R7?? Did you mean to type "R3" or am I missing something?
Assuming you were referring to R3, here is what I would do next:
Turn off the power.
Read resistance from Pin 1 of OK2 to Pin 6 ("anode 2") of the MEGA. It would be best to probe from the top side of the board, i.e. probes touching the pins of the ICs themselves, not the solder joints where the IC sockets are soldered to the back of the board.
If you read ~1K ohms then I am stumped for the moment. But if you read something approximating an open circuit then you need to find the bad connection. You have basically 4 choices:
1 - Solder joint of MEGA socket Pin 6
2 & 3 - solder joints both ends of R3
4 - Solder joint of OK2 socket Pin 1
Also while you are at it verify that OK2 Pin 2 has continuity to ground.
If those all check out, make sure R3 is not physically broken or something. Also there is a SLIGHT possibility that something is wrong with one of the IC sockets. Probe the IC pin of interest on the front side of the board and probe the solder joint of the corresponding socket pin on the back - confirm continuity.
That's all I can think of for now. Let me know what happens.
Look into it later when the dust is clearing off the crater.