Re: Fuzzy Trace that Auto-Corrects Itself


That certainly sounds a lot like what's going on with my scope and I wonder if trying to chase supposedly bad parts on the HV board will be fruitless.

I have monitored the focus voltage and it appears that it takes a slightly different voltage to focus it when it is fully warmed up versus when it first begins to display a (fat) trace. It only varies by about 20 volts or so (e.g. 1730VDC to 1750VDC) and the HV supply values don't seem to change much, if any, so perhaps it's just a matter of having to wait on the CRT to be fully functional. I even went so far as to attempt to pre-heat the components on the HV board but it still took about the same time to flatten that huge trace.

Barry - N4BUQ

----- Original Message -----
From: "Dave Wise" <>
Sent: Monday, August 10, 2020 12:29:30 PM
Subject: Re: [TekScopes] Fuzzy Trace that Auto-Corrects Itself

Puts me in mind of my Tek 321A. After sitting power-off for a day or more,
the geometry is wrong. It will gradually straighten out, or if I turn
intensity to max, in a couple seconds. Then it's fine until it's powered
down, when it will gradually go bad again. All the pin voltages are good.
I've concluded that some metal part inside the CRT is floating - perhaps a
weld has come loose. (Floating electrically, not mechanically; nothing is
rattling around.) High beam current charges it to normal potential, and it
gradually discharges again while power is off. That's the story, at least.

Dave Wise
From: <> on behalf of n4buq via <>
Sent: Saturday, August 08, 2020 1:34 PM
Subject: [TekScopes] Fuzzy Trace that Auto-Corrects Itself

I have a Hameg scope and if this question/thread needs to be deleted, then
may the moderator please intervene; however, I think it may be generic
enough for this list and I've run out of other sources so I'll try here.

When first powered on and the CRT starts displaying a trace, it is very fuzzy
and wide - approximatrly 1.5 cm. Given about two minutes, the trace
gradually becomes very sharp, focuses where it should, and stays that way
for as long as the scope is powered up.

During that "warm up" time, the trace gradually becomes focusable near one
extreme of the focus control and then gradually is focusable at/near center
of the knob rotation and stays that way.

If I increase the intensity to fully bright as soon as the display becomes
visible, the fuzziness disappears much faster but still doesn't settle out
fully until about two minutes after power on.

I have replaced the HV resistors in the chain that makes up the focus voltage
divider as well as a couple of other HV resistors in the intensity circuit
but the problem persists.

There are a few 0.1uf 1kV film caps that are part of that focus circuit and I
am thinking it could be one or more of those but not sure.

Anyone have any ideas as to what's going wrong and what to look for?

Again, if this is too far OT, then please let me know and I will not continue
the thread.

Barry - N4BUQ

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