I have two Uster Tester ( this machine used at textile) cards. one is corrupted and the other is the same one that works. Trying to repair the corrupted card by performing comparison test. When I try to make continuity test with the multimeter to the Vcc and GND terminals of the integrated cards on the robust card, only a short beep sounds heared and stops. however, when I test the continuity between Vcc and GND of any integrated circuit on the corrupted card, the beep sounds continuously. as far as I think it has been shorted somewhere in a capacitor. I wonder how quickly I can find this short-circuit element on the circuit
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\$\begingroup\$ A common method is to use a Huntron Tracker or simulate that by testing the diode like behavior of pins with respect to ground by injecting a current limited voltage pulse and scoping the response. (XY mode) \$\endgroup\$– D.A.S.Commented Nov 28, 2019 at 19:39
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1\$\begingroup\$ Depending on layout and complexity, you can cut traces or remove components to isolate areas and test islands. My guess would be a blown diode, transistor, or regulator. \$\endgroup\$– MadHatterCommented Nov 28, 2019 at 19:59
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3\$\begingroup\$ One technique to find power supply shorts is to use a current-limited supply to power the board, then measure between power and ground at various places on the board. The short will be near the place with the lowest voltage. \$\endgroup\$– Peter BennettCommented Nov 28, 2019 at 20:59
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2 Answers
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One method I have used is to allow current to flow from a power supply and then search for hot spots on the board using a thermal imaging camera.
So, you use a current limited power supply connected to VCC. Adjust the current and voltage so a substantial amount of power is flowing into the board. Maybe 100mW or so at first. Use a thermal imaging camera to find hot spots. If nothing sticks out, then allow more power. Maybe 250 mW, then 500 mW, then 1W until you see a hot spot. 1W should be more than enough.
If you don't have a thermal imaging camera, apply rubbing alcohol to the PCB before connecting the power supply. Then, after you apply power, watch for the spot that dries up most quickly. That is the hot spot. (I have never tried the alcohol method, but I hear it works).
Another option is to pull up the schematic and start removing components from the PCB that are connected to VCC. After each component, re-check the impedance. When you remove the faulty component (assuming there is only one component) the impedance will be high again.
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3\$\begingroup\$ The rubbing alcohol method definitely works - I have also had success with freezing spray making the PCB cold/frost and then you can see where it warms up first. \$\endgroup\$ Commented Nov 28, 2019 at 23:34
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\$\begingroup\$ many thanks, also I saw a video, he says he makes CIC circuit to measure short component. the vide is here but turkish, is that possible youtube.com/watch?v=96hGQaqPQT0&feature=youtu.be \$\endgroup\$– mehmetCommented Nov 29, 2019 at 4:30
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Depending on your equipment, you can measure the resistance with a high precision ohm meter what you are trying to measure here is the trace resistance so it will be quite small (milliohms range).
Start by measure the resistance between ground and the positive side of the cap you suspect are shorted. Then measure the second closest. If the resistance increases the first cap is likely shorted. If the resistance drops you are should be closing in on the short.
An alternative is to use a thermal camera, power the circuit and increasing the power, this will increase the temp of the shorted trace after a short while. By following this trace the fault should be easy to find.
