Re: Type 503 Oscilloscope Issues

Dave Casey

The metal cover is a shield for electromagnetic interference (EMI). It may
be needed in order to meet a specification somewhere, but the scope should
work fine without it.

We're not necessarily looking for a bad tube, we're just disabling
different parts of the circuit by removing tubes because they are socketed
and thus easy to remove and reinstall. The actual cause of the problem may
be a resistor or a piece of loose metal that cause a short somwhere, etc.
Finding the tubes that prevent this fault from overloading the -100V bus
will help us determine where the fault is.

Just like V374 and V384 in the horizontal amplifier, there are V474 and
V484 in the vertical amplifier. You can try pulling those. You can also try
pulling V45, V135, V145, and V160 one at a time to see if any of them are
the smoking gun. If none of those point to the issue, then you'll have to
start un-soldering various connections or measuring a lot of different
voltages to suss out the issue.

Another thing to try with all the tubes installed and your meter hooked up
to monitor the -100V bus is to adjust all of the various controls and see
if any one control has a significant impact on the supply voltage. Again,
that doesn't mean that particular control is bad, but it might tell you
which part of the circuit is burning all that power.

It is also still possible that the problem is with the -100V supply itself.
A leaky filter capacitor such as C682 or C684 could be the problem. You
might see if either of these are particularly warm after the scope has been
on for a minute (but turn the scope off before you feel them up - not sure
if the whole can sees -100V or not).

Dave Casey

On Sat, May 6, 2017 at 7:29 PM, enchanter464@... [TekScopes] <
TekScopes@...> wrote:

Some updates on the scope:

First, to Kjo's concern, we did discuss using silver-bearing solder
previously. I have just been using the unused spool that came inside the
scope, which has worked out so far (although I may need to pick up more
later on).

Then to Dave's and John's points, with V334 and V434 removed, the -100 V
bus was at -90.3 V (better than before). In addition, I was able to better
adjust the horizontal on the trace (previously, twisting the knob for the
horizontal position to the left only made the trace move to about halfway
across the screen, but now it can span the entire width).

Of note, only V434 had the protective metal spring covering on top of the
tube (locks into the socket), while V334 lacked the metal covering. I am
not sure if this would affect anything.

With the tubes still removed, I adjusted R641 to see the effect.
Originally, it was turned all the way to the left (counter-clockwise), but
moving it to the exact middle of its range, the -100 V bus read -94.4 V.
Continuing to turn the resistor knob to its rightmost (clockwise) point,
the -100 V bus still only reached about -94.4 V (occasionally reaching -95
V), but mostly stable around -94.4).

Given the above, it was likely that another tube is also affecting the
-100 V bus, so I continued with Dave's advice on V374 and V384. With V334
and V434 plugged back in, and the other two removed, the -100 V bus
measured at -92 V, which is still better than when all tubes are plugged

Are there any other tubes that should be checked, or any better way of
determining which tubes are bad? It seems that all of the tubes that were
removed helped the -100 V bus to some extent, but could this just be due to
simply removing the tube (whether the tube itself is good or bad)?

- Evan

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