Black screen when booting computer. Why?

Question

Iron

Hey!
There is an old computer that just stood idle for several months, before that it worked fine. Recently turned it on - found only a black screen, the BIOS did not boot. Only the monitor button blinks, as when turned off. What to do?
After he opened the system unit, removed the dust, disassembled everything, cleaned the contacts everywhere with the help of alcohol.
Now I assembled it: motherboard + vidyuha + percent + RAM + power supply.
Coolers are working.
Reset the BIOS, pulled out and inserted the battery - the result is the same.
Tested with another vidyuhi (working) - the same result.
I tested the monitor by connecting it to the laptop - it works.
There are no beeps at startup.
On the processor and vidyuhi changed the thermal paste at the same time.
What to do ?

Answer 1

In older computers, electrolytic capacitors usually die. Those. their characteristics are enough to "turn on", but there are no more for the further operation of the system.
First of all, you need to check with a known good PSU. If it does not help, inspect the motherboard for swollen capacitors.

Answer 2

Perhaps the power supply flew, try with another. It was like that, the problem was in it.

Answer 3

just in case, we cut off nonsense. Check if the speaker is connected.
Then we take out the memory and turn it on. If there are no beeps, then with a high probability there is a problem in the power supply, the 3.3V line. One could try to give more information if the motherboard (chip) and power supply were indicated.