Osborne Vixen - Zenith 7" display t-shooting issues

[drlegendre .]

Looks like we're mostly over the hump with the list breakage, so it's time
to post an update on this.

Our friend Chuck cracked the first nut, by essentially solving the mystery
of the HOT. Turns out that the "1070" marking is an abbreviated Zenith
in-house part number; the full version is 121-1070. Also turns out that the
121-1070, like the Zenith 121-1039 in his 12" display apparently crosses to
an NTE 379, which itself may effectively be a BU408. Again, a big round of
thanks for Chuck on this work.. score one for the flame of knowledge.

So based on the specs of the NTE 379 with an avg, beta of 18-24 (min. 6/8 /
max. 30/40), the extant HOT (1070) with a measured beta of 11-12 may
actually be in-spec, albeit on the very low side. But an average of ~18-24
is a far cry from the 150 of the unit in-use.. so that leads me to a
question, of course.

The HOT basically acts as a switch in this application - it's really an
on/off proposition vs. linear amplification. So why is it that the original
HOT wasn't able to transition to a sufficiently low C-E resistance to do
the job? For some reason, I have the odd feeling that this is due to a lack
of base drive, rather than an excessive load imposed by either the HV
supply, yoke and/or the derived tertiary supplies (15V, 50V). Here's my
line of logic:

The beta / Hfe / gain doesn't really matter - if the HOT saturates, it
saturates and falls to a very low C-E resistance. The sub part (NTE 375,
which is a +vertical+ opt. part) is obviously saturating, as we're getting
plenty of deflection & output from all the supplies. If the original 1070
doesn't produce the same results, then it seems the the reasons are pretty
limited...

It can't be due to excessive loading by the tertiary or HV supplies, as
those can only rise as high as a saturated HOT will allow. +It's not as if
the NTE 375 has any significantly lower "on" resistance than the 1070
could, if both are saturated, right?+ So then, if the various supplies &
deflection are OK with the 375, there's nothing a "better" or "stronger"
HOT could do to increase those levels - see my logic?

This points to only one (well, two) possibilities: Either the base drive is
inadequate to saturate the low-gain 1070 (yet fine to saturate the NTE 375,
with 10X the gain) -OR- there is some defect in the 1070 itself that's
squelching what would otherwise be an adequate base drive - i.e. its input
impedance has somehow fallen very low, and way out of spec such that it's
killing the drive.

Your comments are not only appreciated, but basically required.. ;-)

On Tue, Nov 4, 2014 at 12:39 AM, Tothwolf <tothwolf at concentric.net> wrote:

> 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.
>