Grant,
Does the engine stop the instant you turn on the PTO switch? If so, the most likely cause is the seat safety switch not sensing the presence of the operator. If your tractor is below SN 70,000 there is no safety switch...
If it takes a while for the tractor to stop running after the PTO is turned on and it seems to be from the battery getting discharged, then the PTO coil is likely internally shorted and drawing excessive current. Does the ammeter show a heavy discharge when the PTO is switched on?
Chuck