[Coco] UK PAL CoCo1 Repair

[Phill Harvey-Smith]

Sorry meant to send this to the list.....

On 21/09/2017 13:58, Zippster 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.

Yep one of the problems I had with the CoCo and some Dragon motherboards 
during the development of DragonMMC was that the Xilinx CPLD would pull 
the reset line low due to the rather weak pullup on it leading to the 
machine not coming out of reset and having a screen full of random junk.

Fixed it with a diode and reistor between CPLD and reset line.

Also worth checking that the IRQ, FIRQ, NMI and HALT lines are high, (or 
possibly pulsing at 50Hz in the case of IRQ)

 > 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.

Yep check for E, Q pulsing, R/W will generally be pulsing though less 
regularly, ass should Address and Data lines.

 > 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.

Agreed if you are getting a picture with junk on the screen then the 
6847 and probably the SAM also are working in this board. (I know you 
verified them in another machine).

 > 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.

I think he said he tried it in a Dragon and it worked there, which would 
seem to mean it's probably working.

Have you tried blindly typing MOTORON and listening for the relay click? 
If that works the CPU is working and your fault lies somewhere between 
the RAM and the VDG, I believe there is an LS373 latch there.

Something that I have also seen fail is the LS244 buffer between the RAM 
data outputs and the CPU data bus, can't remember if you checked those.

May also be worth checking for shorts / continuity on the data and 
address lines and between the pins of any chip that you have desoldered 
and replaced as it's all too easy to solder splash short between tracks

Note that on the CoCo and Dragon, unlike almost all other micros of the 
period, the data in and data out of the RAM chips are not connected 
together, so need tracing separately.

Cheers.

Phill.