Cost/benefit of repairing a SMPS or junking it and replacing! - Page 1
Forum section: electronics / beginners

author: Mp3
author_rank: Frequent Contributor
timestamp: February 13, 2020, 03:07:52 am
content: I have a very early (not the best design) SMPS from the late 80's.All but one of the caps have lost a considerable amount of capacitace, one resistor burnt out, and the transistors need replacing too. The inductor is also very sketchy looking so i will replace that if I repair the PSU.So I will be spending $35-40 on Mouser to repair this PSU.Alternately, i could find a more modern SMPS. Taking care of 5V is very simple, i could get a small enclosed unit or something like the PS-05-5 by Mean Well.The problem is nobody makes anything for 18v anymore, let alone +/-18V.So i was thinking perhaps I could get a 24v SMPS and 5v SMPS for less than the cost of parts to repair the existing PSU, but then i'd have to hit some 18v regulators with the 24v SMPS signal.However, it won't work that easily, will it? I would need two separate 24v SMPS, since it seems impossible to find a bipolar supply for either 18v or 24v.Provided i have space to install all of them, i could spend $30 and get two 24v SMPS and one 5v SMPS. Or I could spend $40 and simply refurbish the existing PSU with all new parts.Besides the headache of hooking up 3 different SMPS, is there any reason really not to save about $10 and figure out how to best stuff those 3 SMPS in the case?The one i will be either repairing or replacing is very huge for what it is - about half a 1u rackmount case - so i will have room to fit in three small SMPS if i go that route.I am more concerned about the safety of refurbishing this old SMPS design than anything else, really.

author: Mp3
author_rank: Frequent Contributor
timestamp: February 13, 2020, 03:50:20 am
content: Quote from: blueskull on February 13, 2020, 03:14:20 amAbout 1a 5v and not even 500ma +/-18v. I think around 300ma +/- 18v, i will check closely with a bench supply first if i go this route. Might be the better way to go.

author: vk6zgo
author_rank: Super Contributor
timestamp: February 13, 2020, 03:52:50 am
content: Quote from: Mp3 on February 13, 2020, 03:07:52 amIf the SMPS has lasted from the late '80s till now it mustn't be such a poor design.As an old design, it is probably more "repair friendly" than modern ones.Transistors don't die "just for the giggles", so if you decide to go the repair route, take a long, hard, look at that inductor.I remember, years ago, a SMPS in a Sony "Profeel" picture monitor, where the standard appearance of the inductor was black, which hid any sign of overheating.The resistance of the inductor looked reasonable, so I passed it as OK, replaced the resistors & transistors, & all should have been good.At switch on------"Bang!!".Luckily, another Profeel came in for a separate fault, giving me a known "good" sample to compare with.Both inductors still looked much the same resistance wise, but the inductance meter told the story----the inductance was very much less!I then added the inductance value of the good one to the schematic so as nobody would get caught again!It is a bummer if you don't have a good one to compare with, but I have seen suggestions for indirectly checking these things, like exciting it with a square wave, or make an oscillator using the inductor.If the "Q" looks poor around the SMPS operating frequency, the inductor is probably faulty.

author: Mp3
author_rank: Frequent Contributor
timestamp: February 13, 2020, 04:19:40 am
content: I don't have a good one to compare to, but i think i know who i can ask online to check for me on their unitQuote from: vk6zgo on February 13, 2020, 03:52:50 amOK, sounds good. I suppose you are right, they are mostly still working to this day, so they can't be all that bad. They have a reputation for going bad, but i blame decades-old electrolytics and sketchy 80's voltage regulators/inductors which were not able to made to quite the same tolerances as today's.From what i've read online about this particular unit, i've already seen complaints about the inductor not necessarily being up to the task.

author: james_s
author_rank: Super Contributor
timestamp: February 13, 2020, 05:28:53 am
content: In my experience it's almost always the capacitors, new PSUs are not immune from that either, I've had lots of cheap ones die after a few years. If it were me, I'd replace the electrolytic capacitors with reasonable quality replacements and call it good, it will probably run for another 30 years.Much less messing around than trying to modify a modern 24V PSU to fit.

author: jake111
author_rank: Regular Contributor
timestamp: February 13, 2020, 05:41:14 am
content: So basically your filter and output caps are done and the transfer function for this switcher has shifted to the point of instability. As long as your magnetics are good I'd say it's definitely worth a shot, nice to have original equipment in there vs. getting some china supply that will be poorly copied and noisy as hell. What type of equipment is this that needs +/- 18V?

author: Mp3
author_rank: Frequent Contributor
timestamp: February 13, 2020, 08:34:54 am
content: Quote from: jake111 on February 13, 2020, 05:41:14 amOlder audio processing unit.This guy wrote a scathing review of the aforementioned PSU.https://www.electronicspoint.com/forums/threads/yamaha-spx-90-ii-service-notes.152831/#post-1014911Starting to wonder again if maybe after all i should just ditch it for a nice quality 5V SMPS and hope for the best with some 5v to 18v buck boost converters.Seems like actual draw should be about 500ma 5v and ~200ma +18 and ~50 -18.But first i think i'll try swapping electros, transistors, inductor and see if its stable enough to use.If attmpting to repair the original PSU board doesnt go well, ive had some surprisingly good experiences with those buck boost converters. But i am hoping not to need to go that route.

author: vk6zgo
author_rank: Super Contributor
timestamp: February 13, 2020, 11:10:18 am
content: Quote from: Mp3 on February 13, 2020, 08:34:54 amI wouldn't give him a lot of credence, sounds like he was "all at sea".The comment about no control IC is just silly--- plenty of earlier SMPS did not have them.QuoteOne bit of information from the quoted linkisuseful-- the use of phenolic boards, rather than the fibreglass ones used by Sony.Check such boards closely for any damage, charring, lifted tracks, etc.In general, you will get the best results by going over the board out of the equipment if possible, where you can illuminate it properly.