[Coco] CoCo2 black and gray video only -- need help!

[Dave Philipsen]

Yeah, the spec for NTSC color is pretty tight. In fact, it is tighter than most inexpensive crystals. I imagine that’s one of the reasons why the trimmer cap is in the circuit. If the crystal is a little off you can “bend” it slightly to get the frequency where you want it. Like I was saying, you might just try removing that trim cap which is right next to the crystal. It could be that it has aged an now has more capacitance than it used to. Worth a try. 

Dave

> On Oct 22, 2021, at 11:38 PM, Keith Monahan via Coco <coco at maltedmedia.com> wrote:
> 
> Forgive my ramblings -- and thank you for confirming that it's derived from the main crystal.
> 
> Given the requirements for the VDG, this means that the main crystal has to be 14.31814 to 14.39022. My main crystal is showing 14.2857143.
> 
> Main crystal too slow, color burst derived /4 too slow.
> 
> Doesn't that make the tolerance on that main crystal REALLY tight? I mean if the slowest it can be is 14.31814 (in order for colorburst to be >3.579535) then having it be 14.31818 is really close.....and since they fail SLOW.....you think you'd want it spec'd a bit higher?
> 
> Replacing the main crystal is a piece of cake. I was getting confused because the MC1372 datasheet shows an external crystal connected essentially between pins 1 and 2. CLOSE TO THE MC1372, which would be in the can.
> 
> I haven't found a schematic yet for this particular mobo yet so I didn't have the COCO specific setup.
> 
> Thanks for the image as well!
> 
> Rock on. I'll order a new crystal and hopefully be off to the races.
> 
> Keith
>