I've just been looking at this post on the PMinMO forum from another fella that was powering his machine with a pair of piggybacked PC power supplies:

Actually toasted a few......

I have built several boards, all test out using the measurement of voltage and jumpering enable and step.

Solder in chip with low volt iron, set voltage with pot.

Set computer to setup settings given in this section.

Step motor and it steps once and poof!
He's not happy. This keeps happening. Wiring everything up, it's all humming. THen he takes a step... Blat. The consensus is that it's the lashed up PSU he's using. PC PSUs aren't good for CNC work by all accounts. It's resolved in the end though:

It looks like it was the power supply. Tried a linear 24v supply and the driver and interface board works!
Just thought this was worth pointing out.

Charlie