I have a Sorensen DC20-25B 0-20v 0-25A lab power supply. It will work normally for awhile but after drawing around 10amps or more for awhile it will buzz, the output will go somewhat erratic then the ac line fuse will blow. After this every time the fuse is replaced after a few seconds of operation it will blow again. Has anyone an idea what is wrong with it or where I could get a schematic for it? I would appreciate even some information on the basic operation of the circuits used in this unit. Thanks, Bob