Osborne Vixen - Zenith 7" display t-shooting issues

[Tothwolf]

On Mon, 3 Nov 2014, drlegendre . wrote:

> 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
>
> Removed / tested the horiz. opt. transistor (HOT). Neither open nor 
> shorted, but beta measures ~12. Replaced the HOT and scoped the B & C 
> circuits - base drive was +much+ higher (like 10X) level than the output 
> at 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.)
>
> Bingo! Display is now bright & crisp, looks great.. but.. the sub. HOT 
> is running way, way hot. Rose to 170F in 2-3 min, and was steadily 
> climbing. And that's in free air, not sealed up in the case.
>
> 50V supply came up to 42V.. better, still seems too low. So I think you 
> can see my conundrum, here.. Is the +original+ HOT actually OK, and I'm 
> only masking another problem in the circuit by installing a new part 
> with 10X the gain? Why is the new part running so dang hot - and if the 
> orig. is in fact bad, what killed it, anyway?
>
> 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 original HOT - so I can't tell if it's in or out-of-spec. It's 
> marked "SGS 1070 / 8309". I +think+ the 8309 is for March 1983 but who 
> knows.
>
> Any ideas on this one, folks? =)

I managed to track down the OEM manufacturer of the transistor, but it 
looks like you might not need that information now since you've found an 
NTE cross.

According to one of my transistor books, the SGS marking indicates the 
device was made by "SGS-ATES Componenti Electronici S.p.A.". That company 
was also known as Società Generale Semiconduttori, which merged with 
Thomson Semiconductor in 1987, becoming SGS-Thomson, which changed its 
name in 1998 to STMicroelectronics.