Start up then power down and restart problem

[DTrouble99]

I turn it on it starts powering up then sounds like it powers down the shuts off and restarts with the same on repeat no screen visual just keeps repeating please help!!

[BeemerBiker]

This sounds like short circuit protection (SCP) being activated.  The power supply detects a short and shuts down but does not turn off so it keeps repeating.

If a real fault in motherboard then remove any video card, hard drive etc.  Leave only one stick of memory and see if it boots.

While you do not have a standard ATX power supply I suspect HP includes protection and possibly the power supply  is defective and thinks there is a short when there is none.  There are videos and diagrams to show how to bypass the SCP on atx power supplies.  I do not know about custom HP ones.

This happened to me and the system worked when I substitute another power supply that I kept as a spare.

Try to borrow an HP power supply to verify it is not the motherboard.  You may be able to take it to a repair center first and have them verify the power supply is the problem.  If the problem is the SCP the voltages will measure as normal but the system will shutdown when connected to anything, even an automobile headlamp bulb.  A good technician could verify this.