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

[Keith Monahan]

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!