[Coco] CoCo 2 Repair Help

camillus gmail camillus.b.58 at gmail.com
Mon Aug 21 14:31:21 EDT 2017


Hallo zipster,

If you need some help, i can socket all your ic's.
I have done more then 7 cpu socketting and had never one dead.
i even manage to save some of the old cpu's

All you need to do is send me the board, with return label and the needed ic sockets ( I use the machined ones, more expensive but way better  ).

Oh and I do this freely. 
cb


On 8/21/2017 9:46:44 AM, Zippster <zippster278 at gmail.com> wrote:
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 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 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
>> 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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>>> Coco mailing list
>>> Coco at maltedmedia.com
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>>
>>
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>
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