15 May
2008
15 May
'08
12:19 p.m.
On Thursday 15 May 2008 14:38, Ade Vickers wrote:
Hmmm....
OK, stranger & stranger.
The power supply has 3 identical "plugs" (for want of a better word) coming
off the board, each with 4 pins. The voltage on each corresponding pin of
each plug is identical; so I'm assuming they're there to power other
optional (and presumably never built) kit. Anyhoo...
Without my voltmeter, I've done my best to calibrate the 'scope (which I've
never used in anger before) to a sensible scale. From this scale 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.
All three signals are pretty smooth, I can't see
any rectification noise on
them until I get into the millivolt scales - even as far down as I can
adjust the 'scope.
That's good to know.
The crystal is working properly :)
I've not checked the voltages going into the monitor yet - I need to do
some additional dissasembly so I can get the probe in, and it's knocking
off time...
However, the PSU looks good, I'd say.
Except for that one reading that's somewhat rather lower than I'd like to see.
The display is still fritzed :(
No surprise there. :-)
Is there a trimpot adjustment on that power supply board? 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.
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.
--
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