Try another battery. If it still doesn't work it's either the switch or the motor. You can take it apart and hook the motor directly to the battery. If it runs the switch is likely bad. If not the motor is bad. Since it doesn't have servicable brushes you replace the motor. Be careful when you take it apart because the switch is static sensitive and you could damage it! Do the work at a static sensitive station.
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Try another battery. If it still doesn't work it's either the switch or the motor. You can take it apart and hook the motor directly to the battery. If it runs the switch is likely bad. If not the motor is bad. Since it doesn't have servicable brushes you replace the motor. Be careful when you take it apart because the switch is static sensitive and you could damage it! Do the work at a static sensitive station.