Asteroids X axis problem

Just been fixing up my first Asteroids board for ages - multiple
problems, all now fixed ......... except for one.

Whenever an object gets to the far left or right edges of the display
it's still drawn okay BUT as the object passes through the edge it
makes a vertical line at that edge where the object is passing
through - the line is same 'height' as the object too.

Now I've seen something similar before which was caused by a problem
with the UNMDACX12 signal, but in this case that's not the problem as
changing the relevant chips (7404 at H10 and LS191 at C9) doesn't fix
the fault.

This is an early Rev 2 board.

Could of course be the AD561 DAC, or the TL081 or TL082 in the X axis
output circuit, but is there anything else that I've missed before I
go swapping out multiple chips? I can't unfortunately see any faults
when I 'scope out the signals.

Thanks,
Phil

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Received on Thu Mar 11 12:09:05 2010