However, for some reason I came to physically press down on the U502 chip (GST shifter) and for a brief second saw the screen flash with the TOS desktop showing, so after removing it from its PLCC socket, carefully bending those socket-pins out and putting the chip back in it now appears to work OK again! Yet another "stupid" hardware error. Frustrating problems but inexpensive to repair
Oh well, at least I got to clean the SIMM contacts with IPA. Apparently something that was needing judging by the residue left on the Q-tips
