Osborne Vixen - Zenith 7" display t-shooting issues

drlegendre . drlegendre at gmail.com
Mon Nov 3 13:10:57 CST 2014

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? =)

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