[Coco] CoCo 2 Repair Help

[Zippster]

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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