I have the 12" version. I called PSI Technical Service and spoke to Joe who walked me through the process. It's not difficult, but you do have to exercise some care - you could hurt yourself, and potentially damage the lathe if the screwdriver that you are using to adjust the speeds slips and strikes critical components in the controller box.
The basic process is:
1. Turn off the lathe
2. Remove the three screws that attach the control box to the bedways,, and flip the box up so that it rests on the bedways with the metal bottom plate facing up. Remove the four screws hold the metal plate in place, and flip the plate up and out of the way. Note that there is a ground wire attached to the plate - don't remove it.
3. There are three potentiometers on the 'mother board' in the controller that are marked H, L and F. The adjustments you want to make use the H and L potentiometers. You must not make any changes to the setting of the F potentiometer.
4. From the point on, be very careful. In particular, never let the screwdriver come in contact with any part of the controller other than the screw slots on the H and L potentiometers - including the grounded metal plate that forms the bottom of the box.
5. Turn on the lathe, and adjust the speed control on the front panel to its lowest setting. Carefully tweak the L potentiometer to adjust the speed at the lowest end of the range. My experience is that the speed will change very significantly with very minor rotation of the potentiometer - so proceed very tenuously. You can use the tachometer to judge the speed that you are getting. It doesn't matter which pulley you are on as long as you recognize that the speeds are doubled on the high-speed pulley.
6. With the lathe still on, adjust the speed control on the front panel to its highest setting. Carefully tweak the H potentiometer to adjust the speed at the high end of the range. Again, it doesn't matter which pulley you are on.
7. After adjusting H and L, run the front-panel speed control over its entire range to make sure that you are happy with the range of speeds that you are getting, then turn off the lathe.
8. Reattach the metal plate to the controller box, and then flip the box back to its normal position under the bedways, and reattach.
I normally have my lathe adjusted to give me a range of around 50-1800 r/min on the slow pulley, and twice that on the high speed pulley.
