I built this board about 2 years ago and although it's not a chopper it worked well for me. Was driving 127 oz. steppers using a 48V power supply (had to replace the TIP 120s to a high amp version, don't remember the number). I was getting around 60 IPM using acme 1/2-10 leadscrews. I am now using the hobby cnc chopper board for more speed. I am going to use John's board when I convert my South Bend metal lathe over to CNC soon.
Hope you resolve your issues with the board, do the motors lockup, can you turn them when you supply power to the board?