It's either a cold solder in the general area where the J1 connector is. All those resistors / capacitors and the J1 connector itself on the MPU board (2518-17).

But most likely chip U20 as died. It might be on a socket. It's a "4502" or a "MC14502B" a hex buffer/inverter. (Don't mix up with part number MC145028 instead , which is a decoder (last digit is EIGHT instead of B)). It probably isn't U19 at fault ("4011" or "MC14011B" , Quad 2−Input NAND Gate) but one gate is used in the display logic so it's possible.

These parts cost under a dollar each. If U20 isn't on a socket, you should install one after removing the chip.