Re: Need help with 561A

Stan & Patricia Griffiths <w7ni@...>

This sounds like HV arcing problems to me. The 561A has a rather high
(-3300 volts, I think) on the CRT cathode. The CRT filaments are also
elevated to this voltage so the CRT will not arc internally. This means the
power transfomer CRT filament winding must also be raised to -3300 volts.
High humidity will eventually cause this filament winding to arc over
internally inside the transformer. I think I have heard of ONE occasion
where a scope owner with this problem was able to salvage the transformer.
Tek's cure was always to replace the transformer with a new one. You can
check this by first measuring the -3300 and see if it fluctuates during the
arc. If it does, you can isolate the CRT filament winding right at the
transformer terminals by disconnecting BOTH filament winding wires and then
seeing if the arcing stops. If it does, you have found the problem. If you
could find an isolation transformer good for 600 ma and an isolation voltage
of at least 3300 volts, you could insert it between the CRT filament winding
and the CRT filament itself. I have never seen a transformer like this . .
. Or, if you could find a 6.3 VAC, 600 ma filament transformer that could
stand to have 3300 volts applied to it without arcing to ground, you could
supply CRT filament power with a separate transformer. I have never seen
one of those transformers either . . . I HAVE seen some good for 1500 volts,
however, so this solution will work in some other models of scopes such as
the the 530, 540, and 550 series.


----- Original Message -----
From: "n4buq" <>
To: <>
Sent: Sunday, April 18, 2004 1:06 PM
Subject: [TekScopes] Need help with 561A

I recently bought a couple of 561A's, primarily for the plugins and
to have a good supply of spare parts for my existing 561A. While the
plugins seem to work, neither of the "new" scopes work properly, and
I was wondering if someone might know by the symptoms what the
problem might be.

Both of them produce a rather loud ticking noise which coincides with
a flashing effect in the 6AS7 and the OG3 tubes. Both 6AS7's also
glow a fairly bright violet color and I'm not sure if that is
normal. On one of the scopes, when the flashover occurs and I have
the calibrator hooked up to the input, I get a blur of random traces
on the screen.

I had noticed before that my "good" 561A sometimes would tick like
this, but hadn't traced it to the source.

Is this an indication of power supply problems? I suspect filter
caps simply because of their age, but not sure if this is it. I
haven't bought a schematic yet, so it's difficult for me to do much
analysis at this point. I was just figuring this might be a pretty
common symptom and someone might be able to tell me some things to

Also, still looking for a source for the power cords. Are these now


Barry - N4BUQ

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