[Coco] UK PAL CoCo1 Repair

[Zippster]

Oh, and double check the demultiplexer (74LS138) off of the SAM.
This sounds like something a bad device select could cause.

- Ed


> On Sep 21, 2017, at 7:58 AM, Zippster <zippster278 at gmail.com> wrote:
> 
> 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.