This 500 wont charge. What is the black electrical box at the very front of the quad ? The regulator ? Where should I start ? Ideas?

 

Yes its the reg/rectifier,check first that all connections are good. A lot of time one or more connections come loose. The red wire should have power at all times coming into the reg/rectifier,if does then in all probabilty just needs to be replaced,IF battery is good and can hold a charge in the first place. Should be charging 14-14.5 volts at battery.

 

chances are the recifier connection (as mentioned) is either loose or corroded.