Generic question - power supply failure - Page 1
Forum section: electronics / repair

author: darkstar49
author_rank: Frequent Contributor
timestamp: August 31, 2020, 08:42:26 am
content: I know there are numerous threads on the subject, as well as zillions of tutorials, etc...But I'm busy with a power supply (Cherokee/Lineage Power) SP651 that works well for a few minutes, then randomly shuts down...I'm definitely NOT a PSU expert, but I did quite some research on the Net and found nothing helpful... I did not find any hints on what could be wrong with a working PSU that shuts off randomly, not after a few seconds, or during startup, but after several minutes... and it doesn't get worse... there's just something that makes it think after a while that it's time to sign off...I'd be very thankful for any ideas/hints as to what component(s) could produce this kind of failure and/or how to troubleshoot such failure ?

author: wn1fju
author_rank: Frequent Contributor
timestamp: August 31, 2020, 11:24:41 am
content: Could it be a thermal problem? Are any parts getting real hot? After it shuts off, can you immediately restart it or do you have to wait a while (until it cools off)? Also, some better power supplies have thermal cutoff switches.

author: darkstar49
author_rank: Frequent Contributor
timestamp: August 31, 2020, 01:48:19 pm
content: Thermal issue is indeed my best guess as well... and restarting it immediately after a shutdown indeed leads to a pretty quick shutdown.But the question is, given that the fan is working OK, what components could be involved in such a failure ?Are thermistors ageing ?? Is it a good idea to replace those anyway ?? (assuming the labels are readable and I can find a suitable replacement, because as usual, there are no schematics nor parts lists available)

author: Grandchuck
author_rank: Super Contributor
timestamp: August 31, 2020, 02:21:55 pm
content: Some components partly fail. That could be what is going on. A switching transistor might work well enough to produce output but run extra hot due to leakage and thus eventually cause shut down. Does your nose detect any odor that might indicate over heating? (don't use a nose meter up close, especially under power

author: Nusa
author_rank: Super Contributor
timestamp: August 31, 2020, 02:32:11 pm
content: After anything found with visual inspection, the usual thing when dealing with strange symptoms on old stuff is to replace the electrolytic caps. Or at least test them, but that takes more time than replacing them.Pricing a replacement unit is also a useful exercise, as that tells you whether the repair is economical in terms of time and money spent fixing it.

author: tunk
author_rank: Super Contributor
timestamp: August 31, 2020, 02:36:04 pm
content: Bad or cold solder joints?(And if it's the same unit as in your other thread, clean it first.)

author: darkstar49
author_rank: Frequent Contributor
timestamp: August 31, 2020, 02:42:51 pm
content: Quote from: Nusa on August 31, 2020, 02:32:11 pmOh yes !!!! I bought a few spares from Lecroy a while ago, but those PSU's are no longer available, and Lecroy is not selling any spares anymore (tried 2-3 months ago), I'm not even sure they have some... those suit-case models (Xs, Xi and derivates) are now out-of-support, or under 'best effort' support (I know for sure that those motherboards with the PCI/104 slot are now unobtainium, Lecroy has 0 stock, and they're out of production since many, many years).So, even at 4-500 USD a piece, I wouldn't bother... but unfortunately, there's no other way... I also tried to get in touch with Cherokee / Lineage Power (now GE Power), with no luck... (this was a custom design for Lecroy anyway, so I'm pretty sure they wouldn't nor sell anything, nor transmit any technical details/schematics/...).Regarding the caps, yes, that's a standard action on old PSU's... but I was wondering wether the symptoms here are 'compatible' with 'semi-dead' caps... (they all look 100% fine, btw).My assumption was until now that those symptoms were not compatible with problems with the electrolytics, but maybe I'm wrong ??

author: darkstar49
author_rank: Frequent Contributor
timestamp: August 31, 2020, 02:46:07 pm
content: Quote from: tunk on August 31, 2020, 02:36:04 pmyep, same unit, and of course cleaned up !! ;-)Still same symptoms... I could have used the other thread, but thought that the question was in fact more 'generic'... as stated, I found lots of tutorials for PSU repair, but none that mentions a PSU that fails after several minutes... so I created a new thread, as I also hoped to have feedback from people that wouldn't be interested by a Lecroy specific topic.As for the solder joints, nothing obvious, to me at least...

author: darkstar49
author_rank: Frequent Contributor
timestamp: August 31, 2020, 02:57:19 pm
content: Quote from: Grandchuck on August 31, 2020, 02:21:55 pmNo risk with the odor meter, even at close distance, as we all have to wear a mask !!! ;-)But no, couldn't spot anything special... (without mask !). I'll get a thermal cam this week-end, could be interesting as well...??

author: Squoip
author_rank: Contributor
timestamp: September 01, 2020, 03:58:20 am
content: I've been successful with a can of freeze spray when there are hot shutdown problems. I have been able to isolate to a single component by spraying smaller and smaller areas until a short burst on a specific component does the trick.

