DAC output fails low (output = 0 volts DC): Motor will not run.

DAC output fails high (output = V): Motor runs full speed all the time.

IGBT Q1 fails open (collector to emitter): Motor will not run.

Solder bridge (short) between MSB input on U1 and ground: Speeds 0 through 127 work normally, but speeds 128 through 255 just duplicate speeds 0 through 127, respectively.