Switches on for few seconds followed by 4 bleeps, repeats bleeps every few seconds

[Tuga99]

Switches on for few seconds followed by 4 bleeps, repeats bleeps every few seconds

[itsmyname]

@Tuga99 -- do a Google-search for "HP Beep Codes", and try to match your pattern against the HP documents.

It could be a "loose" add-in video-card, or a bad stick of RAM, but could be something worse.

[Prométhée]

hi

https://support.hp.com/us-en/product/hp-compaq-dc7800-ultra-slim-desktop-pc/3459242/manuals

 Illustrated Parts & Service Map - HP Compaq dc7800 Ultra Slim Desktop Business PC

does it look like this?

Power Red 4 blinks, 1 blink every second
followed by a 2 second pause
Power failure (power supply overload)
OR on USDT the wrong external
power supply is being used.

[Tuga99]

Yes, it could br the cause. I've tested the power supply and it only reads 18.6v , it should be 19.5v. Need to try another power supply. Thank you for your help

[Tuga99]

It is not the power supply, have got new one (19.5v)  but problem persists, need help please 

[Prométhée]

Is it the same power supply ..
If so, apart from doing a series of tests, more or less easy, impossible to certify anything
Either you spend a lot of time, testing each element, and you still have to have what it takes
either you go to a pro who would eventually test everything, or almost everything!
how old is your computer, can you buy a new one, the costs to repair the one if could be high

[itsmyname]

@Tuga99 -- need help please 

As an experiment, if you connect the "old" power-supply to a DIFFERENT desktop computer, does this power-supply work properly on that other computer?  If so, I would suspect the motherboard in your computer is causing the issue.