Have you checked with a multimeter? 7.4volts is what the battery puts outs. Why not just use a 240volt invertor? What i think is happing is there isn't enough amps to power it or your dc step down tranformer is putting out 7.1 volts or something. Don't use it at 9volts, you will kill it by frying out the gates in the ic's or buring out power mostfos resistors ect. I wouldn't be using anything thats out more or less then 5% of 7.4 volts to be safe
