Most user's own repair are limited to replacing some burnout ICs and As a practice we don't provide schematic diagram for the PLC. We don't encourage user to poke around the circuit too much since there are 24V and 5V voltages in the circuit board. A bad short circuit to 24V can damage quite a lot of ICs.
For output driver, besides the ULN2803A, you can also try changing the 74HC595. Output 1-4 are controlled by the 74HC595. Output 7 and 8 are controlled by two IRL530 power MOSFETs on a T100MD888+. On a T100MD2424+ output 7 and 8 are controlled IRF9530.