25 Nov
2023
25 Nov
'23
3:47 a.m.
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.
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.
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.
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. Some
details of what I did are here
https://robs-old-computers.com/2023/10/01/vt100-ram-fault/. Although I have
since realised that I didn't replace two of them because they looked like
diodes. I don't think the board I have is the one in the available
printsets.
Thanks
Rob
25 Nov
25 Nov
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
3:12 p.m.
-----Original Message-----
From: Tony Duell <ard.p850ug1(a)gmail.com>
Sent: 25 November 2023 14:17
To: rob(a)jarratt.me.uk; General Discussion: On-Topic and Off-Topic Posts
<cctalk(a)classiccmp.org>
Cc: Rob Jarratt <robert.jarratt(a)ntlworld.com>
Subject: Re: [cctalk] VT100 Monitor Board
On Sat, Nov 25, 2023 at 12:07 PM Rob Jarratt via cctalk
<cctalk(a)classiccmp.org> wrote:
I have checked all the diodes (in circuit) and they appear to be OK. One capacitor may be
suspect but without the full schematic I am not sure. I may have to lift it to check.
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-clic
king-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.
Yes, I may have to do that. Historically I have done a poor job of this because of my
insufficient understanding of electronics.
Check all diodes and capactiors hung off the flyback transformer.
Agreed, will check carefully.
Ring-test the flyback transformer.
You sent me a circuit for this years ago and I built the device, so this is definitely
something else I can do.
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...
Only that I know it can produce lethal voltages and I would rather avoid that if I can.
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
10:34 p.m.
On Sat, Nov 25, 2023 at 11:12 PM Rob Jarratt
<robert.jarratt(a)ntlworld.com> wrote:
Without the schematic you are going to have problems.
With a schematic you can see
(a) Can you isolate certain areas -- for example the vertical deflection syste
It gets easier with experience. Looking at every small monochrome
monitor schematic you can find will give you an idea of the sort of
things to look for.
The high voltage outputs can't supply that much current and are
unlikely to be lethal. Mains, and even worse the rectified mains in an
SMPSU, is a lot more liely to kill you.
That said, work with one hand in your pocket (current flow arm-to-arm
is the most dangerous) and take care.
I don't see how you can debug a monitor without having the flyback in
place. It's part of the highest power circuit on the board. And it
provides voltages for many other areas.
-tony
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.
I have checked all the diodes (in circuit) and they appear to be OK. One capacitor may be
suspect but without the full schematic I am not sure. I may have to lift it to check.
Trace out a
schematic of the board so I know what I am dealing with.
Yes, I may have to do that. Historically I have done a poor job of this because of my
insufficient understanding of electronics. I do wonder
what the problem is with testing it with the flyback connected,
though...
Only that I know it can produce lethal voltages and I would rather avoid that if I can.
29 Nov
29 Nov
10:16 a.m.
-----Original Message-----
From: Tony Duell <ard.p850ug1(a)gmail.com>
Sent: 26 November 2023 06:35
To: rob(a)jarratt.me.uk
Cc: General Discussion: On-Topic and Off-Topic Posts <cctalk(a)classiccmp.org>
Subject: Re: [cctalk] VT100 Monitor Board
On Sat, Nov 25, 2023 at 11:12 PM Rob Jarratt <robert.jarratt(a)ntlworld.com>
wrote:
Hello Tony,
I am working on the schematic and when it is done I will post it for information. However,
I was wondering what you suggest for testing the board once I have done this. Simply
replacing the blown fuse and plugging it in doesn't sound like a good idea. Equally,
not sure if using the light bulb method for limiting current into the PSU is going to help
is it?
Thanks
Rob
> 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.
I have checked all the diodes (in circuit) and they appear to be OK. One
capacitor
may be suspect but without the full schematic I am not sure. I may
have to lift it to check.
Without the schematic you are going to have problems.
With a schematic you can see
(a) Can you isolate certain areas -- for example the vertical deflection syste
because of
my insufficient understanding of electronics.
