1 Jul
2005
1 Jul
'05
9:57 p.m.
I have a VT-131 suffering from video circuit problems. The logic
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.
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.
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?
TIA,
De
2 Jul
2 Jul
3:58 a.m.
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)
That's the diode in the flyback. Just about every VT1xx terminal
has that blackened PCB. (And has probably gone through several
flyback transformers...)
If you've got any other VT1xx terminal, swapping out the flyback/video
stuff that's in common will verify that you're lacking HV for the CRT.
Tim.
2:46 p.m.
If you've got any other VT1xx terminal, swapping
out the flyback/video
stuff that's in common will verify that you're lacking HV for the CRT.
Be careful if you do this. As I mentioned earlier, a bad flyback can
damage the PCB, a bad PCB can damage the flyback.
The problem could be the CRT. Yes, I know that the heater is glowing, but
it's not unheard-of for the cathode connection to break away inside.
Result : No beam current, and thus no illumnination on the screen. I had
this happen on a Volker-Craig terminal.
It is much better to measure the CRT electrode voltages and then work out
what's wrong than to guess and start replacing parts before you have any
clue as to the problem.
-tony
2:42 p.m.
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
7114
days inactive
7114
days old
3 comments
3 participants
participants (3)
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ard＠p850ug1.demon.co.uk -
drb＠msu.edu -
shoppa_classiccmp＠trailing-edge.com