Osborne Vixen - Zenith 7" display t-shooting issues

drlegendre . drlegendre at gmail.com
Mon Nov 3 15:37:29 CST 2014

Tony, All,

On Mon, Nov 3, 2014 at 2:09 PM, tony duell <ard at p850ug1.demon.co.uk> wrote:

> > voltages on the scan-derived power supplies - the 50V in particular was
> > doing about 18-20V
> This suggests to me (along with the overheating new HOT) that something is
> overloading the horizontal output
> stage/flyback transformer. Have you checked for shorted capacitors, etc?
> Have you done a ringing test on the
> flyback? Normally such faults completely kill the stage, but...

Others have likewise suggested that 'something' is loading the circuit. But
so far, I can't seem to find anything that doesn't act the way it should..
 caps, diodes etc. seem to be fine. But I'll give it another close look.

> > Removed / tested the horiz. opt. transistor (HOT). Neither open nor
> > shorted, but beta measures ~12. Replaced the HOT and scoped the B & C
> It is not uncommon for HOTs to have a very low beta.

I've more or less gotten that impression, but.. do me a favor and take a
look at something. Go to http://nerp.net/~legendre/osborne/ and grab the
Osborne service manual. There's an appendix with full service data for the
+other+ 7" monitor that they used - a Motorola unit. Unless I'm wrong,
isn't the HOT in there a Darlington device? Aren't Darls. almost
universally high-gain devices?

> > circuits - base drive was +much+ higher (like 10X) level than the output
> at
> Hang on, the base drive should be something like 0.6V The emitter is
> grounded, after all.
> Normally you get HV spikes on the collector.

If I re-install the old HOT I can re-check it, but IIRC the base drive
waveform was like 10X that at the output. This is what led me to try
subbing it out in the first place.. why would an amp (configured for gain)
have a lower output than input? And since it was clear to me that the opt.
waveform was also indirectly responsible for producing the 50V supply, it
seemed pretty obvious that a bigger waveform was called for, there.

> > 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.)
> I am not convinced those voltage ratings are high enough. Try something
> like a BU406.

Nor am I. Frankly, I don't even see how it's working at all, why it didn't
over-volt and die right off the bat. But whatever.. who knows what that
part +actually+ is? So long as it meets or exceeds the specs on the
package, it's acceptable. Perhaps its actual specs are in fact much higher
than stated?

> My guess, as I said above, is that something is loading this stage rather
> heavily. Often such a fault will cause
> display problems, but not always. Check for shorted/leaky capacitors and
> diodes, shorted turns in the flyback
> transformer (do a ringing test) and so on.

I don't know squat about analog video, or how to perform / interpret a
'ringing test'. Will try searching on that one, but I may need some
instruction on the subject.

Again, if I just had some dang specs I could ohm-out the flyback, know the
actual specs of the orig. HOT, and all that other great stuff that I can't

> > 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
> I doubt a service manual would contain any more information. The circuit
> looks pretty standard and most
> manuals assume you know how to fix the standard version of <whatever>. A
> parts list is likely to just give
> the house numbers of the transistors -- Zenith loved to house-code
> everything.
> Well.. take a look at the service manual. They were good enough to put the
full mfr's data for the Motorola in there.. and it seems that the Zenith
data was also present. But the appendix section for the Zenith is simply
+missing+ from the text! Now how obnoxious is that??

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