PSU issue - Page 1
Forum section: electronics / repair

author: foggy
author_rank: Newbie
timestamp: May 09, 2021, 05:52:30 pm
content: Hi!Quite new to this so please forgive me if I'm not wording things correctly etc!I have a computer power supply. It's not ATX but reasonably similar I think. It's from a small IBM AS/400 model 150 computer. The PSU model is IBM Part# 40H7561 made be Delta Electronics and carries a Delta Part# DPS-250CB A. If I google the DPS part number I get images that don't really look like this PSU!I have two of these computers. One works perfectly, the other does not. I've changed components around between the two computers and the problem definitely follows the PSU.There's a small LCD display panel and associated circuitry that should come on as soon as mains electricity is provided. I'm pretty sure this is powered by the +5V line on the PSU. Once that powers up, it is then possible to switch on the computer properly.The problem is that the panel isn't properly powering up. The LCD is faint and none of the panel buttons do anything. Sometimes, I can cajole the computer into switching on. When it does, it all works fine except for the front panel. It remains dark. So this is my thinking as to how I've isolated the problem to the 5V line.I do however see 5V on the aux line on the PSU. If I short the Power on to GND, the PSU fires up properly and I see correct voltages on the other lines (12V, 5V, -5V etc).There's two capacitors near where the 5V line connects. I've removed and tested them and they show good uF readings as far as I can tell.As I say, if I switch the PSU out for another one, it all works just fine.Is it possible that the 5V line isn't providing enough power? What can I do to check and fix this?cheers!fog

author: BrokenYugo
author_rank: Super Contributor
timestamp: May 10, 2021, 02:28:38 am
content: Sounds to me like there are probably two 5V supplies, a small standby one (where the problem is), and the big supply that runs the computer. I'd trace that aux line back and see where it goes, at least loosely reverse engineer things. Are you checking this aux line for 5V with it connected to the computer? Have you checked it for ripple?As always, pictures always help, both sides of the board, a couple angles on the component side.

author: foggy
author_rank: Newbie
timestamp: May 10, 2021, 04:50:36 pm
content: Hi, thanks for the response.Yes correct there are two 5V's. There's 12V, -12V, -5V, 3.3V also.The 5V in question is indeed the aux and should always be on (so long as there's mains power, obviously). The rest of the lines come on when the PSU is 'switched on', and by that I mean I ground the appropriate pin on one of the connectors.A few photos. The 5V line in question is to the bottom right of the 2nd stage board. I've highlighted with a red arrow in one of the pics. Those two capacitors just above the red wires are the ones I've pulled and tested (for capacitance).On the suspect PSU, I see 5V on the aux line when it's unconnected but it drops to 3.5V when it's connected to the load. On the known good PSU, I see 5V on the same line when unconnected and also 5V when connected to load.With respect to ripple, are you asking if I see any AC on that 5V line? If so, then "possibly". On the known good PSU, I see flat 0VAC on the aux but unreliable results on the suspect PSU. It sort of flips around a bit then settles on 0VAC.

author: foggy
author_rank: Newbie
timestamp: May 10, 2021, 04:51:30 pm
content: full thing

author: foggy
author_rank: Newbie
timestamp: May 10, 2021, 04:52:07 pm
content: back of board

author: foggy
author_rank: Newbie
timestamp: May 10, 2021, 11:10:15 pm
content: Problem isolated to that capacitor to the right of the smaller transformer.It was reading 5Mwhile a known good one was up around 8M.Switched them around and the previously duff PSU worked!

author: wn1fju
author_rank: Frequent Contributor
timestamp: May 11, 2021, 12:26:28 pm
content: While your PSU works now, I wouldn't be surprised if you had problems in the future. Switching power supplies are notorious for leaking/faulty electrolytic capacitors. Your picture shows the PSU is pretty dirty and all it takes is some leaking electrolyte on the board to spoil your day. If I were you, I would give serious consideration to pulling all of the electrolytics, scrubbing the board several times with alcohol and a toothbrush, then replacing the electrolytics with high-quality, brand-named, low ESR, 105 degree-rated capacitors.