Osborne Vixen - Zenith 7" display t-shooting issues

tony duell ard at p850ug1.demon.co.uk
Fri Nov 7 23:37:33 CST 2014

> >, I would want to reduce the number of variables. If it doesn't work
> > with an MJE13007, then what? Is it that this transistor is not suitable, or is there some other
> > fault?
> That's why god made oscilloscopes, Tony :)  But really, a well-designed

You may know what all the waveforms should be, I could probably make a fair guess.
But the OP admits he's new to this stuff, I was just trying to make it simple for him
> If an MJE13007 doesn't work, try another.  If not that, try, say, a
> BU406.  I seem to recall that some early Macs would work with an
> MJE13007 just fine, but others of the same model required the BU406.

In other words the transistor can be critical. 

> Personally, I don't think that Zenith would ever make the mistake of
> requiring selected semiconductors on what is essentially the video
> section of a TV set, something that they made in the millions.  The
> service manual for the DT2 certainly makes no mention of it.

Hmmm... Not a horizontal output stage, but I have bitter memories of the PSU
in a Zenith MDA monitor. This darn PSU combined the reliability of a switcher with
the efficiency of a linear (!). It was a free-running (no regulation) chopper stage feeding
a chopper transformer. The output of that was rectified/smoothed and gave about
20V DC. It was then regulated down to 12V for the monitor by a discrete transistor linear
circuit -- the reference voltage didn't come from a zener diode, it was the drop across the 
green power-on LED.

Anyway this thing would eat choppers. Sometime it would run for a few minutes and then the
chopper would short. I finally found _a_ transistor that lasted. I could find no other faults (caps 
and diodes all OK or replaced, transformer rang OK, etc). I concluded the original transistor had
been selected for something-or-other.

Does your DT2 manual give the industry standard number or just the house number for the
HOT? If the former, then sure, it's not selected. If the latter, it may well have been.

And no, a failed HOT is not going to do more damage to the monitor. It might to the OP's
(in)sanity, though :-)

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