Power Supply renovation (1978) - Page 1
Forum section: electronics / repair

author: ChrisLX200
author_rank: Unknown
timestamp: October 26, 2017, 02:34:44 pm
content: I've owned this old power supply almost since new (bought around 1980 and there's a date maarked on one component 1978), and it's been running 24/7 for about half that time! So yes, I've had my money's worth out of it but if only for sentimental reasons I would like to get it back up to spec.It's output offers +24, +12, -12, +5v and -5 volts, and nothing has failed dramatically but the voltages have drifted. The 5v rail is still good (5.01v), but the 12v rail is down to 11.3v, and the 24v rail outputs 22.3v. I opened the case up and evicted the dead spiders and cleaned the dust away for a better look. Nothing appears physically wrong - I was mainly looking for bad caps. I suppose the advice will be to put new caps in but I've had to order a full set (all but the big Sprague 250v 500uF axials - if they're gone the cost of new ones is going to be a problem). Anyway, with the problem being the 12v and 24v rails (while the 5v is OK) then I think the big filter caps should still be good. Anyway I've included photos of the PCB below.

author: ChrisLX200
author_rank: Unknown
timestamp: October 26, 2017, 02:36:08 pm
content: more pics..

author: JoeO
author_rank: Frequent Contributor
timestamp: October 26, 2017, 07:10:36 pm
content: The date code on the Boschert transformer is 1984.

author: tooki
author_rank: Super Contributor
timestamp: October 26, 2017, 08:11:13 pm
content: There are a bunch of 1984 date codes visible in the pix.Anyway, why would the big caps be a problem? Digi-Key has, for example, 400V 510uF screw terminal caps from Kemet in stock for about $11. Who cares if they don’t have axial ones in stock, just use screw terminal caps and wire some flying leads to the PCB pads.

author: Kleinstein
author_rank: Super Contributor
timestamp: October 26, 2017, 08:22:18 pm
content: The power supply looks like there is no individual regulation for the voltages. So much like most PC supplies ony 1 output is really accurate and the others will change with load / load ratio. So chances are the other voltages go up once there is a load on the 5 V rail. To stay in specs they may need a minimum load.

author: ChrisLX200
author_rank: Unknown
timestamp: October 26, 2017, 08:54:33 pm
content: Too many years ago...Longer than I can remember but meh, it's old and I've owned it 30 years or so (a lifetime!)When I priced up on ebay those (exact) capacitors were only available from the USA @ £18.87 + £22.64 postage. Each. But yeah, an alternative would have to be used but I found nothing like that on ebay. As said, I don't think they're the problem.In the past it always used to output pretty much accurate voltages - within a 100mV per rail anyway. The 12v rail has had by far the most use. The caps should turn up within a few days and I'll replace them, just wondered if there was anything else obvious I should look at..

author: Armadillo
author_rank: Super Contributor
timestamp: October 27, 2017, 03:18:47 am
content: Too many dried out and poor solder joints to count.You should spend no more than $20 for this, otherwise give it rest and buy a new modern used power supply with adjustable voltage, current, limit control, digital LCD etc...Those are just my opinion.

author: Ian.M
author_rank: Super Contributor
timestamp: October 27, 2017, 04:24:49 am
content: I also see a lot of resistors that could be carbon composition ones. Carbon composition resistors are notorious for drift, so you'll need to identify the voltage feedback networks and see if the resistors are out of tolerance.

author: ChrisLX200
author_rank: Unknown
timestamp: October 27, 2017, 08:25:01 am
content: Thanks for looking everyoneI spent £18 on new caps (that was more caps than I need for this job so I'll have spares) so I'll go ahead with the re-cap. I'll also reflow the joints on the bottom. I'm not going to the trouble of replacing the resistors though, a step too far I think - if it doesn't work correctly after the re-cap and re-flow then I'll give up on it.I do have two other decent bench PSUs for more critical work, but this one is just handy for powering 12v/5v modules, LED lights and so on. It takes up very little space because it sits length-ways on a deep shelf. This morning I also received a DPS5005 step-down module with which I will make another PSU so I'm not short of options

author: Armadillo
author_rank: Super Contributor
timestamp: October 27, 2017, 09:20:39 am
content: If you enjoy troubleshooting, you maybe lucky, ended up spending only a dollar on it and can afford extra 17 dollars on a ESR meter and save the landfill of still good working capacitors to dump to waste.I mean it don't make economic sense to spend a lump of time, efforts and money onto recapping and dump it aside if it didn't works. That "magic" will lure you into spending that extra bits more and more..... but here is a solution, uncap that unworthy shit and dump it into the bin..... far away.And maybe simple reflow did the trick....hahahahahahaI never like the idea of just recapping instead of some troubleshooting measurements. Wonder who spread the virus of recapping in the first place?

author: tooki
author_rank: Super Contributor
timestamp: October 27, 2017, 03:07:54 pm
content: Quote from: Armadillo on October 27, 2017, 09:20:39 amUmm, probably the many people who independently arrived at the realization thata) caps are nearly always the first components to failb) caps are cheapc) time is money, and the time to desolder, test, and resolder a $0.04 capacitor is more than the time to just to desolder and replace it with a new $0.04 capacitor and avoid a future second repairIf common sense is a "virus" then I encourage this epidemic.

