[Coco] CoCo 3 Display Problems

Rick Ulland rickulland1 at gmail.com
Fri Sep 25 18:38:08 EDT 2020

It may be a cap problem, but a good cap with a poor ground. I would go 
there first, because it's cheap. Pins 33/34 on the buss connector 
(fourth from back top/bottom), plus the two edge clips, should all be 0v 
under load - ohmmeter don't count, compare voltage while running...

If a motherboard fault leaves one of those points floating above ground, 
but the thing you plug in ties those points back together outside the 
CoCo, there is your instant mystery fix.

Otherwise it's prolly going to be a cap replacement task and those kits 
are fine if you are into that...  I'd make sure I am well grounded first.


On 9/25/20 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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