Anyone interested in our very simple and effective original ST-120 power supply upgrade, just send me your email address to: frank@avahifi.com and I will send you the complete .pdf file which contains the schematic, original power supply board simple modifications, and original schematics too.
We still have a good supply of the necessary Hitachi 2SK135 power transistor but the Exicon 10N20 TO3 case mosfet will work too although the gate resistor should then be 475 ohms.
Note that our complete Dyna 120 rebuild, our Insight+170 gives you a whole new modern amplifier in your case, everything new including the power transformer for $899.
Frank