Strange PSU Problem

  Mark10 13:00 23 Oct 2007

Hello All.
I have two PC's, a Pentium 4 and a Pentium 3. I bought a new PSU for the P4 with intentions to put the old one from the P4 into the P3. However, the new PSU wont work in the P4 but will work in the P3. When its in the P4, the red LED on the mobo lights up and when the power button is pressed the fans turn about an eighth of a turn and then nothing. The LED remains lit. I have checked all the connections multiple times especially the 4 pin P4 connector as that semms the only difference between them. The PSU is currently running quite happily in the P3 so all is well. The P4 is running fine with its old PSU. I was just wondering if anyone has any ideas as it seems rather strange??
Thanks in advance.

  Technotiger 13:08 23 Oct 2007

You have not mentioned what the wattage is of either PSU, nor anything about the two pc's apart from P3 and P4 - what other peripherals are connected to each pc? Drives/printers/externals etc, the more hardware the more power need from PSU's. Also what OS?

  Mark10 13:17 23 Oct 2007

Hi technotiger, Yes sorry about that. Both the PC's had 300w PSU's and the new one is a 350w. They both run XP pro SP2 and the P4 has a printer, graphics card and LAN card while the P3 has a wireless card and graphics card. Both have run OK with 300w supplies so I expected no problem with a 350w. As I said, both are running ok now, I was just curious :)

  jack 13:32 23 Oct 2007

These are both older machines.
In the recent past the power block on PSU's has changed to accommodate the new CPU that require
separate powering from a 4 pin 12v socket additional to the standard 20 pin block.
The power is the same just the connectivity- adaptors are available ans some PSU's come with all afdaptors- some dont
So check out that all that you need to be connected - is.

  keef66 13:43 23 Oct 2007

as jack says, newer psu's come with a 20 + 4 main power connector (often with the extra 4 pin connector attached to the 20 pin one), and a separate 4 pin P4 connector. I don't think the two 4 pin connectors are the same shape, but it might be possible to confuse them?

My psu only has the P4 connector (which I don't need)so I can't compare.

  Technotiger 14:00 23 Oct 2007

Ok, I trust your curiosity is now satisfied :-)
Just a point though, 350w is still rather small by todays' standards. Next time you buy a PSU make sure it is at least 450w or better, this will allow for further future expansion e.g. you might at some time want to add external harddrive(s), maybe for backup purposes - hint, hint!

  keef66 14:23 23 Oct 2007

click here

P4 connector has 2 black + 2 yellow wires, the 4 pin from the 20 + 4 arrangement has red, yellow, orange and black wires

  crosstrainer 14:31 23 Oct 2007

Checking that you have the power switch, hdd, re-set, and led connections correct, they can be a pain to get right. The fan starting one turn can be a symptom of a wrong connection on these fiddly connectors.

  woodchip 16:38 23 Oct 2007

Something else for you to think about, Not all PSU's are made equal. The old PSU's although old may be better built PSU's better components> steer clear of too cheap PSU's it may cost more than its worth if it blows

  Fruit Bat /\0/\ 17:05 23 Oct 2007

If renewing a PSU check:

1. The physical size of your PSU, some are hard to replace due to being a non standard size.
2. The amount of power need from the PSU don't skimp

1. Physical Dimensions

Besides the specs and form factors, the physical dimensions are also important factors in selecting a compatible power supply. Here is an outline of the physical dimensions of most standard power supplies:

# ATX: 6x3.5x5.5", HxWxD. Most common. Uses 4 mounting screws.
# Mini-ATX: 5x3.5x5", HxWxD. Rare size. Uses 4 mounting screws. Can be used in a regular ATX case, but often not the other way around.
# MicroATX: 5x3x4", HxWxD. Use 3 mounting screws. Not interchangeable with ATX or miniATX.
# Flex ATX: Even smaller than Micro ATX. Various sizes according to case specs; often not interchangeable.

Use the data above to determine if a particular power supply would fit your case.

The quality of a power supply can be estimated by its weight. While this is not a true scientific or thorough measurement of the power supply reliability, it is nevertheless a very simple and easy way for ordinary PC users to estimate and compare the quality of a power supply.

2. Power supply calculator click here

Guide to changing PSU
click here
click here

  Mark10 11:33 25 Oct 2007

Thanks everyone. I will check everything again when I get chance. The new PSU came with a 20/24 pin connector where the 4 pin bit for the P4 slides into place beside the 20 pin connector so you can have a 20 or 24 pin arrangement. My mobo has the 4 pin connection seperate from the 20 pin, so I will double re-check and make sure I am getting this right.
Thanks all again.

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