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

[Rocky Hill]

 My CoCo's routinely behave in a way similar to what you are describing when using themwith my one of newer flat screen tv's  that has an rf input and with another one of my newerflat screen tv's when using the composite input. They all work as they should when using them on older CRT tv's with both RF and composite inputs and work fine with one of my newer LG flat screen tv's.I know that some of the old components in the CoCo's might be drifting a bit but in my case I'm convinced my newer tv'sare the ones that are slightly incompatible or have less wiggle room.
have you tried using other displays?

    On Sunday, October 24, 2021, 09:52:54 PM EDT, Keith Monahan via Coco <coco at maltedmedia.com> wrote:  
 
 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.

-- 
Coco mailing list
Coco at maltedmedia.com
https://pairlist5.pair.net/mailman/listinfo/coco