2 Computers, both won't boot/monitor no signal after CPU swap

Hello,

I've got a pretty old PC, and a friend of mine recently gave me his old PC to try and utilize the hardware from that PC to upgrade my own.

The thing that would've upgraded my PC the most was the CPU, and fortunately they have the same socket. So I carefully removed both heatsinks, removed both CPU's and got rid of the old thermal paste. I applied new thermal paste and swapped the newer one into my own PC. After that I tried to boot the computer, but the monitor got no signal. The PC fans run good, but there are no startup beeps and the HD seems to be inactive very quick after starting. I hooked up the PC that was gifted to me (with my old CPU in it), and that had the exact same problems (The monitor, the absence of beeps, and the HD).

I then went to search online for possible solutions, and tried some things to help me solve it;

- I changed the RAM, leaving only 1 in. I've tried all sockets, and all RAM.
- I removed all RAM, disconnected all DVD-Drives and Hard-drives. Still no beeps
- Used an eraser to clean the RAM and remove static
- I've hooked the monitor directly to the motherboard
- Changed monitors
- Changed monitor cables
- Changed videocards, tried both videocards on both motherboards
- Reset Cmos
- Removed the power cable, held power button for 30 seconds, plug power back in.
- Disconnected everything from the motherboards, adding stuff one by one to determine the problem
- Started both PC's fully connected to other hardware (keyboard, mouse, etc..), and started them with only monitor and power connected.
- Change CPU's back, making the setup exactly the same as they were about 3 hours ago.


I feel like I've tried everything I could find online and it comes down to bad motherboards. But it seems almost impossible that I've broken 2 perfectly fine motherboards, especially since I've been very methodical and precise when swapping the CPU.

Does anyone still have some suggestions that I can try?
Help would be greatly appreciated!