author: Armadillo
author_rank: Super Contributor
timestamp: October 27, 2017, 04:14:48 pm
content: Quote from: tooki on October 27, 2017, 03:07:54 pmThe above is a clear example of the wrong notion of cheap when he actually spent £18 with abit of spare caps but excluding the big cap at additional £18.87 + £22.64 postage and still have to worry about it not solving the problem. That piece of common sense can't differentiate 0.04 cents with £18 + £18.87 + £22.64 and the meaning of"recapping",I really don't understand what common misunderstand is that?hahahahahahahahaAnd when he have to troubleshoot again, you tell me about time is money..... hahahahahaah LOL.Nevertheless, I am not against his enjoyment of what he likes, his way....Those are just my opinion for casual chatting....Edit: And probably he has never even seen a square Sprague before.... 0.04 cents cap probably kid sets...

author: Rasz
author_rank: Super Contributor
timestamp: October 27, 2017, 06:24:43 pm
content: Quote from: Armadillo on October 27, 2017, 04:14:48 pmmaybe not 0.04, but I buy Jackcon caps at $0.2 retail over the counter, at that price measuring is a waste of time£18 for ebay caps is a definition of not smart thoedit: actually checked last invoice, Jamicon\Jackcon\Panasonic FC all came out ~$0.2, and thats in actual brick and mortar shop

author: ChrisLX200
author_rank: Unknown
timestamp: October 27, 2017, 08:54:05 pm
content: Quote from: Armadillo onTodayat 03:14:48 AM>Quote from: tooki onTodayat 02:07:54 AM>Quote from: Armadillo onYesterdayat 08:20:39 PMUmm, probably the many people who independently arrived at the realization thata) caps are nearly always the first components to failb) caps are cheapc) time is money, and the time to desolder, test, and resolder a $0.04 capacitor is more than the time to just to desolder and replace it with a new $0.04 capacitor and avoid a future second repairIf common sense is a "virus" then I encourage this epidemic.The above is a clear example of the wrong notion of cheap when he actually spent £18 with abit of spare caps but excluding the big cap at additional £18.87 + £22.64 postage and still have to worry about it not solving the problem. That piece of common sense can't differentiate 0.04 cents with £18 + £18.87 + £22.64 and the meaning of"recapping",I really don't understand what common misunderstand is that?hahahahahahahahaAnd when he have to troubleshoot again, you tell me about time is money..... hahahahahaah LOL.Nevertheless, I am not against his enjoyment of what he likes, his way....Those are just my opinion for casual chatting....Edit: And probably he has never even seen a square Sprague before.... 0.04 cents cap probably kid sets...Don't get dis-heartened son, I'm certain one day you'll find someone who actually gives a fuck what your opinion is...

author: Armadillo
author_rank: Super Contributor
timestamp: October 27, 2017, 09:26:37 pm
content: Quote from: ChrisLX200 on October 27, 2017, 08:54:05 pmHey, no need to be all so vulgar, this is a civilize forum repair discussion forum.... relax.

author: Ian.M
author_rank: Super Contributor
timestamp: October 27, 2017, 10:20:56 pm
content: Work smarter not harder!'Shotgunning' all the caps is rarely necessary, takes too long and isn't particularly cost-effective.Its really quite simple if you have an in-circuit ESR tester. First go around the board making sure all electrolytic caps are discharged (if you want to spend your time repairing the board not the ESR tester!), use a suitable resistor for large caps in the PSU, and simply short small caps. Then test them in-circuit and if in doubt compare with the ESR of a suitable replacement. If any give ambiguous results, desolder one leg, prod it gently to check it isn't resting against the pad or the side of a plated through hole, and check again. (The ESR tester can also be used to check the leg isn't contacting the pad.) Remember the ESR tester cant distinguish between multiple caps in parallel, so again desolder a leg. Its pretty quick to desolder one leg and resolder it afterwards - certainly quicker than completely desoldering the old cap finding the right value, and replacing it properly.Small caps are the most likely to have gone bad - up to about 1/2" diameter, and also larger caps that are stressed by high ripple currents e.g. smoothing caps immediately after a rectifier on the secondary side of a SMPSU, or are stressed by heat from adjacent power resistors or heatsinks. If any small caps are removed, replace with new, don't put the old one back as you have just stressed the leads and bung seal removing it so the odds are the old cap will go bad fairly soon even if it tested good. If you get a bad one and you cant explain it as excessively stressed, replace *ALL* identical ones.SMD caps *should* always be replaced if disturbed so if you feel the need to lift a leg, you'd better have a replacement handy.If you don't have the correct spares in stock, and end up refitting caps or using salvaged ones, for %DEITY%'s sake mark the top of the can *and* the solder joints of the ones you put back with a coloured marker pen, and order the correct replacements. You can soak-test the device while you wait for the parts, then go through it again making sure that everything you marked gets replaced.For non-electrolytic caps you need to know their failure modes: e.g. the infamous RIFA class X caps that crack, let in a little humidity then belch smoke and flames!, when found across mains inputs and in other high energy circuits should *ALWAYS* be replaced on sight. If any cap shows signs of cracking or surface crazing or discolouration, don't trust it.