Repair pcb then future failure? - Page 1
Forum section: electronics / general

author: wagon
author_rank: Frequent Contributor
timestamp: September 05, 2014, 10:59:49 pm
content: I was talking to an electrician customer on Friday about a job I just did for him. I repaired the contactor drive circuit on a MIG welder control pcb. Whilst I expect the repair (and the board in general) to go on working for years, he was not so confident. He is under the impression that a 'repaired' board will fail in another way a few months later. Thoughts?

author: AlfBaz
author_rank: Super Contributor
timestamp: September 05, 2014, 11:26:22 pm
content: My first reaction would be to ask why he thinks that.Secondly, you had this thing apart. What sort of quality was it? What was the failure you had to repair? Is he using it out on site with cheap-arse portable alternators? Is he jamming the nozzle into his steel mug to heat water for his morning tea?

author: wagon
author_rank: Frequent Contributor
timestamp: September 05, 2014, 11:37:49 pm
content: Quote from: AlfBaz on September 05, 2014, 11:26:22 pmI only had the pcb to fix. It was from a WIA welder, those are pretty good quality and the machine was used in a car workshop (I think). They (the electricians) did the remove/replace work. He didn't really have a particular reason to his view, but I presume he's had previous failures that colour his view of things. I have heard it before, from others. I'm just interested if others here have encountered the same attitude and how they dealt with it.

author: tautech
author_rank: Super Contributor
timestamp: September 05, 2014, 11:46:38 pm
content: Depends on what failed.If it was electronic componentry of dubious quality and replaced with a known brand or careful substitution with a better spec'ed device, count on it lasting.Done properly your only risk is deficiencies in the original design.Often a electricians knowledge of thefiner pointsof electronics is sadly lacking.

author: wagon
author_rank: Frequent Contributor
timestamp: September 05, 2014, 11:56:00 pm
content: Quote from: tautech on September 05, 2014, 11:46:38 pmYep. There is a Schottky diode in the little power supply on those boards that fails, and I changed that whilst I was at it, and the triac I used had a 25A rating instead of the original 8A (I didn't have the 8A part in stock). I get rather anxious when I hear words like 'we tried to fix it, but couldn't' or 'my husband had a go at it, he's pretty handy'.

author: rs20
author_rank: Super Contributor
timestamp: September 05, 2014, 11:57:13 pm
content: The customer is probably thinking about his welder just like he thinks about his car. If your car breaks down and you bring it in and one particular thing is fixed, you're still wondering how all the other parts of the car are holding up. Almost every interesting part of a car is under the effects of continuous mechanical abrasion, so their life has quite a strict upper bound.(Solid-state) electronics, on the other hand, doesn't suffer from the effects of aging lubricants and worn out parts. So as an electrical engineer, I'm far more likely to look at a failed board and think that there's 99 perfect, like-new components and 1 failed part. You'd never look at a car that way, even if it was working perfectly. Most laymen will probably think everything's car-like in this respect.

author: Rerouter
author_rank: Super Contributor
timestamp: September 06, 2014, 01:52:41 am
content: The other view point which i commonly deal with is the implied attention to detail of the repair process, and the implication that the repairer wishes to see return business in X years,For the most part when i set out to repair something i identify the reasons behind the fault, and other likely causes for future problems and fix them, even if sometimes it means tacking on some additional component costs, by me going a bit further and finding and repairing faults they did not even ask about it leaves the impression that the entire device was serviced rather than a single fault fixed and everything else ignored as i have found a large number of laborers to do.The other side issue is some people who take on jobs over there heads clean off or remove things that had a purpose, e.g. hot-snot on a connector while they worked on the main PCB, depending on what they are doing could now vibrate loose in a couple months, unintentionally making the device fail a second time shortly after repair, on a similar thread i have equally seen boards that had the protective lacquer scrubbed off to allow for easy re soldering, only to see the metallic dust from its environment short out a non replaceable bit of silicon (EOL) probably closer to 8 months later on that one,

author: AlfBaz
author_rank: Super Contributor
timestamp: September 06, 2014, 02:11:25 am
content: Quote from: Rerouter on September 06, 2014, 01:52:41 amMy Dad once decided to pull apart his chess game. Once he had all the screws out there was this "plastic thingy" that prevented the 2 halves from coming apart so he cut it with a pair of scissors... It was an FPC ribbon

author: wagon
author_rank: Frequent Contributor
timestamp: September 06, 2014, 03:19:29 am
content: Quote from: Rerouter on September 06, 2014, 01:52:41 amWhen I was a photocopier tech I wasn't always Mr Popular because a) I often took longer per call than many of the techs & b) Used more parts. My callback rate was waaaay down, however, and some of the machines went from PM to PM without any breakdown calls. I wonder why?

