OK...Have setup external power supply as per Clay's guide (spare Peter Chou p/s), run +12, +5 & GND to 1J17...Same error....

Ive done the old finger on chip method ie 'heat checking' the PIA's....U51 is definately warmer than the others.  I suspect it is bad.  U42 was not hot, but I think I will replace both.  Question for the experts (a) am I on the right track & (b) should I just buy the 6821 PIA's or sockets as well?