Miller Welding generally gives the schematics for their machines in the operating/service manuals, which are available free on the Miller site. Find a Miller machine that does about what you need, then look on the site for that model number.
Sorry about that. My previous experience was with a tig unit, circa 1983. No PCBs to speak of, and they still have the diagrams with all the resistors, capacitors, and pots labeled.
The newer units, as you observed, just have a PCB. If you have a valid serial number, you could try asking Miller for the circuit of a specific PCB part number.
