[Coco] Broken CC3

[George Ramsower]

> On Friday 12 September 2008, George Ramsower wrote:
>>----- Original Message -----
>>From: "Gene Heskett"
>>
>>> On Friday 12 September 2008, George Ramsower wrote:
>>>>Darn,
>>>>
>>>> My CNC coco broke.
>>>>
>>>> It starts up and displays the usual stuff. I can ?mem and it reports
>>>>correctly. However, when I boot OS9, it crashes.
>>>>
>>>> I appears to crash when it loads the first boot sector.
>>>>
>>>>I tried other boot disks with the same results.
>>>>I replaced the CPU, RAM board, reseated the GIME, polished the edge
>>>>connector on the disk controller, reseated the ROM in the controller 
>>>>and
>>>>all gives the same results.
>>>>
>>>> Suggestions?
>>>
>>> This smells like a memory problem as os9 put it in all ram mode. 
>>> Perhaps
>>> there is a problem with the memory above 32k?
>>
>> This is why I replaced the memory board from another coco. I was thinking
>>the same thing.
>> The re-seating of things was the first step, then the memory, then the 
>> CPU.
>> I think the next thing to try is another GIME chip. I've never had one 
>> fail
>>before.
>> I think I'll try that now and see.
>>
>>George
>>
> Another thought George.  do you have any hardware that could exert an IRQ
> before it has been initialized to do so?
>
> -- 
> Cheers, Gene

There are only two boards on the expansion port. The disk controller and the 
I/O board. The I/O board generates no IRQs but I disconnected it anyway for 
this problem.
 I tried another GIME chip and the results are the same. I'm wondering if it 
might be a chip that controls the memory that has failed. I don't know where 
the schematic is for the CC3. I've got the book stashed away somewhere.

 Does anyone remember why we modified the circuit at IC-9 to add a 74LS02?

 I'll try another '02 there, since it's socketed.

George