell of burning from V620, which got very hot around the tube, and
smell/heat lingered even while the scope discharged unplugged.

b.) At the +500 V, the voltage ramped up to 560 V, and then decreased to
500 V once V692 kick on (and began to buzz), but was steady.

c.) At the +12.6 V bus, voltage fluctuated around 5.2 V, going as low as
4.0 V, but shooting up to around 8.6 V when V692 made sputtering sounds.

d.) At the +100 V bus, voltage was at 55 V, fluctuating between 50 V and
58 V, with spikes at 63 V.

e.) At the +250 V bus, voltage fluctuated around 150 V, going as low as
125 V (when V692 make sputtering sounds), and as high as 160 V.

f.) Lastly, the +85 V bus fluctuated around 90 V, dipping down to around
70 V, and spiking up to 110 V.

For all of the above readings, since V620 was showing signs of burning, I
only had the scope on for less than a minute (once V692 warmed up),
discharging off between each test. In addition, I did not check the -3000 V
bus since I do not have appropriate leads to test such high voltage.

At this point, is this still likely a transformer issue, and if so, what
do you suggest on how to further assess the scope?

- Evan

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