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

[Keith Monahan]

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


On 10/22/2021 11:56 PM, Keith Monahan via Coco wrote:
> Thanks Dave.
> 
> I just checked the 6847T1 spec sheet...... Yeah it's saying 3.579535 to 
> 3.597555. And I'm 3.577817. So I'm out. I was searching for some NTSC 
> spec value, but couldn't find it. I guess more important is THIS CHIP 
> that's actually consuming the clock is saying it's too low.
> 
> https://www.techtravels.org/wp-content/uploads/2021/10/colorburst_annotated.jpg 
> 
> 
> Is that the crystal there?
> 
> The problem isn't so much the cost of the crystal. It's that I have to 
> unsolder the whole can to get to it!
> 
> Thanks
> Keith
> 
> 
> 
> On 10/22/2021 11:40 PM, Dave Philipsen wrote:
>> The NTSC spec says  +3.579545 MHz ± 0.0003 per cent which is about +/- 
>> 10 Hz so you’re quite away off if you’re 1728 Hz slow. From Digi-Key 
>> the price of a new crystal is less than a dollar. I bet that it’s just 
>> old and has drifted.
>>
>>
>> -Dave Philipsen
>>
>>> On Oct 22, 2021, at 10:03 PM, Keith Monahan via Coco 
>>> <coco at maltedmedia.com> wrote:
>>>
>>> Dave,
>>>
>>> Thanks much for the reply!
>>>
>>> I used my scope to sample pin 1, clock output, out of the MC1372P at 
>>> 2GS/s, in averaging mode, with 64k averages.
>>>
>>> I'm getting a period of 279.5ns, or 3577817.53 hz. So 1728hz slow. I 
>>> don't know this stuff that well. Is 1.7khz on the color burst a big 
>>> deal?
>