I replaced the HAL with a GAL I programmed and it made absolutely no difference. That lead me to suspect he MMU (344-0010-B) of being fauklty and I was able to get hold of a replacement MMU temporarily. I swapped the MMU over and it sprang to life! The only remaining fault is thet the keyboard input is garbled, however as I have not replaced he Keyboard ROM and the original gets pretty hot, I think I can guess what the problem is. My attention now is on how to replave the MMU.