Getting an ATX power supply (24 pin) to run an old MB

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ScubaBart

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I have an old Abit VH6T motherboard I am trying to bench test but a known working power supply will not turn on when connected to the MB. The power supply works in the system I am borrowing it from so I know it functions properly. I have memory, a boot drive and a video card/monitor connected to the motherboard along with the PSU but when I short the two power pins on the MB, nothing happens.

In troubleshooting, I did a quick test on the PSU by shorting pins 15/16 (as found on multiple sites and here) but the PSU does not respond using that method. This PSU has a 24 pin connector and also a 6 pin auxiliary output connector (3.3v x 3 and com x 3 I think) but there is no header for that type of connection on this MB. Is there some other connection required to operate the PSU that has that auxiliary connector?

I'm under the impression that the header pins on the MB identified as the power switch connections should connect to pin 15 (or some other ground) and pin 16 on the PSU but since shorting those pins directly on the PSU does nothing, doing it through the MB is doing nothing as well.

I would expect that even if the MB had issues, that it should function enough to operate the PSU but is seems there is a disconnect in what the MB does and what the PSU expects.

Any thoughts on why this is? Or any other troubleshooting I can perform?

Thanks for any assistance..... Doug
 
Never had a problem with backwards compatibility back in the day when the 20+4 ATX board connector was launched using it without the additional 4 pin plug connected. I wouldn't have thought it would stop it from firing up with shorting the pins out as you say as done that literally 1000's of times testing PSU's & ejecting optical media from slot drives without having to connect up to a complete system. I just Google'd the board to make sure that there is not additional CPU connector which some boards of the age had on them (I remember a Duron system which had a board in it that had an additional 4 pin CPU connector back when most boards never needed more than the basic ATX connection).

Not powering on when shorting the pins out raises my suspicions more than anything TBH as it should still spin up when the pins are shorted out which a paperclip or needle nose pliers.
 
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Yea, that's what gets me too, the PSU goes on instantly when installed in the original system but won't go when shorted. I even tried another PSU form a different system and got the same result. Both of those PSU's are out of old Dell boxes if that makes any difference.

Not sure why I would need to convert to a 22 pin connector. The header on the MB in question is 24 pin and matches fine with the plug on the PSU.

Am thinking I may try to run a wire out of the pin 16 slot so I can ground it while it's plugged into the board. In fact, I may do that test plugged into the known working system first and then on the test MB.

Any other thoughts are still appreciated.
 
ScubaBart said:
Yea, that's what gets me too, the PSU goes on instantly when installed in the original system but won't go when shorted.
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Make sure you're shorting the right pins. Pin 16 is a green wire, pin 15 is a black ground wire, but any ground will work.

ScubaBart said:
Am thinking I may try to run a wire out of the pin 16 slot so I can ground it while it's plugged into the board. In fact, I may do that test plugged into the known working system first and then on the test MB.
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DO NOT do this. If the motherboard is not powering on from a known good supply, then something is wrong with the motherboard. Either there's a problem with the circuitry that deals with the front panel connector, or there's a short somewhere on the board. Forcing power into the board while a fault condition exists will almost certainly lead to something going on fire or exploding.

If the motherboard doesn't power on with everything removed except the CPU (and you know the CPU is good), then the motherboard is dead.

owcraftsman said:
Googled the manual it showed a 22 pin atx
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There was never a 22 pin ATX standard. ATX was 20 pins originally, the four extra pins were added almost a decade later, usually as a detachable brick because some motherboards still use the 20 pin standard. I have no idea where you got the 22 pin thing from.
 
GiGaBiTe said:
Make sure you're shorting the right pins. Pin 16 is a green wire, pin 15 is a black ground wire, but any ground will work.
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By pin count, I am on pin 16 and I tried pin 15 as well as other common grounds. Looking at the connector though the pin 16 wire is not green. In fact there are no green wires. See photos below of both sides of the 24 pin connector and also the 6 pin auxiliary connector. As I indicated, this is out of an older Dell system. Perhaps their PSU's are constructed differently? It certainly doesn't match the color coding presented when doing a search for 24 pin ATX pinout. Guess I will check around for a PSU that has the conventional color coding.




IMG_4358.JPG
IMG_4359.JPG
IMG_4360.JPG
[/QUOTE]
 
Ok, Clearly I have lost the ability to count. My header and PSU connector are 20 pin, not 24.

Still, there is no green wire and shorting pins 13 and 14 or 14 and another ground doesn't spin the PSU.
 
Success!!! Finally found a traditional 20 pin ATX PSU and was able to boot the MB successfully.

I would still like to learn though what the deal is with the Dell PSU that has the non-traditional pin-out if anyone knows.

Otherwise, thanks for all the replies as that is what allowed me to better troubleshoot my situation.
 
ScubaBart said:
I would still like to learn though what the deal is with the Dell PSU that has the non-traditional pin-out if anyone knows.
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Dell was particularly famous for using their own proprietary pinout for many years, but it was common with other PC manufacturers, as well (Apple had 3 different connectors for their PowerMac G4s). Nothing like being incompatible with off-the-shelf parts to lock customers in to overpriced replacements and service contracts, but is often disguised as "convenience" or "features," like the fan signal pin in some models. Fortunately, adapters were easy to make and are still available quite cheaply to make ATX PSUs work in their boards. The converse however is not exactly true, but who wants to use an expensive piece of garbage OEM PSU when better units are available for the same price? The differences are largely wire locations which can generally be rearranged into the right order with some extra wires to compensate for different numbers of connectors.
 
ScubaBart said:
I would still like to learn though what the deal is with the Dell PSU that has the non-traditional pin-out if anyone knows.

Otherwise, thanks for all the replies as that is what allowed me to better troubleshoot my situation.
Click to expand...