author: darkstar49
author_rank: Frequent Contributor
timestamp: September 01, 2020, 01:01:41 pm
content: Quote from: Grandchuck on August 31, 2020, 02:21:55 pmStop !!! I've got it now with the PSU outside the casing, and geee... yes, it smells !!!!!!!!!But I couldn't say what it smells like... as stated, I'm no PSU expert.There's no smoke, just the smell, but quite strong... I'd say it smells like 'electronics stuff', just much stronger... although I know this is pretty childish and doesn't help much...(and yes, I'm running it with a big fan blowing at it !!)

author: Grandchuck
author_rank: Super Contributor
timestamp: September 01, 2020, 02:32:22 pm
content: Thermal camera time.

author: darkstar49
author_rank: Frequent Contributor
timestamp: September 02, 2020, 12:19:20 pm
content: Still don't have the 'real' cam... but some Flir One gave me maybe some hints...And strangely enough, the smell is almost gone today (I started it yesterday for the first time since quite a while...).Anyone knows what temps are 'normal' for what kind of component ?On 2 relatively big resistors near line entry (just after what looks like a relay), I had 130°C (the little Flir One might not be the most precise tool, but still quite a lot), connected to the mains, but idle...Also had the same kind of temp on a resistor near what I believe to be the rectifier bridge ?? For the remaining part, nothing went upside from 65°C... (although the scope was not pumping a lot, as it has a boot disk failure, but it had it yesterday, too.Any comments welcome !

author: darkstar49
author_rank: Frequent Contributor
timestamp: September 02, 2020, 02:24:21 pm
content: Damn !! this repair is not going the way it should... was probably way too tired yesterday night, and was then wondering why the scope had a boot disk failure (on top of smelling like hell... I guess the PSU had nothing to do with the burning smell)Now I've got it...had mis-inserted the IDE cable into the 2.5" diskI'll be very lucky if the mobo is not fried... these are 100% unobtainium !!!!!!

author: tautech
author_rank: Super Contributor
timestamp: September 05, 2020, 03:11:08 am
content: My gut feeling is dead or dying mainboard caps.The PSU current sense circuitry is shutting the PSU down.If you can get it to run for a couple of minutes and there is no HV present run your fingers over a few caps and you should find some getting hot.

author: darkstar49
author_rank: Frequent Contributor
timestamp: September 07, 2020, 11:50:51 am
content: Quote from: tautech on September 05, 2020, 03:11:08 amI've got another PSU (not the same revision, but same type), and everything works fine with that one, thus I assume that the problem is with the PSU.I couldn't get the thermal cam yet, so I've put this aside for the moment. Many components are glued (incl. caps) and not so easy to access, so I'm a bit reluctant to start swapping caps just to be sure... I'll first try to spot any abnormally hot components under load, and then refine the diagnostic.

author: Nusa
author_rank: Super Contributor
timestamp: September 07, 2020, 05:57:01 pm
content: When you get it, take some images of the working supply in operation, so you have a baseline on what's normal.

author: tautech
author_rank: Super Contributor
timestamp: September 07, 2020, 08:25:56 pm
content: Quote from: darkstar49 on September 07, 2020, 11:50:51 amOh well, if nothing else that narrows the field some.Very likely it's caps then or a faulty joint that's breaking down with heat.For SMPS I generally focus around the controller IC particularly its VCC storage cap. They seem to drift way outta spec with age and go high ESR and/or low capacitance. Pull and measure is all you can do with these as they never show signs of degradation.

author: Miti
author_rank: Super Contributor
timestamp: September 08, 2020, 02:05:25 am
content: My bet is on the caps. Their ESR went up and they heat up from the ripples, further increasing the ESR. Touch the top of the low voltage caps with the finger to check their temperature.

author: Rasz
author_rank: Super Contributor
timestamp: September 08, 2020, 05:33:48 am
content: >2 relatively big resistorsthey are big for a reason, but I would replace capacitors directly near them, living next to source of heat tends to dry caps pretty fast> works well for a few minutes, then randomly shuts downwhat shuts down? have you monitored its rails when it does that (ideally using another scope)? does PSU stop producing prgood signal before going down (im guessing its ATX like )?

