SD3: Repetier connects but cannot move in x,y,z or heat extruder/plate

My SD3 has been gathering dust for almost 5 years. I started doing some printing at work and decided to see if I could get my old SD3 up and running. I replaced the power supply because the stock one had some water damage, and I confirmed I'm getting 12V to the board with the new PSU.

Repetier-server connects to the printer, but the only command that seems to have any effect is 'Emergency Stop' (SD3 makes some noises in response). When I try to move in the x, y, or z direction, I get the following responses on the console:

15:19:58.772: Warning: Communication timeout - resetting communication buffer.
15:19:58.772: Connection status: Buffered:36, Manual Commands: 26, Job Commands: 0
15:19:58.772: Buffer used:36 Enforced free byte:11 lines stored:4

I also will sometimes get the following:

15:19:58.790: echo:Unknown command:""

I was wondering if anyone knew of a guide to troubleshooting this sort of issue. I have a multimeter to check if power is getting out of the board, but I suspect it's the board that is no longer working. Any advice/experience would be greatly appreciated.
Thanks!