Tube or not Tube - test-drb@ccmp.vtda.org

22 Aug 2007

 Hi

Strangly enough I do have an EHT meter. It's a legacy of building my own
colour TV when colour came to the UK in the 1970's.

Rod

-----Original Message-----
From: cctech-bounces at classiccmp.org
[mailto:cctech-bounces at classiccmp.org] On Behalf Of Tony Duell
Sent: 22 August 2007 22:38
To: cctalk at classiccmp.org
Subject: Re: Tube or not Tube

...

  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 
Yes. It's almost impossible to get a 'bare' CRT, since often replacing a
CRT is regarded as uneconomic, so nobody stocks the spares. I know that
when I needed a CRT from a Volker-Craig terminal, I bought a cheap
monochrome TV and raided it for the CRT.

...
  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. 
There are baspically 2 types of mono CRT. Thick necks with a B8H base
and a 6.3V heater and thin neckes with a modifed B7G base and an 11.5V
heater. Almost all mono monitor and termial CRTs have a 90 degree
deflection angle.

...
       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 
I would recoemnd against this method. When the contact is made, the peak
current can be quite high. High enough to damage the connation between
the anode cap and the anode coating inside the CRT. And since that
current will flwo where it feels like, it can, if you're not careful,
damage semiconductors all aover the chassis.

What I do is use my EHT meter. It has a resistance of 800MOhms, and is
desigend to handle up to 50kV. Connect the ground lead of the meter to
the CRT earth (the metal contacts that touch the outer aquadag coating),
and put the probe under the anode cap. Hold it there for a minute or so
and the CRT will be discharged

...
  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. 
Normally, once you've freed the edgesof the cape from the CRT glass, you
can move the whole thing over to compress one side of the contact and
release the other. This avoids having to touch the high voltage terminal
(even though it should be discharged.)

[...]

...
  yourself). Once the yoke is level, shut the terminal
off and tighten 
 the yoke so it won't move. 

But not too tight. You don't want to crack the new CRT...

The old rule (not to be taken seriously, of course) was to tighten the
clamp until the CRT implodes, then back off a quarter of a turn ;-)

-tony