Won't turn off!

I work for a non-profit refurbishing computers for the poor. My latest project is an Emachines 550. Among other things, the power supply and processor fans won't turn off unless the unit is unplugged. Additionally, when the unit is powered down and plugged in, the light on the optical mouse flashes.

I postulated that there might be a problem with the power switch, so I disconnected it from the mobo and plugged the unit in. Same problems.

The power supply is a Ilssan ISP 120s and I can't figure out what the mobo is. Whatever they put in the 550.

Any ideas what could be causing this problem?

 

I have no idea what the original manufacturer used as a power supply in your unit but I would guess that the previous owner tryed replacing it with a cheap one he bought online. Since the older OEM machines tended to use proprietary supplies that were wired differently than the stuff you can buy online (presumably to force you to buy from them), Changing a power supply yourself required some skill in switching wires around to mimic the original supply. And since these OEM units often failed, power supply first (they used cheap $15 supplies and charged anywhere from $80-$125 for a replacement), its very common to find older machines with a replaced supply that they could never get to work. Without a schematic color coded wiring diagram or the original supply to see where the colored wires went, its a tough task to get it right. You might get lucky and find a pin placement diagram for the motherboard but you will first have to identify the mobo by what was listed as being used in that model. Sometimes if you examing the motherboard closely you can find a hint as to who made it. Try looking on the sides of the pci or isa slots and in between them for markings. Also you might have some luck using an application like aida32 or everest home from a boot disk to id the mobo.

There are several types of power sources, some of which are on all the time, like to the mouse so you can use the mouses movement to start the pc or the keyboard for that matter. Thats what makes me think that the replacement supply is putting the power in the wrong places, so that the on all the time power is going to the fans and and the power getting to the optical mouse is not correct either

 

Hmm, the motherboard is different in that it has no 4 pin connector for the CPU, just the 20 pin (and the connectors for HDD, CD drives and floppy disk). When I called emachines customer support, they confirmed that the computer shipped with the Ilssan power supply (or at least a power supply of the same model as the one that is currently installed).

 

In that case I would probably remove the motherboard from the case and see if anything (screw) falls out from behind. You don't always have to remove the board to check this since typically a screw will get lodged behind the board from the bottom of the case where it would fall to. Then when someone lays it on its side it can travel up under the mobo so sometimes tugging gently on the bottom of the board in an area as far away from mounting screws as you can get, will cause it to fall. A screw short behind the board can cause anything from nothing to complete burnout depending on what it touches. Putting a screw where one doesn't belong can also cause problems, but it assumes the previous owner fooled around.

 

Thanks for your help Appzalien. Incidentally, I unplugged the unit before grounding myself and killed the entire unit.

Lesson learned.