I have three self-built computers which will power up right now but not post. Two of these computers were working previously, but were bumped and then stopped working. The other is a new computer. I have stripped them down to just a power supply (which has been tested to verify that it works), a motherboard, a CPU, a heatsink, a fan, and the speaker. The system powers up - fans turn on, lights on motherboard turn on, hard drives click if connected- but no post occurs. Any help would certainly be appreciated.

Any beeps?
W/ no video card, you should be getting some beeps. If no video card, and no beeps, ensure that your speaker connection is connected correctly -- if wrong-way-around it won't beep.

If speaker is correct, and no beeps, then either mobo or cpu is toast. Possibly both.
But on all three computers? At once? Any lightning come by your way recently?

Since lights and fans work, PSUs seem OK, and you do say its "known good".

What mobos? CPU?

What is the warranty status of the parts?