[Coco] CoCo 2 with Random Characters on Screen at Power Up

[Danny Amborn]

Well back at it again after after feeling under the weather for a
while.  I replaced the following and tested all the chips in both
computers to make sure they were good or bad.

mc6809ep - Tested Bad.  Had a burn mark in the middle of the chip. 
mc6883p - Tested Good.
mc6822p -Tested Good.
sc77526p - Tested Good.  Swapped this just because it was socketed. 
mc6847p - Tested Bad

I also removed the extended basic rom then I swapped the basic rom to
the non-working computer with no change.  I also replaced the ram
chips a while back.

Currently I have the known working 6809 from the good computer
installed.  I thought I had the problem fixed when I swapped the
mc6847p.  When I installed the mc6847p into the working computer it
showed the same random characters on the screen so I know its bad.
When I took the chip from the working computer and installed it in the
non-working computer it works for like 10 seconds showing the basic
revision before going back to the random characters.  Shutting down
and powering back on again will show the basic revision again for a
second or so and then back to the mess again.  FWIW I was carefull
when pulling the chips and from habbit I alway make sure all the pins
are secure in their sockets.  I tried pulling and reseating the
mc6847p but it did the same thing.  There is a mc6821p chip but its
not socketed in my working computer so I couldn't swap it.  I haven't
swapped the power supply yet either.  

Any ideas?  If not I will just have to keep this one for parts or
until I stumble across a fix.

--

Dan Amborn
slackware64 at embarqmail.com