Barry you are looking at the wrong part. You need to test the input to the servo encoders check it with your volt meter to grnd you should have 5vdc. If not look for a fuse blown .
NOW are you sure you have not changed anything before you started having problems?? If not the you need to look at the power to the motor encoders that is the only thing that will cause all 3 motors to run away at power up.
If you disconnect the pc from the drives will it still run away when you power up the machine?
DID your machine come fully assembled of have you built it from a kit, or from scratch?
(;-) TP