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

[Dave Philipsen]

And, yes, I was referring to the variable cap nearest the crystal. All timing in the computer is derived from that one crystal and most everything will allow quite a bit of tolerance except the color burst. That seems to be why your computer is running just fine but with the color burst off that much the circuitry in the tv probably just gives up on trying to extract any color information. 

-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?
> 
> To be clear, you're talking about the TC1 closest to the 14mhz crystal? The TC1 I spoke about was inside the RF can. Similar part but completely different section of the board.
> 
> The 14.31818 crystal is measuring 14.2857143.
> 
> I'm bad with the math there. ppm and all that business. Do either of those sound (too far) out of spec?
> 
> Thanks,
> Keith
> 
> 
> 
>> On 10/22/2021 9:34 PM, Dave Philipsen wrote:
>> 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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