Well it’s a funny old game where a measure of success is a chime and a Sad Mac code compared to dead silence. But I'll take that as a half step forward(ish).
So Mr 360alaska kindly offered a PMGR chip from one of his PB100 boards and that arrived yesterday. I swapped it out for the presumed dead one (after some discussion about part number interchangeability) and fired up the power full of anticipation but silence reigned and the power draw on the lab supply was zero. Couple of PMGR reset button holds and still nothing. Sad Mac Face.
Coincidentally my digikey parts order came in too and in that were some replacement 74AC02/10/157 based on techknight's comments in another thread. I had not been able to determine a thermal issue with any of these using fingers or IR camera but I figured cheap as chips test so swapped them out. Power up board, nothing, no power draw, more Sad Mac Face.
OK, maybe the PB100 isn't compatible or something, let's put back the original PMGR chip. Powered it up and I see current draw from the supply. Reset the PMGR, and hit the keyboard. CHIME. Then Sad Chime. Looked at the display to see if it was the old 00003 code as before but new one - 0014 with subcode CD38. Which according to my list says PMGR unable to turn all the power on to the board, and the CD38 suggests a handshaking communication problem...
Of course if I can get past this, I might still be back at the Bad RAM 0003 code, but for now at least something is alive. You got to take those small wins....