I don't know about your chances of finding one but I have a schematic. Apparently the 3400 and 4400 series mixers use the same power supply. It's pretty complicated, looks like something you'ld find in a 70's Tektronix scope. Twenty four transistors. Outputs are +48@0.1A for phantom power, +/- 18V@2A, +6V@2A, and an unregulated voltage about 14V fused at 1.5A for lamps.