Help sought in locating short circuit [Sorted] - Page 1
Forum section: electronics / repair

author: max.wwwang
author_rank: Frequent Contributor
timestamp: November 14, 2024, 07:48:00 pm
content: Another repair project, the motherboard of sewing machine PFAFF 7570.It features traditional linear power supply with two secondary coils in the power transformer, 25V AC each. Each secondary coil is protected by a 1.25A fuse (appears to be slow blow). I have now identified one of the fuses as blown. Checking my luck, without the same replacement fuse in hand, I welded in a fuse holder through wires and a normal (quick blow) 800mA; blown straight away once energised. Sourced 1.25A fuses (still quick blow), tested with one and again without outgoing cables connected, the same. Surety no need for more fuses to sacrifice for now.It’s apparent there is short circuit downstream. Problem is, how to pinpoint it? I have done a careful visual inspection but have not located an obvious culprit. I’ve also pulled off all of the smaller electrolytic caps but am yet to source their replacement and put them back (each separately tested with a component tester, no obvious culprit there).Now I’m seeking help in locating the spot of short circuit. My plan for now, is to put a 30 ohm or so in series with another fuse and, if this time the fuse holds despite the short circuit downstream, probe around.Would you think this makes sense? My logic is to restrict the current to within 1.25A anyway with the added resistor (I don’t have a current limiting lab supply). My concern is that, given the existence of fault, the circuit the ps supplies will receive virtually 0 V (or perhaps two diode forward bias voltages that is about 1.2V) anyway so probably not too helpful to my probing, except the spot of short circuit sticks its head out by itself due to abnormal sustained current of around 1A.[Correction: I got it wrong here, the short circuit current is not necessary about 1A, but certainly than intended or expected by design.]Not sure this is worth a try, and if ok before the caps are back.I have not pulled out the big boy power supply filter cap, hoping it’s innocent.Photo of the board for a general idea of the thing.

author: Andy Chee
author_rank: Super Contributor
timestamp: November 14, 2024, 08:13:37 pm
content: You need a low ohms meter.Essentially you need to measure multiple places on the board. If you measure 0.1 ohms at one location, and 0.01 ohms in another location, you know you're moving closer to the general location of the short. If you measure 0.5 ohms, then you know you're moving in the wrong direction.Regular multimeters may have trouble with accuracy at low ohms.Once you've homed in onto the general area, that's where close inspection and component removal comes into play.

author: max.wwwang
author_rank: Frequent Contributor
timestamp: November 14, 2024, 08:35:02 pm
content: Quote from: Andy Chee on November 14, 2024, 08:13:37 pmThanks. I think this means that the filter electrolytic cap must be out of the circuit, otherwise I can’t see a way of accurately measuring the (load) resistance and without needlessly draining the battery of a DMM (if powered by battery).

author: CapLeaker
author_rank: Regular Contributor
timestamp: November 14, 2024, 09:19:00 pm
content: Feed power with a bench power supply into the shorted rail on the output into the smoothing capacitor. Adjust it to what ever that rail supposed to have VDC then crank one amp into it. After a little while something will get hot. You can use the IPA or thermal camera trick. What ever you do, do not exceed the max voltage of the shorted rail, but you can increase the current somewhat.Problem here is no one knows what this thing is, no schematic or anything. Anyway… my 2 cents Canadian.

author: max.wwwang
author_rank: Frequent Contributor
timestamp: November 14, 2024, 09:27:04 pm
content: Quote from: CapLeaker on November 14, 2024, 09:19:00 pmThank you very much. At least I learned something here. I thought of thermal imaging (but don’t have the gear), but now I’ve learned there is the alternative of IPA (I can imagine how it works by intuition but will google for more detail). Unfortunately I don’t have an adjustable lab supply either (might source one in the near future). Do you think my plan is at least safe, and if so, worth a try?

