Yes, I would agree that focusing on that side of the big PCB is a good idea. It could be as simple as a bad solder joint that isn't allowing adequate current draw. Since you have Speedball 1.1, the R1 resistors are where they should be, and the PN2907 and 2N2222 are impossible to mix up (your small board is also working, which excludes these possibilities).
I would bet on the center leg of the TIP50 on the offending side not being soldered well enough.
-PB