Now the great thing about this hack is that it'll probably work with other automatic strobes too. Just this one seems to have enough space inside for the potentiometer knob and the removable plastic cap which makes it very easy to do.
It is a great way to get a manual flash on the cheap, and it can also be modded to use a different power source for faster recycling. I've triggered it with my 40d and they both still work, but don't take my word for it: http://www.botzilla.com/photo/strobeVolts.html
Follow the instructions below at your own risk.
You will need:
If you use a smaller value potentiometer, you will only be able to control the weaker side of the power range.
You will also need a fine phillips screwdriver, soldering iron and some solder.
Disconnect the control module from the flash by pressing the button on the side.
Remove the 4 small screws and pull gently, there is a ribbon cable holding the pieces together.
On the front side of the unit there is a phototransistor that needs removing. (This will mean flash won't be able to work automatically. If you need manual and automatic you can drill a new hole for the knob and use a switch in parallel with the phototransistor)
If there is any glue, cut through it with a stanley and press the bits of plastic together. The sensor should just pop out.
Now you can desolder the wires and prepare to mount the potentiometer. The potentiometer has a pin on the front to stop it from moving. Drill a small hole for it (or burn one with a soldering iron.)
Remove the nut and the washer from the potentiometer and solder it to the wires from the light sensor. Then mount.
Now carefully reposition the top, so that the ribbon cable is not pressed between the top and the potentiometer.
Make sure that the switch fits into the plastic runner, press the top and reposition the nut.
Well done, now test.
Woo-hoo manual control flash for under £10
If your flash fires full power with the control module attached, you've most probably got a connection problem. One of the units I've got had a bit of glue between the pcb an the spring of the pin which was easy to fix by cleaning the contacts (just use a pencil eraser)
If your flash doesn't fire and the pilot light doesn't come on but you can hear the whining sound — try new batteries. If not then most likely one of the capacitors inside the flash needs replacing but make sure you know what you are doing.
If the flash doesn't power on and doesn't whine — try to clean contacts with sand paper including the battery door. Old batteries can leak if left inside the flash which'll most likely prevent it from charging.