On Sat, Nov 25, 2023 at 12:07 PM Rob Jarratt via cctalk <cctalk@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
