Osborne Vixen - Zenith 7" display t-shooting issues

drlegendre . drlegendre at gmail.com
Thu Nov 6 18:37:41 CST 2014

Hi Simon, All - for an update (below)

On Thu, Nov 6, 2014 at 1:16 PM, Simon Claessen <simski at dds.nl> wrote:

> why do you keep looking at the hot? a transistor fails or works.
> transformers and caps can show the problems you describe. did I mention the
> small 1kv caps around the whole stage?

You know the saying.. when all you have is a hammer, everything looks like
a nail.. ;-) IOW, it's largely due to my lack of experience.. I'm sticking
to the stuff I know, and when it comes to analog video, the horizontal
output section is about the only area I have any understanding. Also, I do
know a horizontal issue when I see one (vertical line) and when you add-in
the low tertiary supplies, the HOT is at the core of it.

As for the rest of the caps around the board, I've checked them all with my
eyes (for liquid leakage / blow-ups), an ohmmeter (for static shorts /
electrical leakage) +and+ a quality in-circuit ESR meter (Capacitor Wizard)
and can't find a bogie.

Not to get twitchy, now, but I heartily challenge your assertion that
transistors "either work or don't".. transistors have more than the two
(hard short / hard open) common failure modes - it's in the literature.

** General update commences here **

But the thing is, I've eliminated the tertiary supplies.. by lifting the
diode / wires as I did, they are disconnected from the horiz. section. And
in this state, the 50V supply still crashed on me, did so this afternoon -
and yes, I have replaced all caps connected to the 50V line. It is clearly
a thermal intermittent issue - things are OK when cold, and go haywire
after a few minutes time.

One reason that I'm still suspecting the HOT, is that the entire time I had
the NTE 375 replacement installed, the 50V supply never crashed. No, I
didn't run it for that long, but based on how fast things go bad (when it
warms up) I really should have seen something when the 375 was in place.
Oh, and to eliminate questions about shorts in the horiz. yoke, I pulled
the connection while the 50V supply was in the crashed state - and the
supply rose by perhaps a volt or two - so the yoke is clearly not the
source of a short-circuit.

I still suspect the HOT or components to the left (aka base drive). I need
to let the system get good & cool before I hook up the scope to watch what
happens to the base drive when the 50V supply crashes. if it's stable, then
it's either the HOT or possibly the flyback diode or the (what's it
called?) safety cap? The "special" 400V cap in parallel with the flyback
diode? I'm running out of stuff it could be.. of course, there's always the
horiz drive transformer, ugh.

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