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

Fabio Trevisan
 

Hi Ed,
Thank you very much...
I will comment next to your lines.
Rgrds,
Fabio

2016-10-20 0:11 GMT-02:00 edbreya@... [TekScopes] <
TekScopes@...>:
Hi Fabio,

Good move to make your own CRT rejuvenator - it shouldn't be too hard to
replicate the function. I think a lot of people in the tek and HP groups
will be interested in >your rresults. I don't have any B&K 470 stuff here
at the lake, but I have a huge box of everything related back home. I can
make the heater-boost measurement you >requested, but it may be out a week
or so before I can get around to it. I'm kind of curious about that myself.

Two days ago it started a heat wave here in Sao Paulo... I wish I also
would be by the lake these days... Yesterday we got thermal sensation of 43
Celsius... Way too hot for me.
One week or so is no-time for someone trying to rescue a 40 years old Tek.

I did manage to re-find the schematic on-line, so my memory is somewhat
refreshed on what I did on that project. It was apparently back in January
when I was into >fixing 576 CRTs. Before continuing on, there is an
alternative to use a curve tracer as the rejuvenating supply. Do you have
or have access to a curve tracer like a 576 or >577? If so, it could make
for a pretty sophisticated setup where you could see more of what's going
on, and all you would have to worry about is the heater supply and
control. Likewise, an adjustable medium voltage DC supply around 300-500 V
would do, along with some external parts.

I assume you're thinking about making a dedicated-purpose unit from
transformer(s) and miscellaneous parts, kind of like the commercial ones. I
had been thinking >about rejuvanation issues for a couple of years prior.
My original plan was to go the curve tracer route, but I stumbled upon the
B&K 470s last summer at a flea market. I >got a whole box of them for $5 -
three carcasses and a bunch of CRT socket adapters, and some documentation.
Out of the three, I managed to restore two.
I don't have a curve tracer, although it's on my plans to have one, for
selecting transistors for audio equipment... I sill have plans to actually
USE oscilloscopes, besides just repairing them :-) (Although I must admit,
repairing them "per se" is becoming an addiction).
I don't think I will go this far (to take the "curve-tracer" road), at
least not for this project.
But thinking on what you're saying, if from one side it makes perfect sense
to take the curve-tracer approach, because it's much more elegant and
technically accurate, from the other side, I tend to agree with your
previous message in that you say that, and I quote you:

"Unfortunately, this isn't amenable to experimentation and scientific
method, because you can't do repeatable experiments - the CRT is on a
one-way trip to degradation and ultimate failure, and each attempt would
change how it behaves, and how long it may last".

I`m skeptical (although hopeful) that one can devise a "curve-tracer"
driven rejuvenator in which it can iteratively apply different degrees of
rejuvenation while at the same time evaluating the traces in order to
determine the next step...
And skeptical not because I don't believe that one could design such device
but because - as you mentioned - every rejuvenation action taken, modify
the way how the DUT will behave in the next step.

That's when I started studying the commercial ones for ideas. I convinced
myself that since these things were typically used on big color CRTs, they
were probably a little >too intense for small instrument-sized ones. I
modified one unit with larger series resistors - the exact current-limiting
ones you referred to - and left the other unit in >stock condition. When I
actually got around to zapping the 576 CRTs, the modified unit didn't work!
Not because of poor workmanship on my part, but because the cable >from the
unit to the CRT adapter was intermittent, and didn't show until actual use.
These units had been used a lot (probably in a TV shop), so the cables and
accessories were pretty beat up. So, I used the stock unit, and it worked
well enough to get the job done, although its cable was bad too - a little
jiggling around got it >OK.

I was also thinking exactly what you thought (because color tube CRTs work
at much higher currents) to start trying the Clean-Up, not with the 45mA
that the BK470 does in its Clean/Balance function (225 V pk / 5K), or the
90mA it does in the Rejuvenate function, but with a much lower setting
instead (maybe 10mA, which is already 10 fold the maximum current this CRT .
To support that, here is an article about Sencore's CR7000 in that it tells
that its milder re-activate function is a current of 1mA for 30sec.

https://na.suzohapp.com/images/pdf/SencoreBlowsAwayCRTFailuresWithCR70001.pdf
I suppose that, at least in this particular case it doesn't turn-off the
heater, because the emission wouldn't last for 30 seconds with the heater
off anyway.

It's just fortunate (for me) that you realized that your "modified" BK470
didn't work due to an intermittent cable, because it gives me hopes that I
can achieve the rejuvenation I need without having to resort to the higher
currents.
I really fear destroying this thing! because those 464 Storage CRTs seems
to be hard to find and it's the only Tek scope I got.


So, I managed to do the two CRTs, and then tried it on an HP8566 display
(it seemed to work), then assigned the units to "fix/modify" status, where
they sit in a box with >all the related items. Eventually I plan to replace
the cables with a whole new kind of interconnect system. I have also
gathered at least one of each type CRT socket to >make adapters to cover my
various Tek and HP stuff. I anticipate having more CRTs to rejuvenate going
forward, so it's worth some effort to make a good working >system.

I'll put up some more info maybe tomorrow as I remember things. One thing
to note especially considering the cable issues I had, is that there can be
quite a wiring >voltage drop from the heater supply to the CRT socket,
complicating things a bit more.

Ed

I hope that with the results of my experiment I will be able to contribute
with your next cases... I think it's going to happen more and more from now
on.

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