<div>
an aged, vulnerable hard disk is the first thing came into my mind after reading through your question. the HD is better to be replaced with a brand new one or at least be fully tested and physically checked.
</div>


<div>
I have similar experience of fixing a mini PC which have similar symptoms like yours.<br />
<br />
The fix is on the PSU or the connection to the PSU and you need to have electro meter to test the current and voltage before and after the connection to the PSU.
</div>


<div>
Hmmm.<br />
<br />
Test it with no hard disk in the machine. &nbsp;Does it get to the BIOS that way? &nbsp;Every time?
</div>


<div>
I have replaced the CMOS Battery with a new one.<br />
<br />
I have taken the hard drive out of the computer totally.<br />
<br />
I have also taken the RAM out of the computer totally.<br />
When RAM is out and computer turned on the red light flashes and there is 5 beeps and nothing on screen. &nbsp;OK it detects no RAM as it should.<br />
<br />
Put RAM in and still no hard drive, nothing on screen light is white, after about 30 seconds fans start ramping up. &nbsp;Have to turn off with long press on power switch.<br />
<br />
The power cable is only a cable. &nbsp;The PSU is built into the Case, &nbsp;There is no separate PSU box.<br />
<br />
The only way to get it going it turn off, drain residual power. &nbsp;Try the a few times and it will start.<br />
<br />
So I can now only think it must be systemboard or onboard graphics card hardware.
</div>


<div>
Have you tested with another screen?<br />
<br />
Remove heatsink and check thermal compound. &nbsp;(Unlikely but won't hurt).<br />
<br />
Test computer with big desktop fan blowing into it onto motherboard (I'm looking for overheating).
</div>


<div>
The information below is taken from Page 121 of Maintenance &amp;&nbsp;Service Guide of your computer.<br />
<br />
<a href="https://filedb.experts-exchange.com/incoming/2016/08_w36/1114383/System-does-not-power-on-and-the-LED.jpg" target="_blank" title="System does not power on and the LEDs on the front of the computer are not flashing (238 KB)"><img alt="System does not power on and the LEDs on the front of the computer are not flashing" class="file-block lazyload" style="max-width: 800px;" data-src="https://filedb.experts-exchange.com/incoming/2016/08_w36/800_1114383/System-does-not-power-on-and-the-LED.jpg" /></a><a href="https://filedb.experts-exchange.com/incoming/2016/08_w36/1114384/HP_8300_Maintenance-Service_Guide_Re.pdf" target="_blank" class="file-inline" title="Maintenance & Service Guide - HP Compaq Elite 8300 Touch All-in-One Business PC HP Compaq Elite 8300 All-in-One Business PC HP Compaq Pro 6300 All-in-One Business PC (15.3 MB)"><svg xmlns="http://www.w3.org/2000/svg" viewBox="0 0 512 512"><path d="M67.508 468.467c-58.005-58.013-58.016-151.92 0-209.943l225.011-225.04c44.643-44.645 117.279-44.645 161.92 0 44.743 44.749 44.753 117.186 0 161.944l-189.465 189.49c-31.41 31.413-82.518 31.412-113.926.001-31.479-31.482-31.49-82.453 0-113.944L311.51 110.491c4.687-4.687 12.286-4.687 16.972 0l16.967 16.971c4.685 4.686 4.685 12.283 0 16.969L184.983 304.917c-12.724 12.724-12.73 33.328 0 46.058 12.696 12.697 33.356 12.699 46.054-.001l189.465-189.489c25.987-25.989 25.994-68.06.001-94.056-25.931-25.934-68.119-25.932-94.049 0l-225.01 225.039c-39.249 39.252-39.258 102.795-.001 142.057 39.285 39.29 102.885 39.287 142.162-.028A739446.174 739446.174 0 0 1 439.497 238.49c4.686-4.687 12.282-4.684 16.969.004l16.967 16.971c4.685 4.686 4.689 12.279.004 16.965a755654.128 755654.128 0 0 0-195.881 195.996c-58.034 58.092-152.004 58.093-210.048.041z"/></svg>HP_8300_Maintenance-Service_Guide_Re.pdf</a>
</div>


