[Coco] CoCo 2 Repair Help

[Zippster]

Ok, then it pretty much has to be RAM, the data bus buffer, the VDG latch, or the
IO select IC off of the SAM.  Most likely RAM.

- Ed



> On Aug 21, 2017, at 8:07 AM, Frank Thompson <frank.thompson.iv at gmail.com> wrote:
> 
> The socketed chips included (from memory, typing this from work):
> 
> IC9:  main processor
> IC12, IC13:  Basic ROM / Extended ROM
> IC8:  VDG
> IC1:  SALT
> IC22:  SAM
> IC7:  PIA
> 
> Must have been a few more, just can't recall which ones...
> 
> On Mon, Aug 21, 2017 at 8:52 AM, Zippster <zippster278 at gmail.com> wrote:
> 
>> Hi Frank,
>> 
>> You are doing the right thing going for the RAM next IMO.
>> 
>> Which chips were socketed on the machine?
>> 
>> - Ed
>> 
>> 
>>> On Aug 21, 2017, at 7:09 AM, Frank Thompson <frank.thompson.iv at gmail.com>
>> wrote:
>>> 
>>> 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!
>>> 
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>> 
>> 
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