Any idea how to check if the voltage regulator is working?
Disconnect the battery.
Get a 12 Volt headlight
hook the headlight up to the battery leads
Start the engine
Does the light glow at idle? If yes, rev the engine
If no, rev the engine
Does the light get brighter?
If yes, measure the voltage across the light terminals while revving the engine. It should read about 7 volts
If it does, alles ist in ordnung
If it doesn't, measure the voltage across the output wire of the dynamo and it's case
It should read the same as the leads to the headlight (roughly)
If it doesn't, read the voltage from the field terminal to the case
If there is no voltage, or very little, get another regulator or check the continuity of the field wire from the regulator
Hook some jumper wires to the battery, hook the appropriate polarity jumper to the dynamo case, (ground) and touch the other battery lead to the field terminal.
Do not leave it on for more than 15 seconds or so, and while it is connected, rev the engine and check the voltage of the dynamo output. (terminal to case)
It should go way up
If it doesn't, rebuild the dynamo. If it does, the dynamo is working and the fault of no charge is probably the regulator
Most importantly, make sure you wired the regulator correctly, they do not handle reversed polarity voltages well
For extra credit, BING "flashing the field" and ponder the vagaries of electrical components.
Have fun JR