15 May
2008
15 May
'08
12:32 p.m.
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...
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?
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...
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...
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. 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?
Cheers!
Ade.
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