Question No signal. Tried everything.

May 28, 2020
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For about three months now every time i go to plug in my computer to a monitor it displays "no signal"

in those three months ive:
-bought a new cpu and motherboard
-tested the pc on a working monitor
-dissembled and rebuilt about a dozen times
-switched out the display cables multiple times and tried a different GPU
-swapped out the power supply with another working power supply
- tried the cleaning the RAM with an eraser trick

nothing seems to work and I haven't been able to take it to a repair shop due to the quarantine
It might also help to know that the computer itself turns on for about 20 seconds at a time before rebooting by itself. No matter what I try I always get the same results