Optiplex 780 not booting up until pressing back psu test button

Question

Hello,

My dell Optiplex is not booting unless i press back psu button, without pressing PSU back button it's just blinks diagnostic light 3 . Sometimes it also show solid amber light when turning it on by pressing back PSU button.

Waiting for response ASAP.

Note:I know it is 12 years old PC so please only reply if you have solution.

redxps630 · Accepted Answer

If you save the bios setting changes you made and keep power cord plugged in, you should be able to see the changes stay after restart. Are you saying your changes do not stay after restart (and do you keep power plugged in during this)? If you have only one hdd in it, it really doesn’t matter what sata mode you have (AHCI vs IDE), I think. As long as OS on hdd can boot, it makes no difference.
If you had soldered a new bios of board, the motherboard might be altered (my guess). I would not change motherboard, definitely not in a DT. If you can still use it just get the most of its remaining life.

IGME GAMER · Answer

Edit: My PSU fan is also not working. Thank you for replying redxps630. Does it mean i have to replace my PSU?

redxps630 · Answer

explanation: 'for some reason the computer is not automatically connecting the green sense wire on the power supply to ground, so the power supply does not start up. Pressing the BIST button shorts the green wire to ground, and then everything works just fine.'

redxps630 · Answer

Do you have 780 MT? If yes I suggest you try a different standard ATX psu if available just to confirm the motherboard works normally before you order a replacement psu. 780 MT uses standard 24 pin ATX psu, whereas DT and SFF use mini sata 24 pin.

It is unclear where in the circuit a ground is missing. If psu fan is not spinning when the system is powered on, it is certainly abnormal.

IGME GAMER · Answer

Hi,

Thanks for being responsive,

redxps630,

It's not MT it is DT. I also want to tell you that when the diagnostic light three blinks the internal amber light in mobo also seems to blink and whenever i remove my CMOS battery and keep CMOS battery out of the mobo it boots normally (I also changed the CMOS battery but it doesn't work).

IGME GAMER · Answer

Hi Redxps630,

Yes you're right but If I continue like this, at every startup i will get windows bsod(Blue Screen of Death) because my present SATA controller is AHCI controller but at every Bios reset it returns to Legacy controller. Isn't there any way i can fix it without replacing mobo.

One more thing : I have replaced bios chip recently due to BIOS corruption. Can this be cause of such behaviour?

Thanks again.

redxps630 · Answer

That is interesting. Do you mean when system has no CMOs battery, you do not need to press BIST button any more, system boots and works normally by just pressing power button?  Well if that is true you have found a work around.  If you keep power cord plugged in, you do not really need CMOs battery to keep CMOs setting saved.

IGME GAMER · Answer

Thank you so much for your assistance!

IGME GAMER · Answer

My bios changes are saved even after restarting the PC but when I completely disconnect the power, it loses its settings. Still, you're correct, I will make full use of the current system.

Thanks again.

redxps630 · Answer

Re: My bios changes are saved even after restarting the PC but when I completely disconnect the power, it loses its settings.

that makes sense. Just keep it plugged in now without CMOs battery so you can power on without pressing BIST.

IGME GAMER · Answer

Thank you so much for replying to all my queries.

My PC is fixed now. I just disable fast boot in my Bios and voila! Everything works fine. PC is now booting with CMOS battery and without having to press BIST button.

Thanks again for your help.

Optiplex Desktops

Optiplex 780 not booting up until pressing back psu test button

Was this post helpful?