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I have a Soundcraft EPM6 sound mixer. Recently I noticed a click noise in the right channel of a stereo input. The click seems to happen at each period of the sound wave (I feed a 2 Hz signal, and was able to count two clicks per second.) Each channel has these components:

control What I have done
gain The noise is not affected by the position of this knob
high/low eq The noise does get affected by the eq.
aux1/2 When I route sound to these output, the noise is present
balance The noise is only present in the right channel
pre-fader level switch The noise remains if I monitor the sound pre-fader
mute Kills the noise
fader It does affect the level of the noise

After troubleshooting to discover at which point of the sound path the noise is introduced, I think it is generated before the eq. This area corresponds to the lower left corner in the schematic presented below.

Soundcraft EPM6 stereo input circuit diagram

My knowledge in electronics is pretty basic. I get what a potentiometer and a couple of other components are, but I don't know how capacitors, resistors, and the opamps work together to modify the signal.

What steps should I follow from this point, to find out the problem, and eventually fix it?

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    \$\begingroup\$ The only components that are prone to fail are ICs and aluminium electrolytic caps. And as IC1400A and IC1400B are one IC, I would try and exchange C1405 for a fresh one. \$\endgroup\$
    – Janka
    Aug 24, 2021 at 20:22
  • \$\begingroup\$ By C1405, do you mean IC1400, or the pot P1405? \$\endgroup\$
    – onlycparra
    Aug 24, 2021 at 21:12
  • \$\begingroup\$ No, I mean the aluminium electrolytic cap C1405. Because if IC1400 was at failure, it's likely that both channels would show the problem. \$\endgroup\$
    – Janka
    Aug 25, 2021 at 0:51
  • \$\begingroup\$ true true, thanks. I will try it and update you with the results. (the annoying part is to take apart more than 120 knobs, screws and nuts...) \$\endgroup\$
    – onlycparra
    Aug 25, 2021 at 1:38

1 Answer 1

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I’m going to go out on a limb and say replace IC1400. The gain control is in the feedback loop and so may not affect the amplitude of the click, and it’s unlikely that any failure mode of the passive components would result in crossover distortion. Replacing an IC somewhat speculatively will take a few minutes and cost little, and may well resolve the problem without further ado. Tip: If this is a DIL package device mounted through-hole then it may be easiest to snip the pins and remove them one by one.

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  • \$\begingroup\$ Thanks for the answer. They are SMD. Do you know how/where I could get a couple of IC, without having to buy 1000 at the time? \$\endgroup\$
    – onlycparra
    Aug 24, 2021 at 21:08
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    \$\begingroup\$ You can get TL072 for a buck apiece at Digikey or Mouser, but you could first test the theory by swapping IC1400 and IC1401. If the symptom doesn't change then it's not a bad opamp. \$\endgroup\$
    – td127
    Aug 25, 2021 at 2:58
  • \$\begingroup\$ True but it may be difficult to tell the difference as there’s not a great deal of gain between the two locations. \$\endgroup\$
    – Frog
    Aug 25, 2021 at 7:19
  • \$\begingroup\$ True but OP seems to believe the problem can be be pinpointed to "before the EQ", so can discern which location is the problem. Personally, I think it at least as likely to be a loose wire or bad solder joint than a bad component. \$\endgroup\$
    – td127
    Aug 26, 2021 at 17:07
  • \$\begingroup\$ can a bad capacitor produce this kind of problems? \$\endgroup\$
    – onlycparra
    Sep 3, 2021 at 21:20

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