15 May
2008
15 May
'08
12:50 p.m.
On Thursday 15 May 2008 15:32, Ade Vickers wrote:
Yep, figured that one.
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.
Well..... as my multimeter is flat & I don't have a signal generator, I
calibrated the 'scope using a 1.5v AA battery. Thus, accuracy is far from
guaranteed...
Even still, it shouldn't be fluctuating either.
Hm.
Hopefully I'll find my Osborne tech docs one of these days.
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.
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.
--
Member of the toughest, meanest, deadliest, most unrelenting -- and
ablest -- form of life in this section of space, ?a critter that can
be killed but can't be tamed. ?--Robert A. Heinlein, "The Puppet Masters"
-
Information is more dangerous than cannon to a society ruled by lies. --James
M Dakin
Roy J. Tellason wrote:
OK. I'm not sure how it gets there, as there is no connection in the plug
to take it (and there's only the single 3-wire plug leaving the PSU
board...
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 have
concluded that, with respect to GND:
Pin 1 has approx -13.5v - but is not connected to anything
That's the nominal -12V, and it's used in the RS232 port and
nowhere else that I can recall. Pin 2 has approx +12.0v
That's the nominal +12, and is used in the RS232 port and
the disk drives. Pin 3 has
approx - 0.1v (and is a bit rough, but only on
the millivolt scale)
Measured with respect to where? That's probably your ground pin. Pin 4 has approx + 4.5v (higher, actually, somewhere
between 4.5v and 5.0v
That should be +5 give or take a quarter volt and no more,
if it goes down to as low as 4.5V you have a bit of a problem
there, which could account for some of the erratic behavior
you're seieng. As I say, it IS a little higher than 4.5v - it may be
4.75, but I couldn't
be sure of that. I will find a new battery for the multimeter tonight...
Yeah, I'd see where that ends up because it being off could affect everything
like we're seeing here.
Is there a
trimpot adjustment on that power supply board?
Nope, nothing adjustable at all. I used to work
on those machines (we had "the shop" in a building that had
an Osborne dealer in it so I saw more of those than any other CP/M boxes)
but it's been rather a while since I saw one. I just looked in one set of
files and can't seem to find any schematics or other stuff, though I know
I have one somewhere, so I guess I'm working from memory here.
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... You might
locate the biggest caps on that power supply board
(they'll have the highest voltage rating and be on the primary side of
things, probably two of them) and give some consideration to replacing
those.
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?