Osborne Vixen - Zenith 7" display t-shooting issues

tony duell ard at p850ug1.demon.co.uk
Wed Nov 5 23:47:20 CST 2014

> 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:

Well, it's possible. Normally insufficient base drive is due to a defective coupling component in the
base circuit, but there really aren't any here (I assume the schematic you gave is the one for the monitor
in question). It could be a fault with the driver stage (Q102, etc), but that is unlikely. I did have a 
horizontal drive transformer fail once in a Zenith monitor, but that removed the scan and HV totally.

The next thing to consider is that the gain (beta, hfe, etc) of a transistor depends on the collector 
current. The higher the current, in general, the lower the gain. I wonder if the original HOT is too
low a gain for this circuit, I assume you tested it at a very low current.
One other thing bothers me. With the original HOT you got a vertical line on the screen. In other words you
had the high voltage (probably low), but almost no deflection. That is a very odd fault, it normally points to
 a problem in the yoke circuit. I would check the components in this area (especially the capacitor CX112, 
which is often a problem).

Could you put the old HOT back in and look at the base drive with a 'scope? What do you see on the base?

Can you get a few new HOTs (BU406, BU408, etc)? Try one. What does it do?


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