Osborne Vixen - Zenith 7" display t-shooting issues

drlegendre . drlegendre at gmail.com
Thu Nov 6 20:06:57 CST 2014

** Update - I think it's nailed **

I let the unit cool down sufficiently such that things were "working" again
- but this time, I was ready. I had the base & collector of the HOT
instrumented, as well as the first cap in the 50V supply. Recall that both
15V & 50V tertiary supplies are otherwise disconnected from the horiz.
section where they are produced.

With the 50V supply running fine at 44V, the signals at the HOT base
measured 212mV P-P, and the collector 15V P-P. This ignores any HF spikes,
I'm just concerned with the significant power region of the signals.

After a few minutes, the 50V supply crashed.. and guess what? The HOT base
signal actually increased a bit, to around 250mV P-P while the collector
fell by at least an order of magnitude.

So please correct me if my conclusions are unwarranted, but I believe the
situation is quite clear, now. The  HOT has a thermal intermittent in the
E-C circuit - I just can't see any other explanation for this behavior. The
base signal didn't flicker, it actually +rose+ as the collector signal went
away. It's as if the E-C circuit is going high-impedance while the B-C
circuit remains intact.

Also - if something (like C116) were shorting the C to ground, we wouldn't
see a +rise+ in the base waveform.. we'd only see the base signal increase
if the load on the collector were to go away - eh?

IOW, It's just gotta be the HOT. From the get-go.

On Thu, Nov 6, 2014 at 6:39 PM, drlegendre . <drlegendre at gmail.com> wrote:

> Forgot to mention - this afternoon, when the 50V supply crashed while I
> was monitoring it, I actually +heard+ it happen.. there was a very faint
> little buzzy-sizzle sound "zzzzzztt" as the supply failed. Wish I knew what
> made the noise.. but it could have just been audible harmonics coming from
> the yoke or some other effective transducer, rather than noise produced by
> the faulty component itself.
> On Thu, Nov 6, 2014 at 6:37 PM, drlegendre . <drlegendre at gmail.com> wrote:
>> 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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