brightening CRTs

[Tony Duell]

> 
> 
> So, I've acquired a Wyse 99gt with no screen burn, but I need to turn the
> contrast up all the way to be visible.  Can anyone point me to documents
> or other resources on brightening the CRT?


Firstly, clean the screen if you've not done so already. You can laugh, 
but it caught me once (after spending a morning with a 'scope looking at 
a VT100's video amplifier...)

If you've got a low emission, CRT there are a few tricks that might help. 
Firstly, if there's a 'g2' or 'screen' control on the PCB, turn it up a 
bit. It won;'t do any damage, and it might get a bit more brightness out 
of the CRT.

Increasing the CRT heater voltage can help too, although you do run the 
risk of buring out the header and ruining the CRT. If there's a series 
inductor on the CRT base PCB, short it out (it's often enough). If 
there's a series resistor, drop the value a bit (shorting it out is 
probably going too far. A fw turns of wire round the flyback transformer 
core, connected in series with the heater, can provide a bit of a voltage 
increse too, but, again, be careful (and you are increasing the load on 
the horizontal output stage and flyback if you try this, which might lead 
to ehm failing).

There used to be things called CRT boosters. Typivally, they worked by 
over-running the header (about 8-10V for a 6.3V heater), and then 
applying a high voltage between the cathode and first grid. The strong 
electric field at the surface of the cathode rips said surface off and 
exposes fresh emissibe material.

It worked reasonably well on older CRTs, but then the manufacturers put 
less emissive material in in the first place so you couldn't really 
expose a new layer.

It may work, it may totally ruin the CRT,

In fact, what abvout replacing the CRT? Is there anything special about it?

-tony