14 May
2008
14 May
'08
5:19 p.m.
Hi folks,
I just dug my old Osborne OCC1 (1st model, in the beige vacuum-formed ABS
case), only to find all is not well. Actually, "finding" all is not well is
a bit of a white lie - I already knew it was in trouble, from when I last
tried to boot it about 2 years ago...
Unfortunately, the intervening 2 years have failed to fix the problem, which
is that the video seems to have no horizontal hold.
[...]
Now.... If I pull the termination block off the External Video connector,
the screen goes out (as one would expect); push it back on & the screen
comes back on with the display as steady as a rock -- unfortunately, it's
crashed the computer... From this, I deduce that it must be something in the
mainboard electronics that's failed (a cap, maybe?), rather than something
in the monitor unit.
Now, I don;t know this machine at all, but I am assuming the mainboad
contains the video and sync generation circuitry, while the 'monitor'
contains the deflection circuits that lock to said sync signals, the
video amplifier, HV generator, etc.
Are the signals between the monitor and mainboard separate syncs and
video or a composite video signal?
It is my expeireince that this sort of prolem is caused by a failing
component arouund the horizontal oscillator / PLL / sync circuit. It's
unlikely to be a digial problem. And this is a horizontal problem
(picture breaks up into sloping likes)not a vertical problem (picture
rolls), right?
If you do have seaprate signals to the monitor, start by 'scoping the
horizontal sync signal while the monitor is malfunctioning. Is it the
right frequency and stable?
Then, if there's a horizotnal frequency control on the monitor PCB (it
might be an inductor!), try carefully tweaking it. The idea is to (a) see
if the thing will jump into lock, or if not, at least if you can get it
to slope both ways (that is, can the horizotnal oscillator run above and
elow the right frequency, if so, it's a good bet it'll work _at_ the
right frequency, and that the problem is in the sync circuit).
-tony