Re: Type 503 Oscilloscope Issues

Dave Casey

It certainly sounds like your meter is loading the +85V bus and killing it,
but your story is not consistent. You measured the +85V bus again when you
took voltage readings at R640 and saw it read 91.2V. What's different
between how you performed these two measurements? Was the scope switched on
after you attached the probe at V659 but already up and running when you
measured R640? Perhaps the minor loading of your DMM is not enough to kill
the supply but is enough to keep it from starting.
What warping of the trace are you talking about? It looks pretty good in
the photos/video you've posted.

Dave Casey

On Thu, May 4, 2017 at 11:52 AM, enchanter464@... [TekScopes] <
TekScopes@...> wrote:

So in response to the previous points:

1. I did mean V692, regarding it not lighting up. Normally, the -3000 V
tube comes on after a minute, once the scope warms up. However, with the
probe clipped to +85V bus, the tube never turns on. But looking now at
V659, that also does not turn on if the probe is attached (when normally is
it a bright orange glow).
Re-measuring R692, the resistor comes to 4.4 ohm, which is in its normal
range. I am not sure why the previous reading was so high.

2. The resistance across Pin 1 to Pin 5 of V659 measures at 4.5 ohm with
the tube removed. With the tube installed, the resistance from Pin 1/5 to
ground is 100.2 kOhm. No visible short is present. The voltage on both pins
is the same, and the value has not changed since last I reported it
(between 8-9 V).

3. Voltages measured from both sides of the resistors:
R640 -> +11 V from the left, +91.2 V from the right
R641 -> +0.8 V from the right pin (and trace appears, tubes normal), +5.1
V from the left pin (dim trace appears, V692 not fully glowing, faint
R642 -> -87.3 V from the left, +1.7 V from the right
R644 -> The left side measured at -0.3 V, while the right side gave no
reading (0 V). This may be due to probe position, since I could not put the
probe in the tight space very well while avoiding touching any of the other
wires (had to place on the middle pin of R641, which is soldered to R644).

Note that R641 was a bit hard to turn initially when I tested it a week
ago, but now it is fairly easy to adjust.

I have updated the schematic with the new resistor readings and voltages,
and can be found with the previous link:
4vv9VNR68bkhTUdG9 Note that R624
corresponds to the resistor that is directly across Pins 6 and 8 of the
socket for V620, but it is not shown on the schematic. Pin 6 appears to
just lead to the grounding bracket.

Also of note, using the scope in X-Y mode, the -Input of the Horizontal
adjust is very skewed/out of proportion, while the +/-Input of the Vertical
adjust and the +Input of the Horizontal adjust appear to work as normal,
minus the general warping of the trace image (not sure if this issue is
specifically related to the issues on the other parts of the scope).

- Evan

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