Tube or not Tube

Mr Ian Primus ian_primus at yahoo.com
Wed Aug 22 05:39:49 CDT 2007


> Before doing anything drastic it crossed my mind
> that somebody at some
> may have changed the tube in a DEC terminal.
> There are two issues with this one.
>  
>     a) Where do you get the replacement tube from.

Typically, from something else. Like other terminals,
12" monochrome PC monitors, and portable black and
white television sets. A lot of these tubes are
interchangeable. I've run across some that aren't, but
for the most part, with black and white tubes, if the
base fits, the neck and deflection angle is the same,
and the heater is the same voltage, it's a good chance
it will work.

>     b) How do you fit it.

#include <std_disclaimer.h>

Carefully. :) You want to be sure to discharge the
high voltage first (in both the terminal, and whatever
you're stealing a tube from!). To do this, attach a
length of wire to the metal shaft of a flat blade
screwdriver, and ground the wire to the chassis of the
terminal/whatever. Hold the insulated plastic handle
of the screwdriver and slide it under the rubber
suction cup of the picture tube, until you feel the
metal clip inside. Typically, this will be accompanied
by a *CRACK* as the high voltage discharges.
(Although, it is possible that the HV dissapated
earlier, depends on the terminal) Do this twice just
to be sure.

To remove the clip, peel up the rubber suction cup,
and you'll see it's a hooked clip that fits into a
hole in the tube. Squeeze the clip together and gently
remove it.

Obviously, you'll want to disconnect the other wires
leading to the tube - the base socket, and the yoke
connector. Disconnect the yoke at the board, and leave
it on the tube as you remove it. Unbolt the tube from
the frame, and pull it out, careful not to break the
neck. Also watch out for ground leads.

The new tube needs to physically fit and mount in the
terminal. There are two basic types of picture tube -
the tubes with ears that bolt to something, and the
tubes without ears, that use a metal ring to hold them
to whatever. (VT100's have ears, VT220's don't). Your
replacement tube must be of the same type in order to
fit.

The yoke needs to stay with the terminal. This is the
copper coil thing on the back of the tube. Loosen the
screws that clamp it to the neck of the tube and
carefully remove it. Remember what way is up. You'll
need to move this to your new tube. Try to get it on
straight - it controls deflection, and if it's on
crooked, the picture will be crooked. It's easy to
adjust later though.

Reassemble terminal with new tube. Cross your fingers
and hit the switch. If it works, adjust your yoke for
a level picture (carefully, you don't want to zap
yourself). Once the yoke is level, shut the terminal
off and tighten the yoke so it won't move.

> If anybody has tried it I would be pleased to hear
> how they got on.

I have replaced the tubes in a couple things, and had
pretty good luck. I replaced the tube in a VT100 with
one from a PC monochrome monitor, and it works well.
One time, I even swapped a worn out IBM monitor tube
back into a television. It was the only TV ever with
WordPerfect screen burn. And those long persistance
phosphors make TV look really weird. I kept it around
as a joke for a while - I said that it was a black and
white TV I converted to color. (Green is a color).

-Ian

* Be careful messing with high voltage stuff! Keep one
hand behind your back when discharging tubes, and be
sure your discharge tool is properly grounded to the
metal chassis of the device. There's several kV
potentially stored in there. I don't believe there's
enough in a terminal to kill you, but it'll hurt like
hell. Best to be as careful as you can.



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