If A4 is stuck at 0 (or 1) then the SP and copy location would overlap, eg F5F0H would become F5E0H and overwrite the stack area - at least 6 bytes of it. The RAM modules are made up of 4 ICs so it it possible that only the top most RAM has a fault the others could be fine. If the RAM operation occurs in the other 3 RAM ICs then there would not be a problem. The other CALL 7EE1H works because there are no other RAM ops that could damage the stack.
As the Address lines are shared with the ROM and it seems to work fine it really only leaves the RAM module or decoder IC. I am tending towards the RAM module. Would be nice to somehow run a RAM test :) F5F0H, 1111 0101 111? 0000B start of RAM copy destination F690H, 1111 0110 1001 0000B end of RAM copy destination F5E6H, 1111 0101 111? 0110B start of stack, grows down.
