Got bit by a failing PSU that looked fine on the surface

Had a weird one at the shop last week. A customer brought in a gaming rig that would crash randomly under load, but boot up fine and pass basic tests. I checked temps, swapped RAM, even ran a disk check. Everything looked okay until I hooked it up to my spare PSU tester and saw the 12V rail was dropping to 11.4V during a stress test. The power supply was one of those modular units with no visible bulging caps or burnt smell. Swapped in a known good unit and the system ran stable for three hours straight. I still keep a spare PSU tester on hand but this one nearly tricked me into chasing ghosts. Anybody else run into a power supply that hid its failure like that?

3 comments

3 Comments

margaretm231mo ago

Actually, those cheap PSU testers can be a bit misleading sometimes. They only check a static load, not how the PSU handles real world spikes under heavy gaming. I've had units pass a tester but still cause crashes because the transient response was shot. Your 11.4V reading is a solid clue though, that's definitely a failing unit. But just a heads up, if you ever see it dip below 11.0V or spike above 12.6V, that's when you really know it's dead. Happy you caught it, those ghost hunting sessions are the worst when you're trying to get a build out the door.

lily_singh21mo ago

Oh man, the 3.3V rail is the one nobody talks about in these threads. I had a Seasonic unit once where the 12V looked perfect on a tester but the 3.3V was sitting at 3.1V under a light load. Caused random USB dropouts and SATA drives not being detected (like, the worst kind of intermittent problem). Most people only watch the 12V rail since that's what the GPU and CPU use, but the motherboard, RAM, and SSD controllers are super sensitive to that 3.3V line. When that rail starts sagging, you get the weirdest ghost issues that don't show up in a standard crash log. Might be worth checking if margaretm23's problem child PSU had a wonky 3.3V that the tester missed entirely.

the_james5d ago

Yeah @margaretm23 the whole ghost hunting thing reminds me of this one build I did where the PC would only crash when the AC kicked on in the room, turns out the PSU was picking up line noise from the compressor.