Osborne Vixen - Zenith 7" display t-shooting issues

[drlegendre .]

Tony, All,

On Thu, Nov 6, 2014 at 11:55 PM, tony duell <ard at p850ug1.demon.co.uk> wrote:

>
> That base amplitude sounds a bit low to me, I would have expected about
> 0.6V. But without
> seeing the waveform it's hard to be sure..
>

I don't have enough experience to rate it one way or the other. All I'll
say, is that (again) these readings were taken when I knew I had precious
little time to take +any+ kind of measurement,  before things went away -
and I was neglecting / ignoring any spiky / HF components. Didn't have time
to analyze the minutiae of the signal to any depth.

My primary goal was to establish an easily-acquired reference to compare
with any changes that might (well, that I knew would) occur. Oh, and it's
like a 10MHz scope (Telequipment D54). Limited bandwidth and capability,
but it does have a really excellent trigger setup..


> BUT...
>
> Remember that the forward voltage drop of a semiconductor junction -- like
> the B-E junction of
> the HOT -- depends on temperature. Diodes have actually been used as
> temperature
> sensors due to this effect.


Sure, alongside output transistors in audio amps, for example.


> So if the HOT is overheating for any reason the base drive
> may appear to increase.
>
> Yes, it's worth replacing the HOT. But don't be too surprised if it
> doesn't cure it.


I will. And no, nothing will really surprise me at this juncture, lol. But
I do suspect that a new HOT will more than likely cure what ails it..

Time will tell. Anyone got a spare? =) Otherwise, I'll hit up eBay or
whatnot.. I'll also check with the local electronics jobber, but they tend
to have mad markups on this stuff. Wouldn't be surprised if they wanted $20
for a simple NTE 379.