Question No Bios/ PC wont Post

Nov 29, 2020
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I am using a Prebuild PC from Dell; XPS435MT
Components are;

CPU:i7 920
GPU:Radeon HD 7850
HDD: Seagate 1 TB
SSD:Samsung 860 Evo
PSU: Original 350w/new EVGA 500B 80+
RAM: 6GB ddr3 1066

Hello guys, my PC wont post since i got a new psu for it. My old psu had a -5v in 24 pin and in the new one it was blank. I cannot open my pc since i got the new psu.
No beeps (there is a integrated speaker)even though no ram is installed.
So i turn the pc on with the new PSU, all if the fans spin up, but the monitor stays blank. It says no video detected or something similar to that.

Things i already tried;
*took of the gpu
*tried 1 ram at a time
*took out all rams
*Cleared cmos several times
  • checked all wiring and connections.
  • tried with an another Monitor
  • took out the cpu cleaned the pins and put it back
  • took out the motherboard from the case
  • put the old psu in( now it wont post with the old psu
I am out of ideas. Please help.
 
Last edited:

Karadjgne

Titan
Ambassador
Older 20pin psus sometimes included a -5v wire from the psu (it was optional) . Newer 20+4 psus almost universally do not. That rail was dropped as useless since nothing uses it anymore.

Older style psus also used higher 3.3v and 5v usage, newer psus have cut back on those rails, concentrating on the 12v rail, since the vast majority of use in a pc is now 12v. About the only things left for 3.3v is the motherboard logic circuitry and for 5v it's the usb ports and drives. None of which requires 30+Amps realistically.

The only potential problem with using a new PSU with an old computer is today's PSUs usually do not provide minus 5V output (they either don't have the white wire going to pin 20 at all, or they leave it not connected inside). Even some of ATX-I PSUs may not have "-5V", which was made optional in ATX12V version 1.2 spec as of 2002. The minus 5V rail was used by ISA cards. It was phased out when ISA were replaced by PCI. If you have an old computer with ISA slots, it will still need this buss. In this case, if you can't find a power supply with such a rail, you can derive it from -12V by adding a negative linear regulator such as MC79M05. Since minus 5V buss can draw up to 0.3A current according to ATX spec v.1, this regulator will dissipate up to (12-5)װ.3=2.1W. This may be too much for a stand alone device, so may you need to attach it to a little heatsink.
https://www.smps.us/mobile/20-to-24pin-atx.html

Something like this would also work, you'd just have to fine tune it with a multimeter for -5v, but uses a +12v input. The output wire will be that white wire stripped from the old psu.
Converter
 
Last edited:
Nov 29, 2020
9
0
10
0
Older 20pin psus sometimes included a -5v wire from the psu (it was optional) . Newer 20+4 psus almost universally do not. That rail was dropped as useless since nothing uses it anymore.

Older style psus also used higher 3.3v and 5v usage, newer psus have cut back on those rails, concentrating on the 12v rail, since the vast majority of use in a pc is now 12v. About the only things left for 3.3v is the motherboard logic circuitry and for 5v it's the usb ports and drives. None of which requires 30+Amps realistically.


https://www.smps.us/mobile/20-to-24pin-atx.html

Something like this would also work, you'd just have to fine tune it with a multimeter for -5v, but uses a +12v input. The output wire will be that white wire stripped from the old psu.
Converter
the old psu had a 20+4 pin connector but with a -5v rail and the new one doesnt have it. How am i going to fine tune the voltage?
 

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