> -----Original Message-----
> From: Tony Duell <[email protected]>
> Sent: 25 November 2023 14:17
> To: [email protected]; General Discussion: On-Topic and Off-Topic Posts
> <[email protected]>
> Cc: Rob Jarratt <[email protected]>
> Subject: Re: [cctalk] VT100 Monitor Board
>
> On Sat, Nov 25, 2023 at 12:07 PM Rob Jarratt via cctalk
> <[email protected]> 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-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.
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.
>
> 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
Thanks for all the advice Tony.
Regards
Rob
