How I fixed my waterpik. This waterpik is made for 110V AC (North America). Someone inadvertently plug it into a 220V AC outlet and burns out the resistor inside the circuit board. All I did was to replace a new resistor and it works like a charm. The newer resister is bigger in physical size but it would still work (the bigger size can even handle more Amps).
The new resistor also has more Ohms value (56 Ohms instead of 47 Ohms). But the Ohm value is close enough to be insignificant.