Built new computer. Monitor no signal input problem?

Are ALL the motherboard attachment screws installed, some of these are also grounds so they are critical. If the motherboard has onboard video then pull the dedicated card and check for signal. Is the video card seated correctly in the PCI- E slot ? Square 4-pin power plug attached to card ? Process of elimination, discover what's working to determine what's not.

You do seem to have tried everything, especially if the on board doesn't work it does look like a dud mobo, are there any indicator lights on the mobo?

There are certain requirements that have to be met that "can affect" the outcome. Be kind, I do not know all of what you have tried or not. Check each of the following as each one can cause your issue. - yes, the keyboard is a mandatory component for a PC to boot. Some MB BIOS mfg allow a setting to be changed to not freeze on "keyboard errors" but you need a kybd to make that setting. it must be plugged in, at first. however, you may still have a monitor signal in some cases. - must use an ESD strap as not to fry the components (not an option). You probably just, I am just mentioning it here - the motherboard must be properly fastened to the backplane: need stand-offs if the design of the case requires it (the gold colored posts you mentioned), every MB hole MUST have a screw in it and into a standoff into the case. This is because the MB requires that all "grounding points" be grounded in order to close the circuit for current to flow in all parts of the board and properly. (paramount!) - all components must be free of dust, oils, fingerprints and be securely seated into all slots - your PSU must have enough wattage to cover your Video Adapter even after wattage is reduced by the other components. In other words, if a GPU required 500W then a 500w PSU would not be enough and would fail. - verify the data cable is good (look at the pins on both ends of the cable to ensure none are bent even. - the video adapter, cable or monitor may simply be bad Please think about each of these line items, there are more possibilities - I simply am not there with you to see exactly what is going on. Note: are you getting a BIOS POST Success "beep" from the PC speaker. If not, you DO have a hardware problem!