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

[Arthur Flexser]

I would now try putting the 6847 back into your previously working CoCo and
seeing if it still functions okay.  It sounds possible that the bad CoCo
may be damaging 6847s that were previously okay.  This is particularly
possible if you tried the previously working 6847 in the bad computer
while that computer still had the bad 6809 in it.  Or you had the bad 6809
swapped into the good computer and the presence of the bad 6809 maybe
damaged its 6847 when you turned that computer on.  You may now need to
order one 6809 and two 6847s to put things right, if I'm surmising
correctly that the bad 6809 may be a 6847-killer.

Art


On Sun, Jan 26, 2014 at 7:52 PM, Danny Amborn <slackware64 at embarqmail.com>wrote:

>
>
> 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
>
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