Osbourne OCC1 problem

Tony Duell ard at p850ug1.demon.co.uk
Wed May 14 16:19:20 CDT 2008

> 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).


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