Recaping an old switching power brick? - Page 1
Forum section: electronics / repair

author: e-pirate
author_rank: Regular Contributor
timestamp: February 28, 2021, 03:41:11 pm
content: Hi folks!A couple of days ago I have a power outage and after some 3 hours of downtime, my old and trusty home server refused to power on. This is a fanless machine with an external power "brick". I quickly sorted out that the PSU is in response. Lucky I'm, this one is not welded (as they like to do) and I was able to take it apart. Since everything was perfect until the main power went off, the electrolytic capacitors were my first suspect. Since there are only 5 of them including one big 120 uF 400 V I didn't bothered to desolder and measure them and purchased all. But got curious about condition of the old and got my BK Precision RLC meter out of the box. It came out, that all except the big HV cap have ESR at least to times higher, then new caps, so they were immediately replaced with new nichi. But the old big 120 uF 400V actually has LOWER ESR and higher capacitance value comparing with the new one. So, I scratched my head a little bit and assembled the PSU with all caps replaced excluding the big one. And PSU switched on measuring 12.1 V unloaded which is perfect. It looks like there is no reason to replace the biggie or I'm missing something? Any ideas?

author: TheMG
author_rank: Frequent Contributor
timestamp: February 28, 2021, 04:20:38 pm
content: The primary filter caps seldom go bad, it is not subjected to the same kind of high ripple currents and frequencies as the secondary filter caps.Lower ESR is typiccal for higher voltage caps. Look at a chart of typical ESR values and you'll find ESR decreases with both higher capacitance and higher voltage rating.I'll also add this: don't overlook any smaller electrolytics, especially those around the main switching IC on the primary side (typically around 10 to 100uF). Those frequently go bad too and are a common cause of a power supply failing to start up.

author: e-pirate
author_rank: Regular Contributor
timestamp: February 28, 2021, 05:18:17 pm
content: Quote from: TheMG on February 28, 2021, 04:20:38 pmThis was the case, actually. I missed the little sucker as it was covered by the surrounding shield, I replaced all caps, but the PSU didn't switched on. So I started looking and found 68 uF x 35V cap sitting next to the transformer. Since I didn't bought a new one, I got a replacement from used caps. Unfortunately, I had no 68 uF, so I replaced it with a 100 uF 50 V nichi. The PSU started, but I had to check the datasheet for UC3843A if I can go with the higher value cap. The reference design actually suggests 120 uF in that particular place.

author: TheMG
author_rank: Frequent Contributor
timestamp: February 28, 2021, 06:25:42 pm
content: Quote from: e-pirate on February 28, 2021, 05:18:17 pmThe only real drawback I can think of is that with a larger capacitor the power supply will have a longer turn-on delay, since it will take slightly longer for the resistor to charge it to the voltage that the UC3843A requires to start.But with a computer power brick, that's not really an issue, as turn-on delay really doesn't matter in this instance.

author: e-pirate
author_rank: Regular Contributor
timestamp: February 28, 2021, 07:04:13 pm
content: Quote from: TheMG on February 28, 2021, 06:25:42 pmAnd y-y-yes, you are right once again. The start delay increased, I noticed this when I started the PSU for the first time after recaping. It took, maybe.. half a second, I don't know... I even remember my first thought "not caps? really??" after I powered it on, but the led didn't light on immediately. But this is not an issue, actually. This PSU never switched off. + half a second start delay once upon several years - definitely not a problem. Thanks for your advises and dispelling my doubts.