AFAIK the only components on the 400/800 that use -5VDC are the 4116 DRAM and the power LED, if the 4116 are eliminated by a RAM upgrade and the power LED rewired to use +5VDC this should eliminate the need for -5VDC. I also believe that +12VDC is only used by the 4116 DRAM, the color adjustment potentiometer, and SIO pin 12.