A few weeks ago I received a dead 707 (no lights, screen, or sound). I'm usually not one to ask for help, but I'm totally stumped. B asically, plug it in, hit the switch, and nothing happens. 

I am using a working power supply (center negative), but I do believe the previous owner was trying to use a center positive. 

Some things I've noticed: Intermittently  there is a large voltage sag. Sometimes I am reading 12v into the 5v regulator and 5v on the output, but sometimes I am reading around 0.5v on the regulator input, but still 12v on the psu itself.  When 5v is working correctly, the -10v rail also looks good. 

I did find d12 was blown, but the directly connected transistor (Q31) reads correctly on my meter. 

When the power works correctly: 

CPU reset is stable

Clock is reading 4mhz

and I am seeing activity on the data lines at the cpu. 

I've also unplugged all ribbon cables from the main pcb and the problem still occurs. 

Any suggestions would be awesome. Thanks!!