Osborne OCC1 problem

[Ade Vickers]

Roy J. Tellason wrote :

> Hm,  could be I'm mis-remembering slightly here.  Or it could 
> be used in later stuff.  I recall seeing only one or two of 
> the tan case units,  most of them were the later ones.

I sold my non-tan cased Ozzie, on the basis it wasn't rare enough for my
liking! Of course, that one worked perfectly...

> > Measured with respect to Earth (green wire, goes everywhere 
> including 
> > to the mainboard). As far as I can tell, this acts as GND for the 
> > whole machine?
> 
> Shielding more than anything else,  I'd guess.

OK, that would make sense. In which case, I'd expect the differential
between pins 3 & 4 to be approx 5v, but will confirm tomorrow.

> 
> Even still,  it shouldn't be fluctuating either.
> 

My mistake - there was no fluctuation, except in the optics of the sensor
(me & my eyes). The voltage output was a perfectly flat line (except when
the 'scope sensitivity was turned to 20mv/cm, at which point I could just
about see some rectification effects - but getting the 'scope to trigger on
the peaks was really hard work; it's a pretty flat line.

The signal on Pin 3 (-0.1v I guessed at) was a little noiser, but seemed to
be random noise. Having said that, I didn't "zoom in" on it so much.

If, as you imply, Pin 3 is GND and Pin 4 is +5v; then the difference between
those two is much closer to 5v than between Pin 4 & Earth.

> >
> > Your assistance is much appreciated. I've still got to try Tony's 
> > suggestion of checking out the H-sync signal, but I need to 
> find it first...
> 
> It'll be on that connector on the front,  and prety obvious 
> which one it is because of the repetition rate,  as opposed 
> to the much lower rate for the vertical sync,  and the much 
> erratic nature of the video.  And if it's *way* off in 
> frequency or changes a lot or has a lot of jitter than we're 
> probably looking at the power supply not being right.

Should I probe these with reference to the GND (Earth/shield) or the 0v line
(pin 3, as above)?

> > I shall consider it - is there any (non-destructive: I know you can 
> > pump 240vac into it, then when the magic smoke escapes 
> conclude it's 
> > broken :)) way to test an electrolytic cap?
> 
> With some equipment,  yeah.  The least I'd want is to use an 
> isolation transformer to plug the computer into and then 
> scope across the main filter caps and see what you get,  but 
> I wouldn't care to mess around in there without an isolation 
> transformer.
> 
> If they're bulgy on top they're bad.

They look brand new TBH. Not even any dust on them (probably because the PSU
is mounted component-side-down in the case). That, and the case itself is
pretty tightly sealed when the unit is assembled & closed up.

Cheers,
Ade.

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