It's a simple DC motor control circuit using two SCRs to supply full wave rectified voltage to a DC motor. A uP controls the phasing for speed control through an opto to the driver circuit which is referenced entirely to the motor (no ground or earth). The problem is the noise test which injects 150 nsec pulses of 500, 1000, 2000 and 4000 volts into the AC input. At 2k and 4k, the motor is being disturbed, so I think the pulses are triggering the SCRs. What is the normal noise suppression that is done on these devices? An links on this?