[vcf-midatlantic] VT320 CRT replacement?

[Ian Primus]

The VT320 CRT is kind of unusual - finding a replacement might be
difficult, as the profile is a bit different than the most common
tubes. I have several surplus tubes, I can check to see if I have a
spare that'll fit a 320, but I don't believe so. The ones I have are
mostly for 220/100 and similar monitors (which is a fairly large
spectrum), and a few other odd types.

That said, just because it's dim doesn't mean it's unusable.
Rejuvinating a CRT does work pretty well - especially for a monochrome
text display. Bring it to the next workshop (and let me know ahead of
time), and I'll bring one of my CRT rejuvinators. By zapping the tube,
you can restore the emissions quite a bit, and, in the case of a mono
terminal tube, you can restore a LOT of life. Tracking can suffer as
the tube ages, but this is mostly a problem for TV tubes, which are
expected to produce gradient images. On a text display, you can get
away with a lot. Case in point, Jonathan had a little CRT in a tiny
display he brought to VCF. The tube was *zorched*. Very weak, lots of
hours, almost no emissions. Rejuvinating brought back a lot of
emissions, but the cutoff point is still toast, it won't track - but
it now produces a perfectly legible, bright, text display, where it
was completely unusable before.

Also, remember, the VT320 has a sub brightness control inside. You
will probably find you can just adjust that and make it more legible
without even touching the tube. But rejuvinating it ought to make it
usable.

I've had very good luck with rejuvination - some tubes don't last, but
many do. It's well over a 50% success rate. The ones that don't
respond well are mostly color tubes from certain manufacturers (Sony,
Zenith from the 90's, etc), and some very old TV tubes.

Another option, to eek the last bit of life out of the tube, is to
boost the filament voltage. This is a last resort, and will shorten
the life, but, if it's unusable and won't rejuve, bumping up the
filament voltage a bit will let you get the last bit of use out of it,
and again, the results can be surprising.

No guarantees, of course - it's always a bit of a gamble whenever you
do any of this, but, I'll wager that one zap on the rejuvinator and a
bit of tweaking to the internal brightness controls, and it'll look
perfectly fine for years.

-Ian

On Sun, May 1, 2016 at 2:52 PM, David Riley via vcf-midatlantic
<vcf-midatlantic at lists.vintagecomputerfederation.org> wrote:
> I have a VT320 with a pretty bad case of Being Old. The screen is so dim that even with all the lights off in the basement, it's difficult to read (and if it gets switched to 132-character mode, next to impossible). I know surplus caches of these CRTs pop up every once in a while, and I know other mono CRTs of the same size are often mechanically and electrically compatible, but I'm a bit at sea. Does anyone have pointers for acquiring such things?
>
> The current tube is amber, but I really don't care what color it is as long as it's legible.
>
> Failing that, how well do rejuvenators really work on these? My understanding is that it's usually a pretty temporary fix at best (like Retr0Brite).
>
>
> - Dave