This is what I did to go from 6.0 volts to the receiver to 7.5, and it was a great improvement in servo strength. I added a 5th AA battery in series.
Go to radio shack and buy the single AA battery box. Part # 270-0401.Its $99 cents. It will have 2 leads out, black and red. Take the red wire on your current battery box, and cut it in half. Wire the red lead of the single AA holder to the receiver power plug red wire. Do a good job, twist them together and solder them, then put shrink tube over the exposed wire. Next take the black lead from the single AA box, and hook it too the red wire coming out of the 4-cell battery pack. You will now have a second box hanging off that will need to be positioned in the place inside the receiver housing for a 3rd servo gear. Use a volt meter, and make sure your getting 7.5 volts output, that way you know its working. I put 5 Ni-MH batteries. The power to the servos was much better and the steering is much more powerful. It is well worth it, and I have been told that it will not damage the receiver. It hasnt messed mine up yet.
Go to radio shack and buy the single AA battery box. Part # 270-0401.Its $99 cents. It will have 2 leads out, black and red. Take the red wire on your current battery box, and cut it in half. Wire the red lead of the single AA holder to the receiver power plug red wire. Do a good job, twist them together and solder them, then put shrink tube over the exposed wire. Next take the black lead from the single AA box, and hook it too the red wire coming out of the 4-cell battery pack. You will now have a second box hanging off that will need to be positioned in the place inside the receiver housing for a 3rd servo gear. Use a volt meter, and make sure your getting 7.5 volts output, that way you know its working. I put 5 Ni-MH batteries. The power to the servos was much better and the steering is much more powerful. It is well worth it, and I have been told that it will not damage the receiver. It hasnt messed mine up yet.