[Coco] CoCo 2 Repair Help

[Frank Thompson]

Hello fellow Coco Nuts!

I recently won a "NIB" Coco 2 from e-bay, and despite many difficulties
completing the transaction (thought I was getting scammed) a seemingly
brand new Coco 2 arrived at my doorstep.

Physically, it appeared brand new but despite its great cosmetic condition
it won't boot properly.  When it powers up, all I get a screen full of
alternating 8s and 0s on the screen.  If I hit reset, the characters can
vary, including various block colors and patterns.

I'm looking for any suggestions to guide my continued troubleshooting.

Here is what I've done so far:

1.  Shotgun approach:  swapped every socketed IC with those from a known
good CoCo 2.  Result:  good coco continues to operate, bad coco refuses.
Conclusion:  it isn't one of the socketed chips.

2.  Despite the garbage on the screen, I tried executing a sound command
and a cload command to see if the computer was actually doing something
(just not able to display info to me).  Result:  no function (no sound, no
relay click for cload)

3.  Per the service manual, I monitored the signals (using a scope)
described in the section about "Processing Problems".  Clock signals E and
Q are present and clean.  NMI, FIRQ, HALT, RESET pins on the processor are
all correct state (high), IRQ pulsing at 60 Hz, Address/Data lines all seem
to be changing states and hitting the appropriate levels.  Result:
 everything seems to be operating correctly.

So based on the book, everything looks good.  My next thought is to start
replacing the RAM, although this will be a pain since they are all soldered
in.  My friend has a nice Hakko desoldering tool to make the job easier,
but I didn't want to jump into that if there is something else I should
look at first.  The only other major IC that isn't socketed is the IC2 PIA,
but based on my reading, I'm not sure if that would prevent me from booting.

Is RAM the next place I should focus?  I have some replacement 4164 RAM on
the way from one of the arcade game internet sites I order from (my other
hobby).  I do plan on soldering in sockets to accept the new RAM.

Also, once I get this machine up an running, is there a RAM stress test
meant for 64k coco 2 machines?  I found one (including source code) for a
Coco 3 but I don't think my coding skills are up to par yet to modify this
program for my machine.  My goal would be once the machine is fully up and
running to place a single suspect RAM chip back into the computer (since
I'll have placed sockets it should be easier) and run a stress test to try
to isolate if it is just one or two bad RAMs rather than throw all 8 away.

Thanks all!