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

[camillus gmail]

hi,

I would check the !MS line too, because this controls the address bus state, basically shutting the vdg chip off ... pin is active low, so see with you scope if there is any action on this pin. I it is low all the time then pull it up temperately to 5v with a 10k resistor, see if this gives you image.

cb  
On 6/3/2017 5:44:27 PM, CoCo Demus <retrocanada76 at gmail.com> wrote:
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
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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