MaxMSP, Futaba s3003, Duemilanove & MacG4 ATX PSU - current draw questions

Hi there,

I am attempting a project that involves MaxMSP control of 11 Futaba s3003 standard servos that in general knowledge claim to draw less than 1A in average conditions. I have tested the sketches (knob, sweep) (Firmata, MaxMSP) using initially just the Arduino on usb power for one servo at a time and all is functional. I realize that for 11 of the same servo i need to bank on 130% more power so I acquired an ATX power supply from a Mac G4 (API-9841)which claims to supply 20A of 5V, as well as 8V of 12V.

Now of course, I did get this PSU free, but from a school technician so it was in demanding but reasonable condition. But I suppose if it's living outside of the computer it was built for then something may have happened along the way.

Firstly, does anyone have experience running servos from an ATX switching supply that has been modified? Presumably this should be a quality, efficient (it's very quiet) way for me to power these servos. I have modified the wiring insofar as I have removed the bundles and organized them by colour, (12V, 5V, 3.3V, G) and thrown it into a breakout board with screw terminals and room for the servo pins, etc, ribbons going over to the arduino for +12V, G and digital outputs.

Main issue is that it can hardly run one servo, or rather, if I stall one it will shut down the DC supply. By Max control, I can run 2 servos (tested one at a time) for about a minute, maybe, and i can run 3 for about 5-10 seconds. This won't really work for me hah. I have tested running the arduino with and without the external supply. A question there*, if arduino IS receiving external power, will it always choose the a) higher voltage and b) non-USB power?

This is to be a unit that lives on a shelf so battery isn't really the route I plan to go. Obviously I could buy something but free is a nice price if it is a reasonable solution. Is there a capacitor placement that can save me? Currently my multimeter is non-functional which I suppose is an issue for these types of projects.

In a day or two I will be getting another clean power supply, perhaps my issue truly is in the 6 year old PSU.

Any insights would be interesting,