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I have two units of a device which I shall call deviceA and deviceB. DeviceB is malfunctioning.

Therefore I compared the resistance values of the same components on the boards of both deviceA and deviceB in order to find the one(s) which have failed. I know this is not the best way to proceed, but being deviceA and deviceB identical, it think it is an acceptable strategy.

I noticed that several components do not share its value with the corresponding component on the other board; I am talking of a 5% to 50% difference.

Then a 8.2v Zener diode, on deviceA it has a 15K ohm resistance, on deviceB its resistance is a some 20 ohm. Therefore I decided to swap the two components in order to check if it was the cause of the defect.

I found now that the swapped components are working the same as before, ie. the 8.2v Zener diode on deviceA it has a 15K ohm resistance, on deviceB its resistance is a some 20 ohm.

Therefore I have to assume there must be some other component on the board which is inducing deviceB strange behaviour.

How would I go on troubleshooting malfunctioning deviceB?

Please let me know thanks.

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    \$\begingroup\$ Look for any other components that are connected in parallel with the zener. Likely candidates are electrolytic or tantalum capacitors or chips (assuming the zener is connected to a power rail used by chips). If you have a schematic, it would help to post it. \$\endgroup\$ – The Photon Jul 27 '15 at 16:13
  • \$\begingroup\$ @ThePhoton Ok, I will check and let you know, probably tomorrow. Thanks. \$\endgroup\$ – geraldCelente Jul 27 '15 at 16:18
  • \$\begingroup\$ The resistance you measure across a Zener diode will vary, depending on the polarity. It will likely be quite low when the anode is positive (diode forward biased), and much higher when the cathode is positive. \$\endgroup\$ – Peter Bennett Jul 27 '15 at 19:46
  • \$\begingroup\$ @ThePhoton Your suggestion did the trick. If you are willing to turn your comment into an answer, I will accept it as a good solution. \$\endgroup\$ – geraldCelente Aug 3 '15 at 14:34
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It's likely the fault is in some other component connected in parallel with the zener. Likely candidates are electrolytic or tantalum capacitors or chips (assuming the zener is connected to a power rail used by chips).

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