I've debugged the 555 - RESET section; following the RES line I discovered two things:
1) In the PET UNIVERSAL Board schematics, sheet 1 of 11, "CPU & MEMORY EXPANSION" there is a print error: the chip connected to pin 3 of LM555 (UD16) is marked as UE5 but this is wrong; is a 74LS04 and is...