I tried to upgrade memory, didn't work so reinstalled original sticks. Now pc boots but no video, just a black screen. I looked for the CMOS chip to change pins but it must be behind video cards, not in plain sight. Any ideas?
Yes, it's behind the video card. I had to do this as well, when I upgraded my ram. I was trying to use 2133mhz ram but my processor (4820k) only supports 1866mhz, so I had to reset the the CMOS, then when I got into the bios, change the ram frequency.