Troubleshooting audio mixers has come up twice now so I thought I would post my thoughts and maybe it will help someone that is experiencing the problem.
Sometimes there are clues to where the problem could be and all you have to do is think logical about it. An example is you have a mixing board with 2 channels that are dead. They happen to be right next to each other. This is a very good clue.
Almost all the time when you see this type of problem you know right away it's gotta be something that is common to both channels. If you have a schematic you just grab that and look to see what it is.
Usually it's going to be an op amp, possibly a switch or a fader, volume control or even a feed from the dc supply. Or even a ground connection that both channels share. Usually it's going to be an op amp.
Now the good thing is going to be you can compare voltage readings against known working channels if you don't have a schematic. Just be real careful not to short anything while doing this or you have more problems. I usually take reading off of components connected to the pins of the op amp.
Look for things like the DC voltage rails being at the outputs of the audio op amp. This usually means that the op amp has broken down. Also a bad op amp can be hot to the touch, when it shorts internally.
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If that's not it, eyeball the circuit board real well after making sure that you have supply voltages (or lack of ) on the op amp. Something could have opened up or pulled away from the circuit board because of shock or excessive heat. Lots of times you can see the problem and you don't need to get your test equipment out.
A bad ground can cause supply rails to sit on the output of an op amp also but usually if this is the problem you have to see why you lost your ground. If the trace is burnt it means something shorted somewhere and that has to be located before putting fresh components in.
But hopefully your problem will be something simple like an open in the circuit or switch, pot, slider or op amp replacement.
Another clue is try to see how far into the board you have a signal from your instrument before it disappears. An example is you might can make the bad channels peak lights come on before the signal disappears later in the circuit.
Or the signal gets all the way to the sends and then disappears, this can eliminate a lot of the circuit. Keep in mind that sometimes if you just look at all the clues first and think about it, the area where a problem exists will become apparent even before you start the repair work.
(for a little more trouble shooting tips, check out my post on clamping FETs, these are used in audio circuits sometimes)