Almost there!

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.