Osborne Vixen - Zenith 7" display t-shooting issues

Simon Claessen simski at dds.nl
Thu Nov 6 13:16:15 CST 2014

why do you keep looking at the hot? a transistor fails or works. 
transformers and caps can show the problems you describe. did I mention 
the small 1kv caps around the whole stage?

On 06-11-14 19:18, drlegendre . wrote:
> 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?

Met vriendelijke Groet,

Simon Claessen

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