Re: Type 503 Oscilloscope Issues

Dave Casey

Evan -

I've sent the schematic to you in a direct e-mail where the attachment
shouldn't get stripped. Let me know if you continue to have issues, and I
can put it on imgur or something.

Before you go trying to solve problems you don't have, I would wire in the
new transformer and see if the scope works again. It probably will. The
issue will just be that the power supply voltages are not to spec. It may
have always had this problem and you weren't aware of it because the scope
worked well enough for your purposes.

It's very likely that at least some of the resistors are now out of
tolerance, but you will have a hard time determining that by measuring them
in circuit as you've done. You are correct that the measurements are skewed
by the other components. One of your EE friends who has done the basic
circuit theory coursework can help you determine approximate measurement
values by performing nodal analyses using the schematic. Even though they
are not familiar with tubes, this being a tube scope doesn't complicate
things very much for these types of measurements. When everything is
powered off and you're just checking resistances with a DMM, it's basically
as though the tubes aren't even there.

The resistor you couldn't measure (???), which is supposed to be 22 ohms,
is R624. Your difficulty in measurement may be due to C624 (the little
yellow electrolytic on the back of V620). You should be able to read get a
solid reading on that resistor if you keep your leads connected long enough
for C624 to charge. If you can never get a stable reading, C624 may be
faulty. The presence or absence of V620 should not affect this measurement
either. If the measurement changes when the tube is removed, then V620 has
developed some internal shorts and needs to be replaced.

You've mis-read the values of several resistors. All those you've marked as
48 are actually supposed to be 47. Violet is 7, black is 0. Due to math and
the way resistors are made, 47 is a common value whereas 48 is not. Google
"standard resistor values" and have a look at the E12 series to get a sense
of the common significant figures you can expect to encounter in this
scope. (or just see here: ).

I will look over your measured values as I have time, but I would recommend
you take a close look at R624 in the meantime.

Dave Casey

On Mon, May 1, 2017 at 11:21 PM, enchanter464@... [TekScopes] <
TekScopes@...> wrote:

It has been a while since last posting, but I have only gotten time
recently to work on the scope (in the midst of final exams).

Taking Dave's advice, I went ahead and measured the resistance of the
resistors surrounding V620 and V634 (basically everything on the ceramic
strips). I have a photo of my measurements here:

I was unable to measure the resistor across V620, which is why there are
"???" for that value. Those values reported in red were the ones outside of
the tolerance range. Suffice to say, the majority of the resistors gave
values that were outside of their range. However, I am not sure if this is
due to how some are soldered together, which could affect the values (for
example, the resistor at V692 giving a resistance value of just the tube
itself, since its expected value is relatively small). In addition, only
one resistor (a 100 ohm) had values higher than its range, while the others
were all below their tolerance limit.

Before looking at getting a replacement tube for V620, would it be best to
replace these resistors? Or is there a better way I can test resistance to
confirm that the values I measured are accurate?

Also, a side note for Dave, but the schematic for wiring the 6.3 VAC
transformer did not get added to your posting. Would you be able to share
the image as a link (since the forums don't support attaching photos)?

- Evan

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