15 May
2008
15 May
'08
11:48 p.m.
Roy J. Tellason wrote :
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.
Should I probe these with reference to the GND (Earth/shield) or the 0v line
(pin 3, as above)?
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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16:44
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.
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. 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.