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Unpowered circuit testing

Discussion in 'Troubleshooting and Repair' started by Coconuttopdrop, Nov 6, 2012.

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  1. Coconuttopdrop


    Sep 7, 2012
    Hi guys!

    So i'm repairing some channels on an old audio mixing board.
    The CH's are detachable from the board, and I have no way of testing them with power.

    What kind of test's can i do with a multimeter and/or a oscilloscope, to compare a full working CH with a faulty CH?

    How can i find the busted components testing the circuit without current?
  2. Harald Kapp

    Harald Kapp Moderator Moderator

    Nov 17, 2011
    First look for visual signs (blown capacitors, burnt resistors etc.). Also inspect the PCB traces for signs of wear or broken traces.

    Second measure suspect components with a multimeter and compare to the good board. You may have to de-solder one side of the component to measure it without influence from the rest of the circuit.

    An Oscilloscope is of no use: Since you have no power you have no waveforms to look at.

  3. shrtrnd


    Jan 15, 2010
    Harald Kapp just gave you the method I use.
    You use the multimeter on the 'ohms' selection, making SURE you have no power to
    the unit under test.
    It's amazing how often you can find defective components using this method by comparing
    the same devices on each of the different channels.
    Incidentally, concerning your last sentence. The Ohmeter inputs it's own extemely
    small current when measuring ohms, that's how the meter obtains its reading.
    Just wanted to say that you were right in wondering how to test busted components in
    testing a circuit, ...but that the answer is that the ohmeter supplies that current you
    were wondering about.
    Good luck with the fix, and the method Harald gave you is extemely effective.
  4. Coconuttopdrop


    Sep 7, 2012
    Thanks guys.

    I managed to find a busted diode but now both channels are not working properly.

    Maybe i busted some op amps while testing, and i have no way to check if the problem is in the op-amps.... and the nightmare continues....

    The op amps are TL074CN (4 for each channel).

    Can op amps and ic's get screwed if you take them out and put them back again in the circuit? Amek EQ .jpg
  5. Harald Kapp

    Harald Kapp Moderator Moderator

    Nov 17, 2011
    Yes they can - if handled not properly (e.g. damaged by electrostatic discharge). Testing an OpAmp is not easy. The TL074 is on the order of 0.50$ / piece. Get yourself a handful of new ones and replace the old ones. With new OpAmps you should be able to exclude these as the source of the problem.

    The busted diode is suspicious. What is the reason for the diode to blow? Can you identify any nearby components that don't look o.k.?

    Are you sure the power supply is functioning properly?

    What are the symptoms of that? Low frequency hum? Hissing? Distortion?
  6. Coconuttopdrop


    Sep 7, 2012
    The Channels have a switch to turn the EQ on.
    When i it that switch, audio stops coming trough.

    I changed the op amps on the Ch that was previously working well, and it is 100% again.

    The Ch that had the busted diode is still not working properly.
    Before changing that diode, i had to change a diode near to the +18.5V supply to the board.
    Those two diodes are connect, and have noting between them.

    The diode that got busted later has, after it, some other diodes and caps (and resistances going to ground) before going to the op amps.
    I checked the caps in circuit using a analog multimeter and they seemed to be ok.

    thanks guys!
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