I just opened up my sovtek Mig 100U today and mapped the preamp out. I was curious to see how much it differed from the 100H if any. The major difference is that the tone stack is taken off the cathode instead of plate driven. The power section looks identical from what i could tell. The second and third cathode resistors are both 1k unbypassed instead of 4.7k. Your tech is either very picky or incompetent to demand a schematic. Goodluck!