Osborne OCC1 problem

Ade Vickers javickers at solutionengineers.com
Thu May 15 18:48:23 CDT 2008

Tony Duell wrote:

> > Hmm; I don't think it is wholly an analogue problem: I would expect 
> > the screen to break into horizontal slopes, as you say - 
> and I've seen 
> > that on badly adjusted monitors in the past (I used to have an old 
> > Philips monitor which could be provoked into this with the 
> front panel adjusters).
> > 
> > What's happening is a corruption of the display, rather than an 
> > adjustment
> A more accurate fault description would be helpful...

Well, the video I linked to is about the best description I could think

When the screen is showing static information, and the disk(s) are not
running, then the display kind of looks like it's got a h-synch problem. Or,
try this: It looks like there's a strong right-to-left motion blur. That's
still not 100% accurate, but I'm no artist & trying to describe it is beyond
my ken.

When the machine is booting from disk, the screen flashes and flickers like
a mad thing: But as I say, it seems to be *digital* noise rather than
analogue. But with some analogue effects thrown in for good measure...

Maybe I should monitor the PSU voltage on the 'scope while the machine's
booting - I hadn't thought of that one. Could be interesting - the bulk of
the computer is currently upside down on the bench to make it easy to access
the bits of the PSU I need to probe... if the 5v line is flapping about,
this could cause the digital effects. Mind you, the fact the machine boots
properly indicates that the logic side of the machine is working fine; and
I'd have thought the disk drive would be more sensitive to 12v fluctuations
than the monitor?

> Does this machine use a 6845 or similar for hre video timing 
> chain?

Dunno, will look tomorrow (err, later today).

> If so, mis-programming that will cause apparent sync 
> problems. So the fault could bever well be proscessor, ROM, 
> or bus related. 

Bearing in mind the machine boots, and works behind its corrupted video
output, would that necessarily eliminate any of those parts from enquiries?

> That's why I'd start by looking at the horizotnal sync pulses 
> when you have the lines and when you have a good display 
> (even if the machine has crashed). If they're the same 
> timing, then the fault is monotor (or PSU?) releated. If not, 
> then the video timing circuitry is palying up _or being 
> mis-configured by software_

Gotcha - will look at that tomorrow.

> > I'll look for it... the monitor circuit board is quite well 
> labeled; 
> > but I'm not sure if the connectors themselves have individually 
> > labelled connections.
> Sony were sometimes kind enough to label connector pins, 
> supply lines, CRT pinouts, etc in the slikscreen, few other 
> manufacturers were.
> If it is a standard pinout, then the connections are 
> 1 Ground
> 2 One end of the contrast control
> 3 t'other end of the contrast control
> 4 Slider of the contrast control
> 5 ground
> 6 Hsync
> 7 +12V power input
> 8 video input
> 9 Vsync
> 10 ground

I've not counted the pins, but one thing - where's the brightness control
go? The machine has external (to the monitor) controls for both contrast &
bright; as well as trimpots for both on the monitor PCB itself.

> > Question: How long should I leave the tube to "cool down" 
> (unplugged) 
> > before I can rummage around it without fear of major electrocution?
> Depends on what you're working on, and the design of the 
> monitor. The PCB-mounted capacitors will normally discharge 
> fairly fast (minute at most), so there'll be no masty 
> votlages on PCB tracks or CRT _pins_ after that. The EHT, 
> stored in the 'capacitor' of the CRT glass can stay for days, 
> but (a) it'll be discharrged by the beam current in any 
> normal monitor (so this is really only a concern if the 
> screen is totally blank),

Shouldn't be a problem; the screen is lit up, so I guess it's discharging.

> () the only way you'll get in 
> contact with it is if you remove the anonde connector and (c) 
> good monitors have a bleeder resistor, often inside the 
> flyback trasnformer.

No plans to remove any connectors inside the monitor....

> If I am going to be removing the EHT connector, I check the 
> residual voltage with an EHT voltmeter (which also happens to 
> discharge it :-)). 
> Otherwise, I don't worry. So far I've never had a shock doing that.

OK, that's good to know. Many thanks again Tony - I'll try to report back
the status of the h-sync tomorrow.


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