i would say check all supply lines, and you have isolated ones too, check if you have all the crystal clocks running .... maybe just maybe a tantalum shorted out ?
i don't know if you have a logic probe you could poke on the front panel connector if you see the 3.3v 12v reset line if stuck, data in out line, and for sure the filament voltages
you have to check all section one after the other, mostly voltages 1st, you have isolated +18 -18 volts lines and a 5 volt for the analog section or redacted out, some open office tool was able to recover it, i put 2 files here
other link for the schematic
https://www.dcddcc.com/blog/34410a/schematic.34410a.pdf it miss the supply section
and lastly you dont change parts for nothing hoping it will work, i dont know your expertise levels