Was just about to say that Dell have propitiatory pin outs on most of their PSU's & you are lucky that you didn't blow the motherboard protection diodes! Have seen this happen plenty of times in the past with less knowledgeable staff that thought they could take a non-Dell PSU & install it in to a Dell system & vice versa.
 
OliverQueen said:
Was just about to say that Dell have propitiatory pin outs on most of their PSU's
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This is incorrect, proprietary PSU pinouts are an extreme minority in their computer line. Most of the proprietary units date to the 1998-2003 time frame. I've worked on thousands of Dell machines over the past 25 years and I've only ever come across two machines that used proprietary PSU pinouts, both were Pentium III machines.
 
For 20-pin ATX, #PWR_ON is pin 14 (the move to 24 pins shifted the counting of the pins at 11-20 to 13-22).

That said, the pictures you shared are most certainly NOT a standard ATX PSU. It looks like the wiring Dell was using back in the Pentium 3 days. If so, it's pin 11 that's the PWR_ON signal. The pins you were shorting at 15-16 were +5v and -5v. If you force that PSU to power up while plugged into an ATX motherboard it WILL fry something.


ScubaBart said:
I'm under the impression that the header pins on the MB identified as the power switch connections should connect to pin 15 (or some other ground) and pin 16 on the PSU but since shorting those pins directly on the PSU does nothing, doing it through the MB is doing nothing as well.
Click to expand...

No, the power switch header connects to a power management circuit on the motheboard that's powered by the constant 5v standby from the PSU. Connecting that pin to the PWR_ON pin in the ATX connector at best would do nothing. at worst it could damage the motherboard or PSU.
 
GiGaBiTe said:
This is incorrect, proprietary PSU pinouts are an extreme minority in their computer line. Most of the proprietary units date to the 1998-2003 time frame. I've worked on thousands of Dell machines over the past 25 years and I've only ever come across two machines that used proprietary PSU pinouts, both were Pentium III machines.
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No, you are incorrect (as Grebuloner has also pointed out in the post before mine so it is way more than "2 machines"). I have worked on thousands of Dell machines over the past 30+ years & have seen non-standard PSU pinouts on motherboards ranging from Socket 7 through to S775 boards. I used to have to swap out PSU's a lot for USAF Service Staff based at several USAF bases in England because they had blown their PSU's by plugging them in to a 240V mains circuit (not changing the voltage switch on the rear of them from 115V when they shipped over here to the standard 240V used in the UK - some service staff were lucky & had 115v mains on their housing on base, but the majority didn't). Always had to order in PSU's from DELL directly for them as the standard OEM PSU's would not work with the motherboards or would blow the protection diodes on the motherboards (which meant either replacing the case, psu & motherboard or buying motherboard & psu from Dell again at a much higher cost). Dell also used to use totally different wire colours from the ATX standard also on most of their supplies.
 
OliverQueen said:
No, you are incorrect (as Grebuloner has also pointed out in the post before mine so it is way more than "2 machines"). I have worked on thousands of Dell machines over the past 30+ years & have seen non-standard PSU pinouts on motherboards ranging from Socket 7 through to S775 boards. I used to have to swap out PSU's a lot for USAF Service Staff based at several USAF bases in England because they had blown their PSU's by plugging them in to a 240V mains circuit (not changing the voltage switch on the rear of them from 115V when they shipped over here to the standard 240V used in the UK - some service staff were lucky & had 115v mains on their housing on base, but the majority didn't). Always had to order in PSU's from DELL directly for them as the standard OEM PSU's would not work with the motherboards or would blow the protection diodes on the motherboards (which meant either replacing the case, psu & motherboard or buying motherboard & psu from Dell again at a much higher cost). Dell also used to use totally different wire colours from the ATX standard also on most of their supplies.
Click to expand...

He's not wrong, and what you said aligns with what he said. They stopped using proprietary wiring in a standard ATX 20-pin socket after s370, but continued to use proprietary sizing (especially on smaller form factors). The Optiplex x020 series were the first Dell systems since 2001 with a non-standard PSU wiring, but at least they used a different socket.

So you were ordering parts from Dell for s478 and s775 machines because nothing else would physically fit, not because they were electrically incompatible. Admittedly, I never paid much attention to wire colors since I don't' usually have a need to mess with individual wires.


(For reference, I've got a Dimension 2400 and 8100, Optiplex GX620, 745, 380, and 390, and Inspiron 530 in my spare parts collection that all have standard ATX wiring. )
 
OliverQueen said:
No, you are incorrect (as Grebuloner has also pointed out in the post before mine so it is way more than "2 machines"). I have worked on thousands of Dell machines over the past 30+ years & have seen non-standard PSU pinouts on motherboards ranging from Socket 7 through to S775 boards. I used to have to swap out PSU's a lot for USAF Service Staff based at several USAF bases in England because they had blown their PSU's by plugging them in to a 240V mains circuit (not changing the voltage switch on the rear of them from 115V when they shipped over here to the standard 240V used in the UK - some service staff were lucky & had 115v mains on their housing on base, but the majority didn't). Always had to order in PSU's from DELL directly for them as the standard OEM PSU's would not work with the motherboards or would blow the protection diodes on the motherboards (which meant either replacing the case, psu & motherboard or buying motherboard & psu from Dell again at a much higher cost). Dell also used to use totally different wire colours from the ATX standard also on most of their supplies.
Click to expand...

As Ryan said, you have a different problem of 120v vs 240v mains, and proprietary PSU sizing. This is not a ATX wiring problem.

I've worked on Dell 386 through Pentium II systems and I never saw one with proprietary AT/ATX pinouts, it was mostly limited in the time frame I specified.
 
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