First off I didn’t know if this should be in dead machines, or monitors. This problem doesn’t really fit in either, so I’ll post in both and if a mod wants to lock one because its not in the right area I understand.
Ok, so this isn’t a big deal as I’m just trying to put together a little working computer for when I have guests, something they can use to check mail, or whatever. I have a lot of old machines, three emachines, and a very old custom computer. So I figured I could put something together and get it working, and the fact is I did have a working computer, and this is what happened. I turned it off, and when I went to turn it back on (I didn’t change any configuration) I hear an electric pop, so I’m sure something got ruined but I have no idea what it could be. Everything “seems” to work. But I don’t have any picture on my monitor.
It’s not the PSU; I’ve tried three different ones. It’s not some problem with interference from input devices, as I tried to turn it on with no input devices (mouse/keyboard). I’ve tried two different motherboards, still no signal; I thought that was really odd…. Unplugged the CD-drive, I don’t know how that could interfere but I thought I would try. It’s not the monitor because I’ve tried two different monitors and both of them work with my main computer. It’s not the monitor cord I’ve tried multiple ones. I tried a graphics card in both motherboards, nothing. Tried no graphics card, still nothing.
The only things that I haven’t, or cant test are the ram, and the hard drive, I doubt it’s the hard drive as one; I don’t see how that could cause a problem with the monitor, and two the “pop” I heard came from the motherboard, I think.
So could the ram be causing problems with no video signal? I don’t think so, but I don’t know what else it could be. I honestly don’t know what to try.

What kind of computer do you have? Please list the specs.
how do you know its not the PSU?

the first thing to do when you turn on the computer is to listen to it.

Do you here the fans turning? Check to see if the fan on the PSU is turning also.

I know its not he PSU becuase i used three different psu's lol. And no there is are no fans running abnormally fast. They are all running though..

What would I do if I heard a pop from the mobo?

I would assume that this is now an ex-mobo. A pop means that an electrical function has died - maybe a capacitor or even a transistor and mobo functions will be incorrect.

Maybe I'm wrong in your case, but I would 100% bin that mobo. RAM might be OK - easily tested on another compatible mobo. Likewise the HDD.

Bin the mobo. Simples.

I agree whole-heartedly with suspishio. Even more so if you smelled any ozoney smell after the pop.