author: darkstar49
author_rank: Frequent Contributor
timestamp: September 08, 2020, 07:45:39 am
content: Quote from: Rasz on September 08, 2020, 05:33:48 amany idea what temp to expect on those resistors, as stated, the FLIR One might not be the most precise, but 140+ °C seems pretty much... is this rather indicating a problem somewhere else, or could the resistors themselves also age and dissipate more heat than they're supposed to ??I'm still waiting to redo the exercise with a 'real' thermal cam, I'm willing to change some caps, but the PSU is relatively complex (see pic above), and components are not easily accessible, so unless the only answer is 'change them all', I'd prefer to be more specific...And again, I'm a PSU rookie (with no intention to become an expert !!).As for the shutdown, no, I didn't monitor the power-good signal. But don't forget there are 4 ATX-like connectors, and the driver is actually the scope's acquisition board, not the PC board (the PC mobo's power-on switch is connected to the acq board), and I've no clue how the startup sequence looks like, nor if and how the PSU and the acq board interact with each other... apart from the board with the switching IC, there are 5 other (vertical) mini-boards with different ICs in the PSU...

author: darkstar49
author_rank: Frequent Contributor
timestamp: September 08, 2020, 07:46:46 am
content: Quote from: Miti on September 08, 2020, 02:05:25 amYep, will do that, but once I get my hands on a 'real' thermal cam, not with the fingers...

author: tautech
author_rank: Super Contributor
timestamp: September 08, 2020, 09:05:36 am
content: Rasz's comment on the PWRgood rail is something that need not be overlooked as that's another little independant SMPS AFAIK. Trace the PWRgood rail backwards and you'll find where it originates.

author: darkstar49
author_rank: Frequent Contributor
timestamp: September 08, 2020, 09:10:17 am
content: Quote from: tautech on September 07, 2020, 08:25:56 pmI'm really not an expert, but there are many IC's in this PSU... as stated, it's not an ATX PSU, it's a custom design, that includes an ATX-compliant output, but there are 4 ATX-like connectors (1 ATX 20-pin, 1 22-pin, 1 18-pin, 1 16-pin).So you're saying that regardless of their aspect, caps can be seriously out-of-specs ?? Not good...is there really no way to locate bad caps in a non-intrusive way ?? This PSU is far from simple and physically quite dense, desoldering all the electrolytics is gonna require a serious amount of time ! Is it possible from the picture above to identify 'more critical/suspect' caps to begin with ?

author: darkstar49
author_rank: Frequent Contributor
timestamp: September 08, 2020, 09:15:25 am
content: Quote from: tautech on September 08, 2020, 09:05:36 amit's not very clear to me what you want me to learn from the power-good line ?? Are you trying to detect whether the PSU shutdowns on its own, or on request of the scope ?

author: tautech
author_rank: Super Contributor
timestamp: September 08, 2020, 09:42:49 am
content: Quote from: darkstar49 on September 08, 2020, 09:10:17 amNo, it's more by gut and experience where you go looking first.Like any circuit some understanding of stress points narrows where to look. Most are well designed with conservative component values and of good quality however the bean counters are always trying to save a few cents therefore reliability suffers especially when crap caps are used.I've seen PC mainboard caps so furking hot they would burn fingers and other caps so furked they're no longer measuring uF but pF !Trouble with SMPS one end of them is rectified mains potential which in our case in NZ is ~325VDC which is not to be trifled withyet the other end of the PCB is often at most 12VDC which I'll happily run fingers over to find hot spots.What's helped me become comfortable with SMPS is study of the controller IC datasheets and especially their Typical application schematics where you can clearly see each block of what makes them tick...pun intended.Forget studying the multi-voltage SMPS so much but the basic single output SMPS and each of the bits that need be right for them to be reliable.Sure you can shotgun many parts and fit new ones but little is learned other than soldering skills however they aren't that hard to diagnose with a DMM and a scope. Cap tests need an ESR meter and I get by with Smart Tweezers for this and most other component tests.Typical failure points in no particular order for non functional or troublesome SMPS. (not those that have gone BANG !)HV dropper resistors to the controller IC VCC.Controller IC VCC cap or flyback diode.Diodes in general.Caps, caps and caps.Did I mention caps ?Study basic operation principles, it really helps understand these things.Quote from: darkstar49 on September 08, 2020, 09:15:25 amPower good line goes down and so does the PSU !2 channel scope in Single shot mode with a ch on PG and the other on a main LV power rail will confirm if the PG is OK and not the reason it's shutting OFF.

author: Rasz
author_rank: Super Contributor
timestamp: September 08, 2020, 09:57:39 pm
content: Quote from: darkstar49 on September 08, 2020, 09:15:25 ampgood + pson + main power rails will tell you what lets go first. Is it the scope monitoring power rails and deciding to shut down? SMPS controller detecting exceeded limits? something else in the supply failing intermittently?bad filter caps will manifest with bad filtering, high ripple or straight sagging under loadare you able to start it back up after shutdown? or do you have to pull the power cord?

author: darkstar49
author_rank: Frequent Contributor
timestamp: September 09, 2020, 06:44:49 am
content: Quote from: Rasz on September 08, 2020, 09:57:39 pmAfter a failure, I can still restart it, but it will fail almost instantly, only after a couple of minutes does it work again for a while... hence my suspicion of a thermal issue...