When you say A lead do you mean the wire going to the ammeter?
I do Musky. On the DVR there are 4 wires. At 12v it's yellow and green FROM the dynamo, and then the brown one to the ammeter/switch and red or black to earth depending on whether you're + or - earth. The 5th wire, white, is suppressed at 12v.
But a Q here is whether with the electronic ignition you have, if you disconnect the power from the regulator to the battery after starting, will the engine just die? It might if the battery is on the dead side of flat and can't supply the necessary without the dynamo connected to boost it.
If the engine
will run off just the battery (or a better one pulled off another bike maybe), then you could disconnect the A wire from the DVR2 (anywhere there's a connector you can get at) and measure the voltage between the end of the wire and earth as the engine runs.
If the thing
won't run with the A wire disconnected you could just do the 'normal' thing and measure voltage across the battery terminals with the engine running to see what happens with varying rpm, without disconnecting anything. But it might not tell you as much as you want if the battery is dead as Monty Python's parrot.
From what you've described at the start of the thread, I reckon either the regulator isn't working and it's buggered the battery due to overcharging (made worse by its small capacity perhaps) or the battery is so fubarred that the regulator has lost its way and is telling the dynamo to generate absurd amounts of current, which it can't do indefinitely without having a hissy fit. But wtfdik?
