SFX ATX PSU - Page 1
Forum section: electronics / repair

author: Grayfox
author_rank: Regular Contributor
timestamp: August 12, 2020, 02:27:24 pm
content: I am restoring an old PC that has an SFX PSU in it that is from the 90sI have removed all the electrolytic caps, there are 2 green caps still on the board.I am having trouble finding the datasheets for 2 lots of capacitors which would be long discontinued by now.One is an Rubycon MH 10uF 50v, I can find Rubycon MH5 and MH7 but these caps don't fit into what the datasheet states.The MH5s are 5mm tall, MH7 are 7mm tall.The caps I have are 12mm tall and the datasheet also doesn't list the voltage and capacitance.The other capacitors are 1200uF 16 Panasonic HFA.While there caps don't say "Panasonic" on them they have an M symbol that have curved legs and this seems to match up with what I have seen on google for panasonic capacitor logo.When getting replacement caps, I know the capacitance value has to be the same as some of these older PSUs used capacitance to regulate voltage, the temp can be equal or better but most I am finding are 105°CHow much will the ripple current effect things?I cant find the exact ripple current values some some being less than 25mA out, but the 2 input filter capacitors are 2x higher.The ESR is also a bit tricky to get exactly right as well.I have attached the proposed replacement capacitors for each removed capacitor.Thanks

author: TheMG
author_rank: Frequent Contributor
timestamp: August 12, 2020, 04:47:49 pm
content: I wouldn't worry too much about matching the ESR spec. Any replacement capacitor from a low-esr series will work just fine, it's not super critical just has to be relatively low (which most capacitors these days are, since most applications use switching circuitry).Tolerances on aluminum electrolytic caps are generally pretty wide to start off with and the power supply design has to account for this, as well as allow proper operation with some level of degradation as usually happens over time.To pull up datasheets of the original caps and try to match the specs, IMHO, is generally a waste of time.

author: David Hess
author_rank: Super Contributor
timestamp: August 12, 2020, 07:34:10 pm
content: One of the things I do when replacing capacitors in ATX power supplies is to match them for lead spacing and diameter, and then take advantage of any available height to use higher voltage, and sometimes higher capacitance, parts for higher ripple current ratings.

author: Grayfox
author_rank: Regular Contributor
timestamp: August 23, 2020, 11:43:28 pm
content: Finished the repair.2 Issues I now have to fix.It instantly powers up when it should power on when the PSU_ON(greenwire) is pulled down to GroundThere is no continuity between the green wire and the case of the PSU+12v(Yellow Wire) is supplying 9v.Tracing back to see where the +12v leads to to see what could cause the issue.+3.3v(Orange Wire), +5v(Red Wire), +5vSB(Purple) are all giving out correct voltages under 0 load.

author: TheMG
author_rank: Frequent Contributor
timestamp: August 24, 2020, 12:52:21 am
content: For the 12V sitting at 9V... did you try putting some load on the 3.3V and 5V? A lot of older power supplies needed some minimum load on 3.3V and 5V otherwise the 12V would be out of spec.

author: Grayfox
author_rank: Regular Contributor
timestamp: August 24, 2020, 01:24:11 am
content: Quote from: TheMG on August 24, 2020, 12:52:21 amNo.Since the PSU turns on without it being installed into the PC(which it shouldnt do), I did not want to even attempt even installing it.As for testing under load, I do have 4 50w 7.5Ohm load resistor I can use.