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

Fabio Trevisan
 

Hello Ed,
I'm getting some advice from you before I could even get to the archives
that David pointed me at!
Thank you both for that.
I've seen from manuals of Sencore and B+K rejuventaors that indeed the
currents involved in this
operations are - at times - much greater than the usual cathode current
(but at times not...)
After the last post from David, I was already convinced that I wouldn't get
any useful rejuvenation
with the scope's own bias...but I was inclined to try the increased heater
basically to assess if there is
anything to gain, following the rational that, if additional heat could
cause the cathode to start emitting
from its dead sweet spot, then I would notice an improved focus and
astigmatism (which are lacking).

Now, with your answer, and believing that you are aware from the previous
posts, that my 464's CRT
as it stands right now, it's not THAT bad),
I`m starting to get to the conclusion that doing just the increased heater
will be an extra effort that
won't pay back.

Just to satisfy my curiosity... Do you think that just increasing the
heater (as I said I was planning to do)
would allow me to do the assessment I described above? Or I wouldn't be
able to see the improvement
in focus and astigmatism that I`m expecting?

It's been great to exchange here...
I haven't had a chance to contribute much so far (or to help), but I have
been actively reading the posts
to see if I can be equally helpful. I hope it will come the time.

Rgrds, Fabio






2016-10-17 14:48 GMT-02:00 edbreya@... [TekScopes] <
TekScopes@...>:



I don't think you will get any rejuvenation results by using the scope's
own cathode supply to furnish the necessary higher current - it won't be
able to deliver enough, so will just crash, and you'll risk damaging it.
Also, the heater is usually elevated to near the cathode supply of a couple
kV or so, so it gets complicated to modify it for this purpose. It's best
to unhook the CRT from the scope circuit and apply everything needed
externally. It's also easier to check for results this way. You can unhook
the rejuvenator and rehook the scope guts and fire it up and see what
happened. If you modify the scope circuitry, you'll have to un-modify it
each time to check it.

The rejuvenation process needs higher heater voltage and quite high
cathode current, maybe up to 100 mA or so, versus the normal running
current of less than 1 mA. You don't need a very high voltage - the
commercial units go to about 500 V, and have some series R to limit the
current. I don't recall the circuit details, but I think there's also a
medium-sized capacitor after the R to provide a high current pulse at the
beginning, then the R limits the steady-state value, in conjunction with
the CRT's K-g voltage drop (50-100 V) and effective conductance.

Each process cycle takes about a minute - maybe thirty seconds of higher
heater voltage preheat applied, then thirty seconds of high cathode current
while the heater is still at higher voltage, then all off. The CRT does not
need to be removed, as long as you can get to the socket connections. Only
three lines are needed - two for the heater with one side connected to K,
and one line to g1 and possibly other elements, to supply the rejuvenation
current.

You can find lots of detailed info by looking back to the archives on this
subject. Good luck.

Ed

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