Electronics > Repair
Help sought in locating short circuit [Sorted]
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max.wwwang:
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.

Andy Chee:
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.
max.wwwang:

--- Quote from: Andy Chee on November 14, 2024, 08:13:37 pm ---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.

--- End quote ---

Thanks. 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).
CapLeaker:
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.
max.wwwang:

--- Quote from: CapLeaker on November 14, 2024, 09:19:00 pm ---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.

--- End quote ---

Thank 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?
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