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

[Dave Philipsen]

Keith, I think the first thing to try would probably be the easiest. 
Replace the 14.31818 MHz crystal, then try replacing the TC1 variable 
cap and the two caps C43, C44. That might just fix it right there. You 
might even try taking TC1 out first to see if that helps. I have heard 
of quirks with that part.  Someone posted not too long ago that simply 
diddling with TC1 caused his machine to start working from a previously 
dead state.

You obviously have a running clock since the computer boots up put 
perhaps the frequency has drifted off enough that you can't get color.


Dave



On 10/22/2021 1:05 PM, Keith Monahan via Coco wrote:
> Hello there!
>
> New member of the list and the worldwide CoCo community! Had one as a 
> kid, and now trying to get a working CoCo going again.
>
> I just bought via ebay a Korean CoCo 2, PCB 20261058, with 64K of ram, 
> 16k ROM with ECB 1.1. This has the flat RF modulator with the switch 
> to the side of the RF connector.
>
> I power it up, relay clicks, black on gray screen is shown with 
> copyright msg etc above. Issuing the different CLS commands changes 
> the output to various brightness but definitely no color.
>
> Fiddle with TC1, which I think is a variable capacitor, with a metal 
> screwdriver, and as soon as I make contact screen changes to blurry 
> text on green background. Adjusting variable inductor(??) L2 does 
> nothing -- even when worked in combination with TC1. As soon as I 
> removed the screwdriver, or certain stopping points along TC1, screen 
> goes back to black on gray.
>
> Placing a dip test clip on the MC1372P is enough to cause the screen 
> to go blurry/green. That's with nothing hooked up. Just a clip on the 
> chip. So some Heisenberg stuff happening here.
>
> MC1372P is getting ~ +5.2v. Pin 6, the color reference voltage is 
> +1.56v. I haven't measured A and B yet(partially because of clip 
> problem above) but I can see transitions/data present on those pins. I 
> just haven't measured to see if they are 1.0/1.5/2.0 yet. Because I 
> can get intermittent but correct results when I fiddle with TC1, it 
> makes me think 6847T1 is putting out the right voltages. I have no 
> clue if the chip generates them or if they are just logic signals 
> through a resistor network or something.
>
> FWIW, when I change the screen color, say CLS 8, which is orange...and 
> then fiddle with TC1 -- get black text on an orange background as 
> expected. CLS 7 produces magenta, etc.
>
> My plan is to replace the RF modulator completely, which I'm really 
> not looking forward to, given the fat solder blobs holding the can 
> down. Going to use a 100/140 watt soldering gun as a friend suggested 
> on those. And then regular iron on the (8) contacts.
>
> I have 4-ch scope, logic analyzer(s) if I gotta dig it out, 
> multimeters, power supplies, loads, blah blah I'm bragging now. Some 
> decent knowledge on digital electronics, but very little on the analog 
> side.
>
> Does anyone have an theories as to what's going on? More 
> troubleshooting steps? Do I chalk it up to a bad rf modulator can and 
> just put a new/old one in there?
>
> Thanks
> Keith
>
> P.S. Coming to CoCofest in November, would love to meet you and say 
> hello!
>


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