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

[Keith Monahan]

Very fair question.

My only way currently is RF through a TV tuner, to composite. I have tried 
three TVs on the composite side. Two worked, one didn't.

I agree with your overall sentiment though. The older analog stuff was more 
forgiving, and because manufacturing tolerances were worse, they needed to be.

I've got no VCRs, no TVs with analog antenna connections, only one CRT 
monitor in the house. I checked ebay/craigslist/etc. I just don't want to 
mess with them.

I don't love this messing about in the fringe edge. This should work 
solidly in the center.

My plan is to add Coco VGA as soon as reasonably possible. I have 
experience designing my own FPGA-based on the fly conversion. I designed a 
converter for the Amiga to VGA. Never perfected it, but it worked fine. I 
may try to slap something together while I wait on the list.

In the meantime, I'd like to get the coco2 back to the proper state and 
running cleanly. No reason I can't bring it to the 21st century. HDMI can 
be done as well. VGA ports are disappearing, too.

When I popped open the coco2 for the first time in 35 years, I was 
surprised to see a pretty clean design. Popped open the RF modulator, and 
it's like yuck. Ugly brown single sided board. I bet that design is from 
1975. If I can eliminate it altogether, I just might.

Thanks for your message!
Keith

On October 24, 2021 11:00:22 PM Rocky Hill via Coco <coco at maltedmedia.com> 
wrote:

> 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.
>
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