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

[Zippster]

Phil,

Since you checked the VDG in another machine, my guess would be the circuit that
stops the clock to get it to 50Hz.  Bad flip-flop in there? (I have no Dragon schematics)

- Ed




> On Jun 3, 2017, at 5:13 PM, Phill Harvey-Smith <afra at ramoth.org.uk> wrote:
> 
> 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.
> 
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