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Help with a P4 that turn on just sometimes

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First post, by astoufu

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Hello, guys, i am trying to revive a really old computer, but having some issues with it even to figure it out whats is wrong.

It's a Pentium 4 HT that i dont use since 2008, but i had some personal projects to go with it. First, i cleaned the computer, replaced the CMOS battery and installed a old PSU that used to work.

The computer does turn on and the fans keep spinning everytime, but it wont give me a video signal, sometimes it doesn't even turn on the keyboard. Even when i turn the PC on without any memory, the PC won't beep. I also tried to turn it on just with memories, without the HDD and CD-ROM Drive, but no luck.

If i try to shut it down, sometimes it will do right way, while others i will have to hold the power button for a few secs, as it really should be.

In my tests yesterday, after 4 hours messing up with this machine, i managed to turn it on 3 times, all randomly. I even managed to get through the Windows boot, but the installation was corrupted by my fault on the past.

I don't know if a "dead motherboard" would be able to turn on sometimes, so i am taking suggestions to what to do next. Maybe should i replace the PSU or something?

Sorry for my bad english

Reply 1 of 2, by RandomStranger

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This is the kind of situation where it's good if you have some spare parts for testing everything separately.

My first guess would be faulty caps on the motherboard or in the PSU which work just enough so they will let the PC turn on properly sometimes.

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Reply 2 of 2, by Roman555

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astoufu wrote on 2022-08-30, 14:59:

I don't know if a "dead motherboard" would be able to turn on sometimes, so i am taking suggestions to what to do next. Maybe should i replace the PSU or something?

Hello.
P4 platform (both S478 and LGA775) is notoriously known for losing BGA contact between a PCB and a north bridge or a CPU socket. It's too tight fasteners of the stock CPU heatsink.
Of course it might be not your case. But if your system doesn't start you can do a test.
I suppose the system is taken out of a computer case and built just on a desktop. A CPU cooler is on place but it is unfastened. So try to press carefully onto a CPU or a NB (it's better to put something like a lump of soft fabric under a mainboard into a place of pressing) to temporarily restore losing contact and then press power-on button.

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