Osborne Vixen - Zenith 7" display t-shooting issues

tony duell ard at p850ug1.demon.co.uk
Mon Nov 3 14:09:57 CST 2014

_________________Firstly an explanation. I am currently moving house. Well, to be more exact, I am in the new house, as are
my classic computers (or what's left of them [1]). But very little is set up. So I am using an rather inconvenient
system to post here. I will therefore not be posting as much as I sometimes do...

[1] The movers assured me they knew what they were doing and told me I didn't know how to handle computer
stuff. Alas they were very, very, wrong. Stuff which I packed and moved (fortunately the rarest machines) has
survived. Solidly built stuff like PDPs has survived because it was designed to stand up to minor misuse. But
countless keyboad are now missing keycaps (some with broken switches). I have a Pro350 I can't get the 
mainboard out of because the chassis is twisted (that is probably restorable). ARGH! And that's just the stuff
I have looked at.

> Need some help on this one.. trying to repair the built-in CRT in an
> Osborne Vixen portable. There's some complexity here, so I'll do my best to
> be succinct.
> The unit presented with a vertical line down the screen, and very low
> voltages on the scan-derived power supplies - the 50V in particular was
> doing about 18-20V

This suggests to me (along with the overheating new HOT) that something is overloading the horizontal output
stage/flyback transformer. Have you checked for shorted capacitors, etc? Have you done a ringing test on the
flyback? Normally such faults completely kill the stage, but...

> Removed / tested the horiz. opt. transistor (HOT). Neither open nor
> shorted, but beta measures ~12. Replaced the HOT and scoped the B & C

It is not uncommon for HOTs to have a very low beta.

> circuits - base drive was +much+ higher (like 10X) level than the output at

Hang on, the base drive should be something like 0.6V The emitter is grounded, after all. 

Normally you get HV spikes on the collector. 

> the collector. So again pulled the HOT and swapped in the only thing I had
> that +seemed+ it might work:
> RCA SK9118 (375) - Pt 25W / Vcbo 200V / Vceo 150V / Vebo 6V / Hfe 150 (typ.)

I am not convinced those voltage ratings are high enough. Try something like a BU406.

My guess, as I said above, is that something is loading this stage rather heavily. Often such a fault will cause
display problems, but not always. Check for shorted/leaky capacitors and diodes, shorted turns in the flyback
transformer (do a ringing test) and so on.

> To make things worse, I can't find +any+ service data or parts list for the
> display; all I have is a schematic. Nor can I find a datasheet for the

I doubt a service manual would contain any more information. The circuit looks pretty standard and most
manuals assume you know how to fix the standard version of <whatever>. A parts list is likely to just give
the house numbers of the transistors -- Zenith loved to house-code everything.


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