My name is Jeannot thewissen.
I got a dell Optiplex from a friend, because he said it wont power on.
So I tested it powers but if you power it on you dont get any video or sound. The only thing that happens is that the fan in a few seconds maxed out on rpm.
I did search google and i found it can be a broken psu or a bad cmos battery. Both replaced. when I replaced the PSU the computer worked for a day. I wanted to install windows today and the same problem again, CPU fan maxed out in few seconds.
It's impossible to say. I'm an electronics enthusiast and I've diagnosed and repaired many faults in equipment in my life, but it's really hard to say it's one thing. It could be two bad PSUs in a row, it could be a fault on the motherboard, it could be something completely separate. If you can't fix the problem yourself, find a friend who can come over and diagnose the fault for you. That or sell it online - someone might buy it wanting to fix it.
for one thing its to old to spend money on ,I just recently repalced 2[with newer used one ] of these for customers of mine, they had similur issues as yours ,one had a bad psu and the other one had leaky capsators .