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

[CoCo Demus]

hmmm check the clock incoming to the VDG. it must be stuck because if a VDG
has a VCC, GND and a CLOCK it will generate video. It's a different clock
from the CPU.

On Sat, Jun 3, 2017 at 3:13 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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