Odd Power Supply/MB situation

[bigcuedaddy]

Long story, please bear with me

I was at a customer home to install additional RAM in their fully-operational PC. Obviously, pretty basic stuff. I powered down the PC, put in the RAM, and reconnected everything and tried to power back up. No powerup. Odd, so I made the obvious double-checks, even triple checks of connections, etc. No go. I prefer not to have PC components and test equipment scattered across a client's floor, so I said I would take it back to my bench and see what is going on.

I run voltage tests on the PSU, and get nothing back on any of the PSU pins on the multimeter. So I install a new PSU, and the unit fires up and all is well. Strange, but not impossible. I check to make sure the new RAM is recognized by Windows, and it is, so I reboot again to make sure the PSU and everything else is working properly, and all is good.

I take the unit back to the client, and power it up. No power! At this point, I am thinking there are gremlins in the client's UPS unit, so we run voltage tests through the multimeter, and all the UPS outlets check good at 120V. We check the power cable, checks good. We plug the PC into a totally different outlet, away from UPS, will not power up. Also, the client's router, modem, monitor and printer are all plugged into the UPS unit and are suffering no ill effects.

So as a last resort, and to satisfy my curiosity, we swap out PSU's from his Compaq in the back room (the one in question is an HP). Both are 24 pin, ATX12V, +P4 connector, should be compatible. No go. So I put the Compaq's PSU back in the Compaq, and now IT wont power up. Bizarre, at least to me.

I take both units back to the bench, and run more voltage tests, this time using a power supply tester. Although I realize they arent highly accurate, they at least will tell me if something is generally wrong. The original PSU that was in the client's computer to begin with now checks good. The new PSU I put in the client's computer checks good. The Compaq PSU we swapped out checks bad.

If it werent for the Compaq PSU going bad, I would be looking at checking the power switch to make sure it is working. However, with the system seemingly blowing the Compaq PSU, I have doubts the switch would be the culprit. At this point, in my mind it points to a bad MB, or at least something not connecting properly or shorted. Although probably not impossible, I have never come upon a situation where a motherboard would blow out a PSU.

I have discussed this nightmare with local tech friends, they have no suggestions. I have a hunch I am making an assumption or two in my troubleshooting that is not valid. My next semi-serious step is to bring in a priest and have the whole thing exorcised .

Any sugestiions? What am I doing wrong?

 

[Styxbound]

My impression is that you're having a problem I've been wrestling with a while now, but still haven't been able to really get a handle on. The typical scenario is try a power supply, it's no-go, put it in a machine which has been working and it fires right up, take it back to the first machine and now it works. There are numerous variations on this theme; the constant is that none of them are predictable and dependable. I'm thinking it has something to do with the power supply getting "turned on" in some way_or off_ but I just haven't been able to nail anything down. Maddening!

 

[bigcuedaddy]

Yeah, but now NOTHING is powering up in either machine. 2 of the three PSU's test good, but these two (previously working) machines just wont power up. At first it was humiliating, thinking that if I cant handle installing RAM I just as well quit and become a mime. Now it's just bizarre. I really don't want to eat $150+ for a new MB, obviously.

 

[B Trevathan]

Have you tried taking the RAM back out and drain the PSUs flea power. Its amazing that sometimes the slightest bump or jar can cause a fully charged PSU to need to be reset.

Did you check the motherboard for any physical damage like bad caps, check close around the RAM slots and the power connectors.

Was there any dust in the RAM slot or anywhere else?

Did you wear a ESD strap?

 

[bigcuedaddy]

I reseated all the RAM modules. I have checked the area for damage, but will check again, good call. I will drain the fp as well, good call there. I didnt specifically check for dust in the memory slot, but will do so. ESD strap on when RAM was installed, but not when we were troubleshooting later on in the client home (I didnt come prepared to fix any pc's, I was originally just going to deliver the fixed one . I thought about ESD afterwards. That would be just my luck. But, all good suggestions. thanks!

 

[B Trevathan]

I would remove the RAM that was added not just reseat it and put the original PSU back in the first computer and drain the flea power. The RAM could have an intermittent short or dust in the slot could be causing a short. Any kind of short could cause the PSUs protections (SCP, OPP, etc.) to be tripped. Not meaning to put you down but you know it was working before you added the RAM and all three PSUs quit working after being inside the computer with the added RAM.

I always carry at least a small set of tools which includes a ESD strap, its not always necessary to wear one but its a good idea and it looks good in front of the customer. You never really know if a ESD has happen because you probably won't feel it, it takes a lot of voltage to actually see or feel a ESD shock but not much voltage to damage an IC chip and ESD damage can cause intermittent problems the computer could work when you return it but not work the next day, so I wear one more often than not.

 

[Styxbound]

The only other thing I can think of not mentioned is clearing the bios. And as you probably know not all machines can be properly cleared by just taking out the battery or setting a jumper; there are a few you have to set a jumper, turn the machine on then off again_specifically what you're not supposed to do with most of them. You just have to check the documentation.

Just for the hell of it you might try taking the "dead" psu, hooking it up to a couple of hard drives, and try turning it on by jumping a ground to green wire on the board connector. Out of the machine of course.

 

[bigcuedaddy]

Well I just bit the bullet and ordered a replacement MB. It was only $50, so I wont lose my shirt over the whole thing. But it sure is ego-battering and frustrating to not be able to figure out something like this, and to think it started out by installing 2 sticks of RAM. That hurts.

 

[fixitdaz]

Have you checked the plug is wired correct if the polarity is reversed it could blow internal fuse in the psu although it was working when you arrived I would get a working old psu and plug it in to a wall outlet first if it works then put it into the ups if it fails its the ups after all its an inverter ups????
Could be correct volts incorrect hz
Check nothing is floating around behind the m/b shorting it out

 

[bigcuedaddy]

BTW, it wound up being the CPU. Found a troubleshooting tip on another forum from someone with exact same issue on exact same PSU. Thanks for all your suggestions!

 

[atlanticjim]

Wow, I just started reading this thread and I was sure that it was going to be a flukey MB problem. How did you finally diagnose the CPU problem?