Re: Type 503 Oscilloscope Issues

Dave Casey
 

Your Radio Shanty meter should be adequate for measuring everything besides
the -3kV, and you can probably learn enough about the cause without
measuring that voltage. Keep in mind that if there is a short in the
transformer, other leads that normally have much smaller potentials on them
may be at -3kV with respect to the scope chassis (and earth ground). It is
always important to consider all the effects of a failure. I would advise
you to not handle your meter while you're taking voltage measurements and
to do your best to isolate the meter from earth ground; use alligator clip
leads to attach the meter to the measurement points while the scope is
unplugged and place the meter on a non-conductive surface. Remember that
you have exposed mains voltage inside the scope case very near where you
intend to measure even when the scope is off. Use a GFCI protected outlet
if at all possible as well.

Obviously, for resistance measurements with the scope unplugged and
discharged, you can just use point probes and go to town.

You can also use your meter to check for hard defects in the tubes of
concern; while much higher voltages are usually used to check tubes for
shorts, you can see a major fault with your DMM. For example, you should
(and likely will, based on your videos) see an open between pins 1 and 2 of
V659.

You will have to unsolder V692 to check it, but you should see only about 6
ohms across the filament (or heater) leads (the two on the same end of the
tube). You should see an open between either of those leads and the plate
lead at the other end of the tube. Here's the datasheet for a little more
detail:
https://frank.pocnet.net/sheets/049/5/5642.pdf
I would not at all be surprised if V692 is now trashed based on that light
show in your video. If you remove yours and compare it to photos of new
tubes online, such as at http://www.radiomuseum.org/tubes/tube_5642.html
you may notice that things have melted. But did T601 cause that, or did
V692 cause it?
If a shorted/arcing T601 is the only thing currently wrong with your scope
(i.e. no collateral damage), then the removal of V692 should restore the
rest of the supplies to their normal voltages. IF YOU DO unsolder anything
from the ceramic strips, it is generally advisable to NOT wick/vacuum all
the old solder out and replace with new. The design of the strips is such
that they work best with silver bearing (~2%) solder, which you typically
won't have on your bench. You may, however, have some in one of your scopes
as Tek often provided small amounts inside the instruments for convenience.
If you get rid of too much of the silver you may lose adhesion to the
ceramic, which is bad for long term reliability of the connection.

You're probably feeling a bit inundated with information and possibilities,
etc. Feel free to report back to the group at each stage of your findings,
and we'll help you narrow things down. It's also very probable that someone
on the list local to you will offer to help you in person if you advertise
your location and ask nicely. I think I can safely say that we are always
happy to see more people learn how to troubleshoot and maintain these
instruments instead of scrapping them for parts.

Dave Casey

On Sun, Apr 2, 2017 at 1:05 AM, enchanter464@... [TekScopes] <
TekScopes@...> wrote:



Thank you everyone for the quick responses and advice.

I had looked over the manual for potential sources early on. The V659 and
V692 did stand out to me, as they flickered sporadically when the scope was
on (and forgot to mention it in my original posting). My personal
RadioShack multimeter only does DC up to 1000 V (and AC at 750 V), but I
can get access to others for higher voltages to check the -3000 (and may
even have an analog one lying around). I have worked with CRTs in computers
before, so I am familiar with procedures for high-voltages and safety.

As for parts, I do have other scopes that things could be taken from,
namely a Type 545B, Type 543A, and a Type 564 Storage, but some of these
are still used, so it could not be anything permanent, and I am not sure
how many of the tubes are compatible. (I am only familiar with the 503
since it was the lightest / most portable one to use.)


For now, I'll let the scope discharge and collect what I can to measure
the various components to trace/confirm the issue.


- Evan






[Non-text portions of this message have been removed]

Join TekScopes@groups.io to automatically receive all group messages.