Osborne Vixen - Zenith 7" display t-shooting issues

tony duell ard at p850ug1.demon.co.uk
Thu Nov 6 13:42:49 CST 2014

> It's not that there was a solid, bright vertical line - the image consisted
> of the contents of the display (an ACSII block message box, plus a line or
> two of text) squashed into the vertical. So there was a short, dashed
> vertical line about 1.5" high, in the upper third of the screen. It was
> dim, and became more dim over the course of a minute or two - eventually
> disappearing totally, appearing to slowly slide upward past the upper limit
> of the screen.

Ah... That sounds like the horizontal output stage stopped working completely (so no scan) and the thing kept 
working on the charge stored in the capacitors.

> More on "disappearing" coming up.. I now realize that it's important.

> >
> > Could you put the old HOT back in and look at the base drive with a
> > 'scope? What do you see on the base?
> >
> Having concluded that the original HOT was reading in-spec, I did
> re-install it as part of my next-step troubleshooting. I then disconnected
> the tertiary supplies (15V & 50V) by lifting leads of W3 & CR104,
> respectively. Guess what? The 50V supply came up to 44V (the highest I've
> seen it).. then the stupid stuff started..
> I re-connected the 15V supply, and checked the 50V - still good. Then
> re-connected the 50V supply - and it's still good.. AND the d*mn display is
> working!! Now how obnoxious is that? Oh, and the HOT was running quite
> cool, barely over 100F.
> But not for long.. soon, the display faded out and the 50V supply crashed
> to 20V.  Ack, it's an intermittent..!
> Replaced C114 & C401 (the stabilizng caps for the 50V & 15V supplies) just
> for drill, to no effect. So this leaves a couple of possibilities.. There's
> an intermittently shorting device in one of the tertiary supplies -or- the

This sounds thermal to me. Somethng is failing when hot. Maybe the HOT. Maybe
a diode or capacitor in the horizontal output stage.


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