[rescue] COCO2 question and looking for a few chips

[Dave McGuire]

On 07/08/2015 09:42 PM, Francois Dion wrote:
> So, I started a RetroTech meet at $WORK. This is mostly about repairing and
> maintaining older tech. We worked some on a COCO2. First issue, the
> character set seems to be coming from the wrong bank. When it's powered on,
> the characters are the graphical set, instead of saying stuff like BASIC
> etc, we see graphic sprites. So it looks like there is a bit that's off
> (high bank of the ASCII set). I've reset the ROMS and VDG and I've also
> cleaned and lubed the pins on those with deoxit cleaner and deoxit lube. It
> didn't help, unfortunately.

  I don't have much experience hacking on Coco hardware.  The first
thing to check, though, is power.  Make sure your 5V line is clean and
within tolerance.

  You could have a stuck bit in either the ROM (address decoder section)
or in what drives the ROM's address lines.

> On top of that, the NTSC signal gets too weak after about 30 seconds and
> the whole thing becomes unreadable. Cocos are not my expertise, but it
> seems like I have a VDG chip going bad? Before I go and order some from
> China and wait months for it to get here, I figured I'd ask here if anybody
> has a spare one, or know a US source for them.

  I'd be surprised if it was the video chip, but it's certainly
possible.  The after-30-seconds part sounds thermal to me.  I'd go
around the board with some freeze spray and see if freezing any specific
component brings it back.  Start in the analog video section first, of
course.

> Also, a coworker is looking for a SID chip for his C64. I know they were
> already hard to find some years back, but I told him I'd check around.

  Good luck with that. :-/

  For stuff like this you should probably join the MARCH mailing list.
Are you familiar with it?

             -Dave

-- 
Dave McGuire, AK4HZ
New Kensington, PA