@s8n i see, you prefer a complicate way :)
you can use only +5v power supply. computer will start without 12v
i have similar one
Follow
I'll check scheme, main board of "yamaha" use only +5V. +12 and -12 go directly to slot, for cartridges.
But video modules was different, simple and old version use +12. It need a very small power, ~0.02A, a easy way - use cheap step-up 5-to-12V board
