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

[Gene Heskett]

On Friday 22 October 2021 14:05:01 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

Let an old fart, retired broadcast engineer, dig around in his 87 yo 
attic and see if I can offer a clue or two.
1. IIRC recalling reading someplace probably 25 years ago, the the korean 
broadcasters used a method of adding color to thier broadcasts that was 
similar to NTSC but used a slightly higher color subcarrier. If you have 
a counter, get close enough but not touching a pin, to pickup the 
generated subcarrier, which for an ntsc receiver should be within 10 HZ 
of 3579545 HZ, or possibly an exact multiple of a power of 2, like 4, 
which would measure as 4x that 3579545 hz. Note I am not using a decimal 
point after the first 3 to indicate megahertz. If you measure several 
hundred hz below that, a common cause would be the age and how well 
sealed that crystal is, because the silver plate that is the electrode 
contact will oxidize if airborn oxygen is allowed into the can, and 
those were BBLB stuff, not inert gas filled or sealed.  The silver 
oxide, in gaining the oxygen atom as it oxidizes, gains weight, enough 
that in 30+ years, its well out of tolerance, slow now in 2021. 

We, as broadcasters were locked into that standard by the math involved, 
but all that math was not available, nor affordable to the makers of 
this stuff when it finally became available long after our beloved 
coco's were out of production, the end result on your monitor screen 
being the dot crawl visible if it was sharp enough caused by the lack of 
mathematical lock when the subcarrier was not locked to everything else.

2. And its possible that the korean standard for ntsc did not follow the
american standard because they wanted to brag they had a sharper pix and 
used a higher subcarrier frequency. The difference was not generally 
visible to the human eye given the loss of resolution in the crt 
displays of the time that were also affordable. The cm8 the shack sold 
was a horrible example of that, so an 80 column screen was so fuzzy it 
was estimated by the eye, not read.

That of course will throw a 48", 15 lb pipewrench into things if being 
connected to a modern, all digital, tv or monitor, as most can't deal 
with an unlocked signal at all well.

So get a counter, sub 500$ ones won't be accurate enough, and measure it. 
My bet is that after 30+ years, its gone at least 100 hz flat. Like a 
piano that hasn't been tuned in 30 years. 

And tell us what you get, for a counter reading, please.

> P.S. Coming to CoCofest in November, would love to meet you and say
> hello!


Cheers, Gene Heskett.
-- 
"There are four boxes to be used in defense of liberty:
 soap, ballot, jury, and ammo. Please use in that order."
-Ed Howdershelt (Author, 1940)
If we desire respect for the law, we must first make the law respectable.
 - Louis D. Brandeis
Genes Web page <http://geneslinuxbox.net:6309/gene>