It gets easier with experience. Looking at every small monochrome monitor
schematic you can find will give you an idea of the sort of things to look for.
if I
can.
The high voltage outputs can't supply that much current and are unlikely to
be lethal. Mains, and even worse the rectified mains in an SMPSU, is a lot
more liely to kill you.
That said, work with one hand in your pocket (current flow arm-to-arm is the
most dangerous) and take care.
I don't see how you can debug a monitor without having the flyback in place.
It's part of the highest power circuit on the board. And it provides voltages for
many other areas.
-tony
Trace out
a schematic of the board so I know what I am dealing with.
Yes, I may have to do that. Historically I have done a poor job of this I do
wonder what the problem is with testing it with the flyback
connected, though...
Only that I know it can produce lethal voltages and I would rather avoid that
10:23 a.m.
On Wed, Nov 29, 2023 at 6:16 PM Rob Jarratt <robert.jarratt(a)ntlworld.com> wrote:
I am working on the schematic and when it is done I
will post it for information. However, I was wondering what you suggest for testing the
board once I have done this. Simply replacing the blown fuse and plugging it in
doesn't sound like a good idea. Equally, not sure if using the light bulb method for
limiting current into the PSU is going to help is it?
You could try a lamp limiter between the 12V power supply and the
monitor board (or across the (empty) fuseholder). Something like a car
headlamp bulb with both filaments in parallel.
-tony
30 Nov
30 Nov
12:43 a.m.
On Wed, 29 Nov 2023, 'General Discussion: On-Topic and Off-Topic Posts' wrote:
I am working on the schematic and when it is done I
will post it for
information. However, I was wondering what you suggest for testing the
What am I missing? Why don't you use the official schematics of the VT100
that also include the schematic of the monitor board?
It is on bitsavers PDF page 43.
/pdf/dec/terminal/vt100/MP00633_VT100_Schematic_Feb82.pdf
Christian
2:34 a.m.
-----Original Message-----
From: Christian Corti via cctalk <cctalk(a)classiccmp.org>
Sent: 30 November 2023 08:43
To: Rob Jarratt via cctalk <cctalk(a)classiccmp.org>
Cc: Christian Corti <cc(a)informatik.uni-stuttgart.de>
Subject: [cctalk] Re: VT100 Monitor Board
On Wed, 29 Nov 2023, 'General Discussion: On-Topic and Off-Topic Posts'
wrote:
My board does not match the one in the schematic.
I am working on the schematic and when it is done
I will post it for
information. However, I was wondering what you suggest for testing the
What am I missing? Why don't you use the official schematics of the VT100
that also include the schematic of the monitor board?
It is on bitsavers PDF page 43.
/pdf/dec/terminal/vt100/MP00633_VT100_Schematic_Feb82.pdf
Christian
3:15 a.m.
Den tors 30 nov. 2023 kl 11:34 skrev Rob Jarratt via cctalk <
cctalk(a)classiccmp.org>gt;:
You know that there is another variant of the monitor board described in a
schematic at page 58, right?
/Mattis
-----Original Message-----
From: Christian Corti via cctalk <cctalk(a)classiccmp.org>
Sent: 30 November 2023 08:43
To: Rob Jarratt via cctalk <cctalk(a)classiccmp.org>
Cc: Christian Corti <cc(a)informatik.uni-stuttgart.de>
Subject: [cctalk] Re: VT100 Monitor Board
On Wed, 29 Nov 2023, 'General Discussion: On-Topic and Off-Topic Posts'
wrote:
My board does not match the one in the schematic.
I am working on the schematic and when it is done
I will post it for
information. However, I was wondering what you suggest for testing the
What am I missing? Why don't you use the official schematics of the VT100
that also include the schematic of the monitor board?
It is on bitsavers PDF page 43.
/pdf/dec/terminal/vt100/MP00633_VT100_Schematic_Feb82.pdf
Christian
161
days inactive
166
days old
8 comments
4 participants
participants (4)
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Christian Corti -
Mattis Lind -
Rob Jarratt -
Tony Duell