Commodore PET 2001 Video Problem - test-drb@ccmp.vtda.org

20 May 2012

Success!  I thought I had done the whole reseating of the chips
in the socket thing, but messing more with the VRAMs as per your
suggestion, seems to have soveld the problem.  I tried swapping them and
that just made things worse, so I swapped them back and it worked great.
 Looks like some oxidation in the sockets/pics.  So I removed the PETvet
and replaced the 6502 and for now, it is running well.  Now to test the
rest of the machine.  Thanks for the help.

http://imgur.com/qclhF

Win

Date: Sat, 19 May 2012 21:35:17 -0400
From: Ethan Dicks <ethan.dicks at gmail.com>
To: "General Discussion: On-Topic and Off-Topic Posts"
       <cctalk at classiccmp.org>
Subject: Re: Commodore PET 2001 Video Problem
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       <CAALmim=SKRsX+YVADoEVrn3BNQ_VJd2MSuT==jK6GAbotaJfqg at mail.gmail.com>
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On Sat, May 19, 2012 at 9:23 PM, Win Heagy <wheagy at gmail.com> wrote:
...
  I've made a bit of progress on this since the last
post and wanted to get
 some additional help if possible. ?I picked up a PETvet board from
 bitfixer.org and installed it. ?The 2001 boots now (usually) and I get a
 screen like this....
 http://imgur.com/fQxH3 
You are quite close now.

...
  The machine works like this, but obviously the
vertical lines are a bit
 annoying. ?The PETvet replaces the RAM and ROM, but does anyone know if
 that includes replacing the video ROM? ?Any next steps are appreciated. 
Your video (character generator) ROM is likely just fine.  It's more
probable that you have a either a bad video SRAM or perhaps a stuck
bit on the shift register that takes the byte from the ROM.

What does the screen look like if you remove the ROM?  Do you still
see the lines?  What if you swap the RAM chips.  Does the vertical
line move?

Since you pulled your RAM for the PETvet, you should have spares to
try in the video RAM spot.  I've seen up to 25% bad SRAMs in old PETs,
so it's a place to focus your efforts.  If you can borrow or have a
spare character generator ROM, it's worth swapping that out, too.

I think you've got it narrowed down to one of 4 chips... ROM, RAM, or
shift register.  The most likely candidate is RAM, in my experience.

-ethan