IBM 5120 monitor issues
ard at p850ug1.demon.co.uk
Wed Oct 17 19:41:01 CDT 2007
> But I'd like to see what I can do about the monitor. The picture
> exhibits a strange behavior that I can't say I've seen before. The
> horizontal retrace seems to "fold" over itself in the middle (like the
> electron gun is doing a "loop" in the center of the screen for each
> line). I've taken a (terrible) picture of the fault and put it at:
> http://yahozna.dyndns.org/scratch/5120disp.jpg). The picture's also a
> bit jumpy. I've tinkered a bit with the various adjustments on the
> CRT's PCB but it doesn't make any appreciable difference.
Now that's unusual, to say the least....
Now, let's think about this.
Firstly, the deflected position of the beam is in the centre of the
screen, right. Normally, the horizotnal output transistor, when turned
on, pulls the beam to the left (and also stores energy in the magnetic
fields of the yoke and hoirzotnal output transofrmer), the beam is then
'released' and goes back towards the centre, and then the stored energy
is used to get it all the way across. OK, that's an oversimplification,
but it'll do for a start.
Now it appears to me you've got some kind of 'ringing' at the crossover
point. The beam is moving back and forth at the centre of the screen.
I'd start checking components around the horizontal output stage/flyback
transformer. Maybe an open capacitor, maybe a damper diode is
open-circuit. Being a Ball Brothers unit, it should be fairly standard
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