author: richnormand
author_rank: Unknown
timestamp: November 14, 2024, 09:37:01 pm
content: I use a can of Freeze Mist or a can upside down air blower can to create a white cold mist on the suspect components.I then apply current on the shorted line and see where it warms up abnormally fast.This works well for shorted tantalum caps but less so for a contact to contact metal dead short that does not dissipate much heat (closer to zero ohm with good thermal capacity)

author: Russ_A
author_rank: Contributor
timestamp: November 14, 2024, 09:42:07 pm
content: I did a power supply replacement on a sewing machine because a sewing machine repair tech said they could no longer get the "official part" from the supplier. They wanted someone to scrap an $8000 machine because of this. So I figured why troubleshoot something that costs $60 when I can just order a similar supply off Digikey? I ordered from them so I could get the datasheet beforehand to confirm that it would fit. I had to drill a few mounting holes because of the different size circuit board. Other than that it was just a matter of plugging everything into it. One of the connectors was the same and I had to modify one a tiny bit. Much easier than trying to revive a power supply.

author: max.wwwang
author_rank: Frequent Contributor
timestamp: November 14, 2024, 10:03:29 pm
content: Quote from: Russ_A on November 14, 2024, 09:42:07 pmWell done good on you!

author: max.wwwang
author_rank: Frequent Contributor
timestamp: November 14, 2024, 10:30:51 pm
content: Quote from: richnormand on November 14, 2024, 09:37:01 pmThanks, another trick learned. Never knew this before, not sure if it’s available where I live.

author: fzabkar
author_rank: Super Contributor
timestamp: November 14, 2024, 10:32:36 pm
content: Perhaps one of the rectifier diodes is leaky? If so, then the associated electrolytic filter capacitor would look like a short circuit to AC, but not DC.

author: max.wwwang
author_rank: Frequent Contributor
timestamp: November 14, 2024, 10:49:11 pm
content: Quote from: fzabkar on November 14, 2024, 10:32:36 pmThanks. I checked the full bridge diodes (without obvious anomalies) but didn’t have a careful and close analysis of the possible failure modes and corresponding behaviour (I didn’t suspect them that much).Certainly that’s a possibility. By leaking do you mean they conduct to someone degree under reverse voltage? If so then certainly a bad one will cause AC short circuit without involving the filter cap (hence I don’t quite understand what you mean by “capacitor would look like a short circuit to AC”).I will have another close look.

author: fzabkar
author_rank: Super Contributor
timestamp: November 14, 2024, 11:03:09 pm
content: Quote from: max.wwwang on November 14, 2024, 10:49:11 pmI have had cases where a leaky diode in a bridge rectifier resulted in very large ripple and unstable behaviour. The diode doesn't need to be a dead short, especially if the capacitor is very large.To answer your question, yes, "leaky" means that there is some conduction in the reverse direction.

author: Russ_A
author_rank: Contributor
timestamp: November 14, 2024, 11:33:15 pm
content: Yeah I get that it isn't directly applicable to your case since it appears the power supply and the control board share a PCB. If the problem is with the power supply maybe you can find a way to disable it and then bypass it with something off the shelf. Just throwing an idea out there.

author: marhuum
author_rank: Contributor
timestamp: November 15, 2024, 12:18:28 am
content: Mere try / effort;obtain thermal imaging, snapshoot or record a (time range on) the moments it gets voltage supplied while getting short so illustrating the current flow on the intended parts or areas

author: max.wwwang
author_rank: Frequent Contributor
timestamp: November 15, 2024, 12:26:46 am
content: Quote from: Russ_A on November 14, 2024, 11:33:15 pmCorrect. The power supply here is hard to break (except perhaps the diode). Most likely the problem sits elsewhere (thought I will look closely into the supply diodes as suggested).Your idea is welcome and certainly appreciated.

