Re: Type 503 Oscilloscope Issues

Dave Casey

Evan -

I think it is probable that you simply shorted the +85V bus to ground with
your test lead and in doing so fused open R659. Just because you had an arc
and heard a pop does not mean you encountered the 3kV supply. Pretty much
any voltage in the scope will arc under the right circumstances, even the
12.6 supply. You should not operate the scope until you've repaired the
+85V bus, as this is the reference voltage used in regulating the other
supplies. Also, because it is such a low current reference voltage, you
should endeavor to measure it with as high impedance meter as possible to
prevent the loading effect you witnessed. In today's low measurements, were
you using the same range setting on the same meter as before? Having the
meter set to a different range today could make the meter load your circuit
under test differently than it did before, which could explain why you saw
a loading effect today but not previously.

When things have cooled down/discharged, you might look at R692 again and
see if it's still measuring high. If V692 is not getting enough heater
current due to R692 being high or a bad connection from the R&R of V692,
then you might not be getting -3kV, which means no trace on the screen.

You might also post a close up picture of your transformer installation.
While I am confident in the diagram I sent you, it hasn't been scrutinized
by the rest of the list. A picture of your installation will either verify
that everything is right or that you or I made a mistake.

Also, be careful checking ripple with the other scope. Make sure you
understand the input limits of your other scope and that you have good
margin with respect to what you're going to measure. Hopefully you also
have probes to use with the other scope - make sure you understand the
de-rating of the safe operating voltage for these probes with respect to
frequency. The ripple you're going to be looking at is at tens of
kilohertz. If you don't have good margins here, you risk toasting your test
scope and possibly yourself.

Dave Casey

On Wed, May 3, 2017 at 12:53 AM, enchanter464@... [TekScopes] <
TekScopes@...> wrote:

Hello all,

With all of the input, I started with the earliest point from Dave on
wiring in the 6.3 VAC transformer to see the effect, without any
replacement resistors (would not be calibrated, but that's okay for now).
However, there was no trace on the display. All of the tubes seemed to be
lighting up to the same intensity as previously, so I went ahead and took
the voltage readings.

Some readings were constant, with no fluctuations:
1. The -100 V bus was at ~ -95 V, which is better than it was previously
(at -85 V).
2. The +250 V bus was at ~275 V, which is still high, but not as high as
previously (300 V).
3. The +100 V bus was at ~130 V, which is higher than before (was at ~117

Now for the odd ones, these all had fluctuated wildly at first, then
approached a steady value, although never set on one permanently (still
going in one direction):

4. The +12.6 V bus came in initially at 13.5 V, but then steadily
increased to ~14.8 V (and increased ever so much if time continued),
overall slightly higher than the last reading (~14 V).

5. The +500 V bus fluctuated wildly around 500 V, with readings that
dipped above and below by almost the same amount (500 V), until a few
minutes after warming up, and then set around 520 V, but steadily decreased
to around 495 V. Previously it was constant at 480 V.

6. The +85 V bus was the most problematic. With the lead clipped in place,
the voltage only came to 9.8 V. In addition, V692 did not turn on with the
probe attached. Previously, this was the only bus that was consistent with
its expected value. However, I made a very stupid mistake, and wanted to
see the difference between the probe being attached and not, and had the
clip still near the wire while the scope was turned back on. This resulted
in a very bright flash of blue that arced to the ground with a loud crack.

Given the last measurement's result, I have not turned the scope back on,
and based on the advice of working with HV in early morning (i.e. Don't), I
will be waiting until tomorrow to assess any damage.

In the meantime, does it seem likely that the -3000 V is now leaking into
V634 (the +85 V bus), and if so, how? The transformer is wired in
correctly, and no changes have been made to the scope apart from this.

Of note, I have secured another oscilloscope for use over the upcoming
weekend, which can be used to read the ripples at the various bus points
(if that is still something worth looking into, as Albert suggested).

- Evan

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