[Coco] Yellow tinge

[Zippster]

Really close Gene.  It is indeed related to color burst. 

The “artifact” circuit in the CoCo2, which uses a 555 timer IC circuit to
pull on one of the color phases (causing the 1372 to generate a pseudo
color burst) gets out of tune, and pulls the phase lower for too long.

This causes the yellow bar on the left.  I’ve also seen a bad diode in this
circuit constantly drag the phase lower than it should be, causing a yellowish tint
over the entire screen.

- Ed


> On Sep 13, 2017, at 6:50 PM, Gene Heskett <gheskett at shentel.net> wrote:
> 
> On Wednesday 13 September 2017 07:06:40 Carlos Camacho wrote:
> 
>> I have two CoCo 2 for sale on eBay and a customer pointed out one of
>> them has a yellow tinge running vertically on the left side. You can't
>> see it on the BASIC screen. You can see it with a white background
>> screen like Canyon Climber. I recall some video composite boards have
>> this issue, but haven't heard of it with regular RF out. It's not the
>> monitor as my other CoCo for sale doesn't have it.
>> 
>> So what is the yellow bar and how to remove it?
> 
> As an old ntsc broadcast engineer, I'd expect its the color burst, which 
> is sent late in the horizontal retrace time, and is nearly a yellow, and 
> which would show up on the left side of the screen, and a monitor that 
> didn't do a good enough job of blanking the video during this time will 
> generally show it.  Whether you see it or not will be quite dependent on 
> the monitor being used.
> 
>> Thanks,
>> 
>> Carlos
> 
> 
> Cheers, Gene Heskett
> -- 
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