author: max.wwwang
author_rank: Frequent Contributor
timestamp: November 15, 2024, 03:26:37 am
content: So my plan for next step is,1. sources replacement new caps for those that have been pulled out and put them back.2. Add a 30 ohm resistor in series with a new fuse and see what happens. (The fuse should not blow because the AC current will be under 1A.)3. Scope the voltage across the added resistor. It should be regular sine wave form. Excess irregularities suggest regulator diode leakage or other fault.4. Diodes good, measure the power rail positive voltage (should be at least a little above 0 - actually al least 1.2V). This voltage will hint on the degree and potentially nature of the short circuit fault.5. Be mindful this powered condition probably should not maintain for too long, but when it’s on, try to find hot spots.Any comments on my plan are welcome.

author: max.wwwang
author_rank: Frequent Contributor
timestamp: November 19, 2024, 02:02:38 am
content: Sourced the replacement for the caps that are out. Before soldering them back, I took some time to do a bit more board tracing, datasheet reading, diagnosing, etc. because I didn’t want to miss the opportunity with the caps off. I don’t want to be in a situation where, when they are back, I hope they were not (and that’s a human tendency as far as I’m aware).The faulty section of the power supply feeds into a L4963 power regulator, which I had a close look, and which had my suspicion at one point. Its output, 5.1V, goes to all of the logic power supply pin of the logic chips or stepper motor driver chips across the board.Checked again and again, no clear signs of short circuit measured with a DMM, either before or after this 4963. Now solder the caps back and see how it goes with a current limiting 30Ohm in series. (And a short section of aluminium flat bar is clamped behind the Darlingtons as improvised heat sink.)As expected, fuse survived this time. No anomalies in the waveform across the added resistor, which ruled out the hypothesis of leaking rectifier diodes.For testing and more as an indicator, the front panel is attached to the motherboard; the panel has a lcd display which previously only lit up but was blank. This time, it has stuff in display! Clearly the uC has managed to work under the less than design condition.In the meantime, I also noticed a peculiar sizzling noise near the transformer (tiny but clearly different from the typical hum of transformer). That distracted me from the planned course of action of finding hot spot. I first thought it was something nasty going on in the 4963 chip; it was only by accident that I noticed the big guy 2200uF 50V axial cap was hot. Then it made more sense that the noise must have been from some sort of cooking in that barrel.There may still be odds, but it’s virtually certain that this big cap is the problem.It will take time to source its replacement (another 1000uF 16V will also be replaced). But will first use a used cylinder type cap of similar specs to see if that is the only problem, by now removing the added resistor.This is the first time I see a big boy power filter cap that failed.

author: richnormand
author_rank: Unknown
timestamp: November 19, 2024, 03:40:31 am
content: Replace that cap but make sure it is fed DC.If there is a lot of ripple or AC due to defective diodes the new one will also overheat.Some DVM will have a DC and AC position that will allow you to measure an AC ripple over DC properly.

author: max.wwwang
author_rank: Frequent Contributor
timestamp: November 19, 2024, 04:13:19 am
content: Quote from: richnormand on November 19, 2024, 03:40:31 amYeah there was a post above suggesting the scenario of excessive AC ripple across the filter cap due to leaking diodes. I still can’t get my head around what exactly this is. I can’t think of a way that due to leaking diodes, the cap is momentarily subject to reverse voltage due to diodes leakage. (Not sure if I’m missing the idea.)A bit of enlightening would be appreciated.

author: kripton2035
author_rank: Super Contributor
timestamp: November 19, 2024, 07:26:24 am
content: build yourself a shorty(with display) like described here :http://kripton2035.free.fr/Projects/shorty-display.htmlit's really easy to find a short. I did another macbook air shorted capacitor this week in a few minutes ...

author: max.wwwang
author_rank: Frequent Contributor
timestamp: November 19, 2024, 09:58:35 pm
content: Quote from: kripton2035 on November 19, 2024, 07:26:24 amThank you. This looks fantastic. Will look into making one.Update, by cutting off the bad guy, hooking up a used 4700uF 63V regular cap, connecting all the cables, removing the added 30Ohm, everything is working the way it should.Now just a matter of putting the new caps and slow blow fuse in when they arrive.