[Coco] CoCo 2 Repair Help

[Frank Thompson]

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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> > Coco at maltedmedia.com
> > https://pairlist5.pair.net/mailman/listinfo/coco
>
>
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