What type of problem is it: video card or mother board?

My monitor displays a ''no input signal'' when connected to modem. I've changed video cards and tried mother board card, still no signal.|||get a new rig and throw out ur 1968 computer|||try inserting video card into another pc, works in other pc, then installed wrong! i don't think it would be motherboard! u could try installing it in another slot! not all the slots can't be bad! i had the almost same problem and later found out that power supply was to small! but it said resoarce error on mine! but might be same case on u! u might not have big enough power supply! but first try above! insert in another computer, then take and try all your pc slots!