Osborne Vixen - Zenith 7" display t-shooting issues

[drlegendre .]

Tony, All,

On Wed, Nov 5, 2014 at 11:47 PM, tony duell <ard at p850ug1.demon.co.uk> wrote:

>
> One other thing bothers me. With the original HOT you got a vertical line
> on the screen. In other words you
> had the high voltage (probably low), but almost no deflection. That is a
> very odd fault, it normally points to
>  a problem in the yoke circuit. I would check the components in this area
> (especially the capacitor CX112,
> which is often a problem).
>

There was more to it than that, but I described it many posts ago, and you
probably missed the details.. here they are again, for anyone who might be
curious.

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.

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
HOT itself is intermittent. Also possible that there's an issue with the
base drive dropping out, but I'm putting my money on the HOT itself. The
base drive question can be eliminated with a scope, but I'm going to
replace the HOT in any event.

Anybody got a spare laying around?