Normally I would assume a power supply issue. I replaced the power supply less than six months back. This is the older machine I replaced with a new one.

I want to repair it but don't know just where to start.

Any feed back?

Billy

Yeah, I'd normally assume a power supply issue as well - try another one if you have one kicking around or can borrow one. PSU's can fry easily if you've got "unclean" power - a good power bar with filtering can help that. Could be defective/under warranty maybe. Got a multimeter you can use to check voltages?

Try disconnecting all peripherals from power and only have the motherboard connected, see if that gets it to go. Try resetting the bios - could be a jumper to do that, could be just removing the battery on the motherboard. Remove and reinstall ram sticks one at a time. Unplug and replug the fans.

Any beeps when you try to start it? Any blinky light codes - maybe an LED on the motherboard or the power switch?

A multi-tester would be a challenge without a manual showing what's on all those pins (up to 40). I purchased a very handy power supply tester a few years aback for maybe $20. It was in Circuit City, now defunct, but they must still be available. Perhaps Tiger Direct.

Do you hear any beeps? For example, if there is no RAM seated correctly, that's what happens, followed by a shutdown. And inside the case, the motherboard may have a single character LED display with a code.

This is the same computer that started shutting down intermittently a while back and I put a new power supply in the machine.
and a new heat sink and new fan.

This is also what prompted me to build the new one.

Turn on the switch and fan lights come on and turn off just as fast. I did not take it apart yet.

I also have a old dell that is doing the same thing, All have been running on a UPS.

I am not really looking forward to rebuilding computers...I am retired...lol

I need to find a real job so I don't have to work so much...lol

Billy

Hi, don’t know if it will help but I’d start by pulling out the C Drive and putting into another computer to test it. (Ensure the C Drive is replaced in the test machine so that it is booting from the drive).

For data safety I’d also disconnect all other drives in the test machine to avoid transferring any nasty viruses etc that might exist.

This will eliminate or confirm the operating system if nothing else. The issue could also be the mother board. The test above should help eliminate or confirm such an issue.

Have fun

My thoughts
Tony.

Switch Mode power supplies will automatically shut down if any fault is detected, or if they cannot produce the correct output voltages (several 5v and 12v rails). Definitely substitution would be the fastest way to determine if the P/S is the problem.

Thanks guys. I opened the computer and started it from the motherboard. Now it is working normally.

I backed up a few things just in case. I have shut it down several times and restarted and everything is normal.

It's a computer...who knows!

The Dell in the garage I will take a look at some other day. It was working fine until I took it down stares so bumping it around may have cause something to disconnect.

I have back up computers for everything so none of this is anything more than an irritation.

Thanks again,

Billy

My favorite saying related to fault-finding: "Problems that go away by themselves, come back by themselves"

Lets hope when It comes back I will have gone away...lol

Billy

There are a few clues here that remind me of a problem I had when the power switch on the case stayed depressed.

Intermittent RAM issue.

Billy

Did you replace or just re-seat?

When I removed the ram chip the computer would start. I installed new ram and it would not start. I had to replace the motherboard.

Sometimes things just fail. Motherboards are pretty reliable but I find I have to replace them every five or six years. Sometimes because they fail and sometimes because I want to upgrade.

I posted this in the wrong thread. This is the Dell Optiplex 990. The other computer that would not start is working without issue for the moment, It is pretty old and I have built a new machine with a i9 intel.

I really don't notice much usable difference from the old i7 intel. It starts faster and some things run a little faster. There were no noticeable changes with BIAB. The cost of the i9 processor was only about $100 more than a new i7.

The Dell is important because it runs my CNC machine. The other old computer is just a backup that I most likely will not make major repairs or upgrades to.

I have a pretty new HP Lap Top that I have never been happy with. I am going to install a SSD and reinstall windows. I have been using it temporally to run the CNC machine but Lap Tops have to many power management issues to run CNC.

Billy

Thanks for the long, detailed answer Billy, but back to the "Intermittent RAM" question:

I couldn't work out the real answer (but I tried ).

The cause of the intermittent ram Issue was a defective slot on the motherboard. Put known good ram chip in that slot and the computer will not start.

I did not try to repair the motherboard. I replaced it.

Billy

Excellent, all clear now. Seems to have been a motherboard issue.

Let's hope it behaves itself from now on.

I’m puzzled that this motherboard did not issue diagnostic beeps if the RAM was not seated correctly.

Me also Matt. But it did not say a lonesome word...lol I assume there was a broken pin in the slot or a cold solder joint somewhere. Not worth my time to chase trace and try to fic something I need a microscope to see.

To tell the truth I have never had issues with any computer I have built within the first five years of use.

I have had any number of issues with name brand computers such as Dell and HP. Mainly due to all the bloat ware included in the OS. Also their frequent use of the cheapest components they can find and sometimes some proprietary thing that is hard to get around.

The Dell Optiplex 990 is a pretty old computer in computer terms.

Billy