[Coco] Very weird VDG problem.....

[Phill Harvey-Smith]

Hi all,

I have on my bench at the moment a friend's UK Dragon 64 board that is 
experiencing a really weird problem with it's VDG which I'm having 
problems figuring out.

So the machine powers on but I get no video, typing "motor on" blind 
clicks the relay so the SAM, 6809 & memory myst be at least partly working.

Putting a scope on the power pins of the VDG I get the expected +5 & 
GND, and the Video clock pin has the expected 3.57MHz clock signal on it 
(generated by the SAM), but none of the VDG pins output anything, the 
video is blank, the syncs are not toggling (/HS and /FS are stuck high) 
neither are any of the DA0..DA12 lines. /MS is high.

Taking the VDG, SAM and CPU out and checking it in another machine 
reveals that they all work, so the fault is a mystery to me, I was under 
the impression that the VDG would output video and toggle the syncs etc 
with just power and a clock.

Has anyone ever seen this before or have any idea what could be the 
problem? Otherwise I suspect that the solution is going to involve a 
scatter gun approach to replacing all the LS logic chips until I get the 
right one (this is a PAL machine so has the circuitry to stop the VDG 
clock and insert extra lines).

Cheers,

Phill.

-- 
Phill Harvey-Smith, Programmer, Hardware hacker, and general eccentric !

"You can twist perceptions, but reality won't budge" -- Rush.