I remixed and installed some X-Y belt tensioning mods. These really made setting tension on the belts super easy after I installed the gear box and idler pulley mods.
The first mod was originally designed by supinemonkey. It adds a fine tension adjustment by simply turning a knob. I made a few improvements and got his blessing to post them as a remix, as he has abandoned his Promega.
The second mod was prompted by the first. The factory method of running an M3 screw down through the rear cable chain bracket and motor mount and terminating it with a lock nut was causing some movement limitations of the motor on that side for tensioning purposes. So, I remixed the bracket to allow the use of a flat head screw inserted from the bottom and a heat insert in the top, which also required using a drill press to countersink the end of the associated motor retaining bolt slot on the underside to make the screw head flush with the bottom of the motor mount. As an added bonus, I also cut a vertical slot through the "bridge" on the cable chain bracket for tightening the bolt under that bridge without repositioning the bracket.
In order to use the new vertical slot, you'll need a screwdriver with a maximum 3mm shank. I bought this one and it worked well: