Thanks to everyone who offered suggestions.

Last night after I wrote my request, I found a short on the back of the
board (with the lead of a cap that had been glued on as an afterthought).
The leads and cap were still under the glue fitted from new, so it wasn't
from physical damage or abuse, it must have just eaten through the track
coating.

I figured I had it all fixed from there, but it made no difference !!!

Anyway, after a suggestion this morning, I made up a plug-in device (to 
replace the eprom) from
another IC socket and fitted links to short the data lines to ground.

I was then able to compare the address lines between good and bad. On the
crook one, where the waveform should have had a space, it was filled up with
clock cycles, and on the good one, the waveform was as I would have expected
...  in effect a square wave.

Long story short, I decided to fit a new Z80 and that fixed the machine.
I've had the Z80 chip on the shelf for the last 25 years. It's good to have
some useful stuff lying around.

So thanks Andrew, Jim, Peter, John, Gus, I appreciate the help.

Regards,   Roger

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