I just repaired a 5 amp power supply that was purchased at a rally in non working condition. I purchased it primary for parts especially ariits variac. Looking at the construction I find it unusual that the company "Power Designs" (model # 3650-S) utilized a variac for voltage control and then piggy backed a wire wound pot on the back for input and feedback for voltage regulation - - - So the variac does most of the "work".  Its a great supply, works well with 24V dynamotors or solid state circuits requiring lower regulated voltages. And as a bonus you can lift it. 
Q. Has anyone any information on similar supplies using a variac/feed back pot ?
k4che