Re: Tek 464 CRT tube presenting "double-peaking" - Is rejuvenation recommended

Fabio Trevisan
 

Hi David,
Now that the failing HV is sorted out and while waiting for the new xformer
to arrive,
I wish to explore further the CRT "double-peaking" and the possibility that
its cathode is either worn-out or is "poisoned" (coated with an insulating
layer of
non-emitting material).
I will comment next to your replies.
Rgrds,
Fabio

> 2016-09-22 0:07 GMT-03:00 David @DWH [TekScopes] <
TekScopes@...>:

>I used a 464 at one time and was never impressed with the performance
>of the CRT and especially so compared to the 7834 I eventually used.
>It is possible that the 464 I used just needed to be refurbished.

I don't have previous experience with any Tek analog storage scope, but in
my first job
there was a 50MHz Philips Storage scope (PM3219) which I also didn't like
to use because the trace looked dimmer and less sharp (and back then, that
one was new).
When I got this Tek and found its trace to be - sort of - disappointing, I
immediately thought
that it was just a shortcoming of this kind of storage CRTs.

But then, while I was searching the net to learn more on the H.V. issue
(which we figured out already),
I stumbled into several descriptions of the "double-peaking" on CRTs, and
realized that it
describes exactly how my 464 is behaving:
1. Overall less than optimal focus.
2. Poor focus tracking (but I check that focus tracking potentiometer is
doing what it should do)
3. Poor astigmatism control...It gets best at almost full C.W. but never
perfect.
4. And last but not least, the "double peaking" effect...

>Before messing with the CRT, I would do the usual checks on the low
>and high voltage power supplies for levels, noise, and ripple. Then I
>would at least do the CRT calibration procedures to see if they
>improve things or reveal a problem.

While checking the H.V. defect, I have been on this page already.
All low voltages check OK. +65 on spot without needing to touch it,
and all others are well within the accepted tolerances.
The H.V. (at least while it doesn't collapse) are all at the correct level.
Performed the CRT bias adjustment as per the manual:
To preset Intensity control to get +20V at TP1443 (X-Y mode) and
adjust R1454 to dimmest visible dot on screen.

After doing that calibration, no change in the quality of the trace or
focus.
From all that, I`m convinced already that my CRT's cathode is either
poisoned or
on its way to Valhalla.

>After that I would try is using a separate heater supply and raising
>it while operating the oscilloscope at maximum brightness and
>defocused. The problem however will be finding a suitable power
>supply which can be floated to the high negative cathode voltage.

There you catch my attention!... What do you mean to achieve, 1 or 2?

1. Just to use a variable driven heater to rule-out or confirm the suspicion
in a more definitive way?
I've read that people who are experienced with this art, can tell that how
bad a
CRT cathode is going by just watching how much the emission drops when you
diminish the heating. If the emission falls dramatically or cuts-off
altogether
by just dropping a few percent of the heater voltage, than it's a bad sign.

2. Or, by over-driving the heater, to use the scope's own biasing to break
the
cathode non-emitting coating, and promote some degree of rejuvenation?
Your very instruction to set Intensity at MAX, while keeping it defocused
makes me conclude that this is exactly what you intend.

If that's the idea... I will definitely go for it... it's much better then
going after
an external CRT rejuvenator or try any DIY approach that can wreck the thing
altogether.
I think the scope's own CRT circuits are less likely to cause any harm and
if
they can achieve some result... it's a win-win.

For the "floating" CRT heater power supply, my idea is a humble LM317
powered
from a 12V battery... I can't think of anything more "floating" than this,
I can vary
it from 1.25V to about 10V (I wouldn't risk going above 9.5 anyway).
A battery of those used in computer no-breaks can power the CRT
heater for hours if necessary...

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