Re: Type 503 Oscilloscope Issues

Dave Casey

Step 1: Get better friends.
Step 2: Read the manual. There's a whole section on troubleshooting the
scope, including the power supply which is where your problems appear to

You should be able to narrow things down by taking a few DC voltage
measurements. BE CERTAIN to use a quality meter and leads for those
measurements, as the voltages present in old tube gear are not to be
handled casually. For example, in the power supply troubleshoot section it
tells you to look for a voltage of around 500V at one location, and 600 or
more at another. These voltages are derived from AC, so they're going to
have ripple on them, especially if the equipment is malfunctioning. Don't
use a meter that isn't rated to at least 600 VAC or you're taking some
risks. If it all possible, clip the probes on before plugging in and
powering on the scope; touch nothing while you record your measurement, and
then turn it off and unplug it before moving on to the next measurement.
That said, the light show going on in the power supply is not normal. V659
(the regulator tube making the fuss in your video) is the vacuum tube
analog of a zener diode with a Vz of 85V. The current through this tube is
limited by R659. You should have a good look at R659 and measure it with
the scope off and unplugged (and give the caps some time to discharge). You
shouldn't notice a difference in measurement whether or not V659 is
installed. It's very possible that your only problem is V659 and, as a
result, a burned R659. Tubes don't make burning smells, so use a bright
light to look for any obvious damage on the Rs, Ls, and Cs (scope doesn't
need to be on for this). You can also measure the DC voltage at pins 1 and
5 of V659 and see if it's regulating properly (should be about 85V, but
could be upwards of 250V if V659 is failing). The blue glow in the 6DQ6
(V620) is extra electrons hitting the glass; this may or may not be
indicative of a problem, but I'd pay closer attention to V659 first; V659
has some feedback to V620 through V634. I would also verify that D652 is
still a diode.

Good luck! Let us know what you find, but be safe!

Dave Casey

On Sat, Apr 1, 2017 at 7:34 PM, enchanter464@... [TekScopes] <
TekScopes@...> wrote:

I am hoping some members may be able to help with my current problem.

I have been having issues with a Type 503 that I recently acquired from an
old electronics/mechanical engineering lab at school (has not been used
since the 1970s). The scope had been working for the past week, but then
started losing quality in XY mode, with the (-) Input horizontal channel
becoming weaker. Yesterday I had run the scope for about 2 hours, and then
all signals cut out, with a smell of something burning. The beam is no
longer able to center itself, being blurred and flickering sporadically.
The scope also makes a loud buzzing sound, which seems to be coming from
one of the voltage regulator tubes (based on flickering and visibly seeing
the blue electron beam on the 6DQ6A tube wall). However, I am not sure if
this is due to a bad tube itself, or a bad capacitor (as I have read that
these are prone to failure).

From old Tektronix manuals and support articles on the 503 model, I have
confirmed that the model I have has the re-grounding fix for the defect
that earlier models were shipped with, and that none of the capacitors have
paper caps (all plastic).

Below are links to videos I took of the oscilloscope while running to help
visualize the issue.


Any help with this is much appreciated, since I am not familiar with
analog tube scopes, and the electronics inside it are beyond anything my
electrical engineering friends can figure out.

- Evan

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