author: tautech
author_rank: Super Contributor
timestamp: September 06, 2014, 04:14:13 am
content: Quote from: wagon on September 06, 2014, 03:19:29 amI don't, especially after a re-read of this thread.QuoteIntimate knowledge of the circuit you have repaired..... who can question that?

author: wagon
author_rank: Frequent Contributor
timestamp: September 06, 2014, 04:16:56 am
content: Quote from: tautech on September 06, 2014, 04:14:13 amWe can only try. Experience teaches us a lot if we are prepared to listen.

author: miguelvp
author_rank: Super Contributor
timestamp: September 06, 2014, 04:26:42 am
content: Everyone knows that when you get anything that doesn't work right has gremlins and you just can't get rid of them so it will continue to fail.Half kidding because there is something to it in my uninformed opinion

author: wagon
author_rank: Frequent Contributor
timestamp: September 06, 2014, 04:37:25 am
content: Get a bigger hammer.

author: AlfBaz
author_rank: Super Contributor
timestamp: September 06, 2014, 04:42:24 am
content: Quote from: miguelvp on September 06, 2014, 04:26:42 amThere was a lot of anecdotal evidence back in the VCR days that if you bought a unit and it had to be repaired under warranty inevitably the unit would continue to fail.The same could be said about white goods so most likely this is where this notion comes from

author: wagon
author_rank: Frequent Contributor
timestamp: September 06, 2014, 04:51:27 am
content: Quote from: AlfBaz on September 06, 2014, 04:42:24 amThere's probablysometruth in this concept.... I work on the basis, with new gear, if it doesn't fail whilst under warranty it will probably run to the end of it's design life : until parts start wearing out. For example, electric fence energisers usually go for years then the big capacitor dies. Change the big cap, goes for a long time again.

author: tautech
author_rank: Super Contributor
timestamp: September 06, 2014, 04:53:01 am
content: Quote from: AlfBaz on September 06, 2014, 04:42:24 amAnd it will continue this way while componentry is used at 95% or more of its max rating for the sake of a few cents extra cost, never mind the possible dramatically increased reliability.Bloody bean counters.The only good from it is for the service/repair industry.

author: wagon
author_rank: Frequent Contributor
timestamp: September 06, 2014, 04:59:51 am
content: Quote from: tautech on September 06, 2014, 04:53:01 amUnless we repair it with better-quality parts, thus doing ourselves out of jobs.

author: SeanB
author_rank: Super Contributor
timestamp: September 06, 2014, 05:18:45 am
content: Funny, I bought a cheap VCR and it had one issue under warranty, so I sent it in ( brake band came loose) and after it was repaired I used it for about a decade more with no other problems. Had to change one belt in that time, but nothing else at all.Another I did modify by adding a bigger heatsink to the 3 hot power transistors in the power supply, as the OEM, Hitachi, had decided to use a small bent aluminium plate with 4 power devices on it. Left the 7805 on that and extended the other 3 pass transistors to outside the case on a scrap heatsink. They all ran cooler, and I never had power supply issues again. Did change a lot of clutch units though, but those were quite cheap and quick to fit, 2 screws and one belt and it all clipped out and in. Removed the auto head cleaner to keep the video heads from being worn out prematurely though.

author: wagon
author_rank: Frequent Contributor
timestamp: September 06, 2014, 05:36:16 am
content: Old auto head cleaner : garbage on, garbage off, garbage on,....

author: wraper
author_rank: Unknown
timestamp: September 06, 2014, 08:12:05 am
content: Quote from: wagon on September 05, 2014, 10:59:49 pmI wish my boss would think such a way. He thinks that every 20 year old (or sometimes even older) junk which I repaired for our own company use should work as reliable as new. And there are not only electronics and mechanics in those devices but also pneumatics, pumps, water valves, IR gas analyzing cells... Ton of the stuff that wears a lot with a time. Not to mention they are mostly operated by trained monkeys.Oh yeah, forgot to mention. Most of those devices are located 1000-6000 km away from my hometherefore I need to trip each time and of course can take only same basic equipment and parts with me.

author: wagon
author_rank: Frequent Contributor
timestamp: September 06, 2014, 08:21:54 am
content: Quote from: wraper on September 06, 2014, 08:12:05 amThere's 5years old, 10years old, and in your case, ancient.