System does not power on and the LEDs on the front of the computer are not flashing

System-does-not-power-on-and-the-LED.jpg

Maintenance & Service Guide - HP Compaq Elite 8300 Touch All-in-One Business PC HP Compaq Elite 8300 All-in-One Business PC HP Compaq Pro 6300 All-in-One Business PC

HP_8300_Maintenance-Service_Guide_Re.pdf

<div>
Thanks All for the suggestions.<br />
<br />
Yes re pasted the CPU and no change.<br />
<br />
I have since rung HP support and found the system is 2 months inside a 3 year warranty. &nbsp;So they will be replacing the systemboard on the system for me!<br />
<br />
Happy Days.<br />
<br />
Thanks again for all your support
</div>


<div>
there was something missing - did you test with 1 ram stick (and every one)?
</div>


<div>
Yes tested with different Ram Sticks. &nbsp;Tested both slots and each one individually.
</div>


<div>
As suspected - mainboard problem. Good that machine was still under warranty.
</div>


<div>
It was system board. &nbsp;I chose fist post that suggested system board as best solution and next post to suggest motherboard as assisted
</div>


<div>
<div class="content wysiwyg-content">
I have a clients HP Compaq Elite 8300 AIO computer with a startup issue.<br />
<br />
When we press the button to turn the computer on there is nothing that comes up on the screen, not even the BIOS screen. &nbsp;If I leave the computer on then after about 45 seconds, the system fan starts winding up and getting quite fast. &nbsp;Its as though the system is heating up and the fan is trying to cool it, but I cannot feel it to touch.<br />
<br />
I then turn it off with the power button. &nbsp;<br />
<br />
If I take the power cable out then drain all residual power, then sometimes when I plug the power cable back in and turn on the system will start. &nbsp;When it starts it works OK. &nbsp;<br />
But the next time you go to start it after a shut down it will not start.<br />
<br />
I have upgraded the BIOS to version 3.02 successfully.<br />
<br />
The computer was hardly ever turned off over the 3 years of its life. &nbsp;So I am wondering if the System Board is degrading, or the onboard VGA is becoming faulty.<br />
<br />
Out of every 5 times we attempt to start-up it will start up once maybe twice.<br />
<br />
I have replaced the RAM and the hard drive but that makes no difference.<br />
<br />
Anyone else encountered such an issue with the HP AIO??<br />
<br />
It was also recently upgraded to Windows 10, but as the issue is a startup issue I have ruled that out as it seems to fail before the BIOS screen comes up even.
</div>
</div>


I have a clients HP Compaq Elite 8300 AIO computer with a startup issue.

When we press the button to turn the computer on there is nothing that comes up on the screen, not even the BIOS screen.  If I leave the computer on then after about 45 seconds, the system fan starts winding up and getting quite fast.  Its as though the system is heating up and the fan is trying to cool it, but I cannot feel it to touch.

I then turn it off with the power button.  

If I take the power cable out then drain all residual power, then sometimes when I plug the power cable back in and turn on the system will start.  When it starts it works OK.  
But the next time you go to start it after a shut down it will not start.

I have upgraded the BIOS to version 3.02 successfully.

The computer was hardly ever turned off over the 3 years of its life.  So I am wondering if the System Board is degrading, or the onboard VGA is becoming faulty.

Out of every 5 times we attempt to start-up it will start up once maybe twice.

I have replaced the RAM and the hard drive but that makes no difference.

Anyone else encountered such an issue with the HP AIO??

It was also recently upgraded to Windows 10, but as the issue is a startup issue I have ruled that out as it seems to fail before the BIOS screen comes up even.

HP Compaq Elite 8300 All In One Startup Issue

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