[Coco] UK PAL CoCo1 Repair

[Zippster]

Well, if power seems goods, and the IC’s seem good, I think I’d take a close look at
the reset circuit if you haven’t already.

If that looks good I might start poking around with the scope, probably starting at
the CPU/SAM looking for the expected signals and bus activity and working out
from there.

It sounds as if the video system is working ok, so you’re probably good there.
It does sound like a CPU/memory problem of some sort. 

You may want to verify the BASIC ROM just to make sure it hasn’t been corrupted/damaged
in some way.  Just to make sure the system isn’t simply crashing on bad ROM code.

- Ed


> On Sep 21, 2017, at 5:17 AM, R Jones <rjones at biketech.org> wrote:
> 
> Thanks for the reply.
> 
> I have checked the voltages at the ICs and they all check out fine.
> 
> This is a 32K machine with the  64Kb ram chips. I removed the ram and checked the voltages on the sockets:
> 
> RAM jumper on 16K
> pin1 -4.8
> pin8 12.0v
> pin9 5.1v
> pin16 gnd 0v
> 
> RAM jumper on 64K
> pin1 +5.1v this is nc on the 64Kb chips
> pin8 +5.1v
> pin9 a7 line, pulses ~4.5v high
> pin16 gnd 0v
> 
> The ICs, components and test points are all numbered differently on this board to those in the reference manual. Although I have checked the supply voltage with the datasheets and all ICs are receiving the correct supply voltages and ground.
> 
> I have bought 2 new sets of ram along with another supposed working set from another coco which I was given some time ago. I have rotated all these chips one at at time and sets at a time and it makes no difference to the output on screen.
> 
> 
> 
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