the CPU walking the bus is a really good sign -- a lot of things have to be going right for that to happen. UK6 is the video ROM, right? pins 2-10, 21, and 23-25 are just the address lines (i.e., inputs) so i wouldn't expect swapping the chip to affect anything there. i'm assuming that you're only observing that behavior with the ROM present? that plus the lack of startup chime seems consistent (imo) with the CPU having trouble accessing memory (which could be caused by any number of things).