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

[Keith Monahan]

I've gone through the service manual of a similar, but not the same, 
color computer 2 NTSC.

Played around a bit more today. Took a look at Color Phase A, Color 
Phase B, and Y. I was able to reproduce all of the video waveforms I 
tried in the service manual. It's pretty nice, they give you a little 
basic program, and then the associated o'scope screenshots. I did green, 
blue, red. Y looks perfect. B has the "3.58mhz burst blip" present in it 
as well, as it should.

I went through the "No color" troubleshooting section. Amazing that this 
exists in the first place.

* Check to see if 3.58mhz clock into the modulator. They say Pin A, I'm 
guessing they mean pin 1. There is no A. The Color Phase A is a 
different pin, and does not/should not have a clock on it. It's there on 
Pin 1, but obvious not perfectly on frequency.

* If the clock is missing, or less than 100mv p-p amplitude.... Not 
missing, and it's almost +4v in amplitude.

* They talk about probable causes for missing clock to be R13 and C61 
--- but I don't think that matches my PCB. And I can't figure out the 
parallels easy enough. And besides, clock isn't missing, but I thought I 
could at least ohm out R13.

* Check Chroma signals. A & B are correct, best I can tell.

What effect would clipping a dip test clip on MC1372 be? Would it add 
extra capacitance to each line? It's essentially a piece of plastic 
which adds an extra couple inches of copper on each individual pin. 
Please note that I see an effect(blurry but with proper colors) with 
just the clip attached, but no o'scope etc connected. I'm sure it adds a 
bit of force (up off the PCB, pins squeezed together?) to the chip. Bad 
solder joint?

I have yet to find a service manual or associated schematic/BOM list for 
a 26-3127B. If anyone knows where to find one? Color Computer Archive 
does not seem to have it, but maybe I'm not searching good enough.

I've got a 50ppm crystal coming tomorrow, and a 10ppm crystal coming 
Wednesday. I've also got some 6-45pf murata trim caps, which I can 
reinstall if necessary.

FWIW, I'm not sold on this crystal being the issue. I think it's good 
practice to solve problems as we find them, and don't get wrong --- I 
hope that's the issue.

I'll post some updates soon.

Thanks,
Keith

On 10/23/2021 12:32 PM, Dave Philipsen wrote:
> Yeah, the truth be known, I suppose the spec is a little more loose than 
> that but I've never sat down and measured at which point the average 
> television will stop displaying color. So that's one good thing about 
> the RGB output on the CoCo 3.  You could be out of tolerance on the 
> crystal but the color would still display ok.