Master Replicas saber running off dc adapter?

[athrun02151]

Hi there,
Not sure if i'm posting this to the right place, or if this has been covered, sorry if it has been.
I'm currently working to convert a Master Replicas Darth Maul from running off 3 x aaa batteries to using a DC adapter for display(not on all the time).
The problem I'm having is that I'm getting noise kinda like audio amp motorboating.
The way I'm setting this up is that I've got wires soldered into the battery pack, so the speaker is still in there (drilled grooves for the power cable). I then am using a 12vdc adapter stepped down to 4.0v (using a resistor for now until proper voltage regulator arrives in post).
I'm not sure if it's because the current needs filtering? Or if it's something else. I've tried laptop power bricks after limiting the voltage and current, but still same problem. Can't hear any of the normal saber sounds, just motorboating. It's a mrfx board I'm trying to power.
Any help would be appreciated!
Thanks

[Skottsaber]

Does the blade scroll on and off perfectly? It sounds like the board is not getting enough current and is resetting rapidly, but if the blade works that isn't the case.

[athrun02151]

I haven't got the blade connected, but attached an led and when power is connected the led turns on for a second, then goes out (the rapid motorboating sort of sound happens throughout the time power is connected). Tried a capacitor across the supply, same problem. Although when I connected the 12v to the battery pack by accident (forgot about the resistor for some reason) the board ran, although the led burnt out.

[Skottsaber]

It sounds like the board is underpowered. It sounds like your power supply is getting enough voltage to the board, but not enough current. Most 12V DC adapters are made for charging consumer electronics, and often only put out relatively small amounts of current at high voltage. It is also possible that your resistor is limiting the current to the board by a significant amount. What value resistor are you using to step down to 4V?
If you have a multimeter I would suggest you measure the current flowing from the power supply and see if it is adequate to power the board.

[athrun02151]

It's around 400ma, how much current does the board require? I'm using a 1.5K ohm 5w resistor. Thanks