DC regulator defective? How can i check ?
 

• The shortest way to tell whether a regulator has gone is to smell on it. If the unit has a burnt smell it has surely gone (invariably due to shortcircuits or overloads)
• the next easy thing is to disconnect the battery, disconnect (if existing) the blue wire from the switch out relay, start the engine and switch on headlights. If headlights shine brightly (at least when you accelerate a little engine revs) than the regulator is working
  
  should the regulator not smell and there be no lights whatsoever, than you have to dig a little deeper as suggested below. Please refrain from any other methods of testing, especially from connecting testing voltages from supplies other than the Powerdynamo magneto.

1. You have to check first whether it is not the generator which is defective. without any output from the generator there is nothing to rectify and regulate.
   
   for this disconnect the 2 black wires which lead from the generator to the regulator.
   Connect here a meter that can read AC (!) voltage.  Start the engine. The meter should indicate in volts ac about revolutions/100 (see Video, will take a little to load). Say at 800 revs you get about 8V. At 5000 revs you get 50 volts. If you get no reading or significantly less the stator or the wires from the stator are defective. (regulator might be also defective, but you can not check now). More information here
   
2. Provided there is AC voltage from the generator, reconnect the 2 black wires to the regulator again and completely disconnect the red and brown wires (which carry the exiting DC voltage) from from rest of the motorcycle wiring 
   
   Connect a headlight bulb (in the correct voltage 6 or 12) directly to the red and brown wires at the regulator. Do not connect anything else there!
   
   Start the engine. If the bulb shines brightly (be careful not to look directly into the light, this will dazzle your eyes!) the regulator is working. If it does not (and you have rechecked wiring of the testing arrangement) the regulator has a problem.
   
3. The charge control function (available only on the larger 12V regulator) is a function independent of rectification and regulation. Even if this is not working, the regulator as such could still be ok. for checking charge control see here.