[Coco] CoCo 3 Display Problems

robert.gault at att.net robert.gault at att.net
Fri Sep 25 17:45:19 EDT 2020

Kurt,You probably should check the coco3 internal voltages with and without the cocosdc inserted.If you don't have the coco3 service manual, it is in the colorcomputerarchive.com and contains a list and picture location for voltage check points.I've noticed that the output for the cmp tv socket on my unit was higher than recommend for my monitor. That could cause problems.Any 60hz signal will mess up TVs if you don't have good shielding on the cable connecting the coco to monitor/TV.RobertSent from my LG Mobile------ Original message------From: Ed SniderDate: Fri, Sep 25, 2020 4:37 PMTo: CoCoList for Color Computer Enthusiasts;Cc: Subject:Re: [Coco] CoCo 3 Display ProblemsDoes kinda sound power related.

I'd probably replace the electrolytic caps and then see where it's at,
though you might try adjusting the trim capacitor on the GIME clock
circuit first to see if that has any effect.

- Ed

> On Sep 25, 2020, at 2:21 PM, Kurt Randall  wrote:
> Just wondering if anyone can help me with a problem I have with a CoCo 3 I
> recently picked up. I'm new to CoCo's -- my 8-bit computer experience is
> with ti99's, Apple IIs, and ZX Spectrums.
> My problem is that the display in both RF and Composite is shaky, but the
> problem goes if I plug in my CoCo SDC. I have one game cartridge, and that
> seems to help too.
> I tried re-seating all the socketed chips, cleaning connections, tried a
> couple different cables, and a couple different screens (LCD tv, CRT
> composite monitors, CRT tv). Without the SDC plugged in, the LCD tv always
> has problems with the top of the screen -- the top moves back and forth
> about 1/4 of the way across the screen, while the bottom is fine. The CRTs
> have shaky horizontal stripes -- it looks like every second line is a bit
> wiggly, but they are usable.
> If I plug in the SDC the CRTs are fine, while the LCD is only fine unless I
> try running a game, then the problem comes back.
> Anything I should be looking at? Capacitors perhaps?
> Thanks!
> -- 
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