I have a used Quadra 700, first time I powered up it not immediately switch on but... after some minutes! And with power button I not able to power off, if i push the switch the mac only reset... And now after some test and cleaning of the pcb it never switch on. I able to switch on only with a bridge pin 9 and 10 of psu.
But I have another problem... this is a screenshot of my screen (without hdd), recap solve all my problems?
If I recall correctly, the Q700 doesn't have many electrolytics outside the PSU. That looks more like bad video memory. Do you have a spare nubus graphics card you could try, instead of the onboard video?
Power switch issue sounds like battery problems. Either it's dead and the PMU's corrupted itself (which would also potentially be why the screen is scrambled since PRAM Is there) or the battery leaked and damaged the power-up circuit (which is unfortunately common).