Osborne Vixen - Zenith 7" display t-shooting issues
simski at dds.nl
Mon Nov 3 13:25:58 CST 2014
check cx111 and cx116/cx117 booster capacitors have a lot of work to do
and a faulty one can be the problem here. the original trasistor could
be ok as there was ht on the tube. as you mentioned a vertical line, the
othe voltages on the tube seem ok, so the capacitors around the crt
should be fine.
On 03-11-14 20:10, 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? =)
Met vriendelijke Groet,
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