25 Nov
2023
25 Nov
'23
6:17 a.m.
On Sat, Nov 25, 2023 at 12:07 PM Rob Jarratt via cctalk
<cctalk(a)classiccmp.org> wrote:
Hello,
As some may recall I have been working on getting a VT100 going again. I
have made good progress and I think the main board is probably OK now (see
here if you are interested:
https://robs-old-computers.com/2023/11/19/vt100-keyboard-constant-clicking-f
ault/). Possibly I still need to replace the NVRAM, but I am leaving that
until I fix the problem I want to describe next.
The problem is that there is no image on the screen. This is because the
monitor board is not doing anything, there is no glow from the neck of the
tube etc. I have found that this is because the fuse on the 12V input to the
monitor board is open circuit.
OK. My first suspicion would be problem round the horizontal output
stage/flyback transformer. But that is just a guess
Of course the worry is, why? There could be a fault on the board. I have
tested the transistors in circuit with a multimeter and they appear to be
OK. I used a bench PSU to give the board 12V and it drew no current (with
all connectors disconnected). I tried again with the round connector
attached to the end of the tube and it drew about 100mA and there was a
faint glow from the neck of the tube.
The CRT has a heater fillament rated at about 11V or so. Most, if not
all, such monitors run it from the12V input via a suitable resistor.
So you know the CRT filament is good but you don't know much else so
far.
I am hesitant just to replace the fuse and try it. I am hoping for some
suggestions on how to test this safely (in particular without involving the
flyback transformer) to find if there is a fault.
I don't think you can do much without the flyback connector plugged
in. You need the flyback transformer to do any sane tests on the
horizontal side, and you need the iron-cored inductor, normally wired
on the same connector, to get the vertical output stage to work. Also
you may need the deflection yoke connected for some tests, the
inductance of the horizontal deflection windings can make quite a
difference (factor of 2) to the voltages produce by the flyback.
The flyback transformer started as a 'something for nothing' idea. To
deflect the electron beam horizontally, you store considerable energy
in the horizontal yok windings. Rather than waste that at the end of
the line, you use it to power other bits of the monitor, like the EHT
for the CRT final anode.
Have you checked the diodes and capacitors associated with the flyback
transformer? If something is shorted there. it's the equivaent of
running a power supply into a short circuit. The input current will go
up.
Does this unit have a horizontal oscllator? Quite a lot do not, they
simply use the horizontal pulses from the logic,suitably amplified, to
drive the horizontal output transistor. This could never have worked
for television as interference pulses could send the thing crazy with
voltages going all over the place, but it's not uncommon in small
monitors. The IBM5151 was like that. If there is no horizontal
oscillator then you need to provide a drive signal of the correct
frequency and duty cycle. Rather than use the logic (which might be
faulty and thus mis-driving the monitor), I've been known to cobble
something up using a 555 timer chip,
Here's roughtly what I would do :
Trace out a schematic of the board so I know what I am dealing with.
Check all diodes and capactiors hung off the flyback transformer.
Ring-test the flyback transformer.
Make a test oscillator to drive it if necessary
Connect it up, run it from a current limited supply. If it tries to
draw too much current, then I've mssed something. Might try lifting
the diodes to disconnect voltage outputs to see if one of those is
loading it.
I do wonder what the problem is with testing it with the flyback
connected, though...
For information, the monitor is an Elston and I pre-emptively replaced all
the electrolytics on the monitor board apart from the non-polar one.
Why?
I have never understood replacing capacitors at random in the hope it
cures the fault. It is much easier to start from a board that once
worked and trace the fault.
-tony