VT-131 prints?

[Tony Duell]

> 
> I have a VT-131 suffering from video circuit problems.  The logic

I think all the VT1xx terminals use much the same video board, but there 
are at least 3 versions of it that were essentially randomly used in all 
models. The VT100 prints that I have show one version, I traced out the 
schemaitc of the one used in my VT100s.

> seems to be ok -- does a self test, led dance, emits a normal beep,
> and ends with the on-line led lit.  The crt filament is glowing.
> Key click happens when keys are pressed.  No screen display.
> I've turned up brightness in set-up, flying blind.  Haven't twiddled
> the pot yet.  So far, I've just taken the lid off, and am looking
> into the still-assembled chassis.

I would start by measuring the CRT voltages. If you have an EHT meter, 
start with the anode connector on the side of the CRT (expect about 10-12 
kV here I think). Expect a few hundred volts on pins 6 and 7 of the CRT 
(g2 and g3, to use the US names). 

> 
> The video board has some very charred-looking spots around the solder
> pads of one component (cap?  spherical white ceramic-like device with
> two stout leads) down and to the right of the brightness adjustment.

That sounds like one of the many diodes that produce supplies from the 
flyback transformer.

> 
> The tech manuals from bitsavers and vt100.net don't give me enough
> info to identify the component in question.
> 
> Anyone have prints they'd be willing to scan bits of for me?  Or read
> off to me by e-mail?  Or have "replace this, kid" or "you're screwed"
> specific knowledge of this scenario?

Alas most of the time the problem is the flyback transformer. Be warned 
that a shorted diode on the video board can kill flybacks, and that a 
defective flyback transformer can kill the horizontal output transistor 
on the video board.

-tony