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Why does my amp not produce a sound from the instrument's signal?

After thinking I found the problem, fixing it, and testing the amp, it still only makes a slight humming sound like most solid state amps. It gets quieter and louder with master controls, and equalization controls alter the pitch of the hum, though you can't hear any signal from the instrument at all even on the highest settings.

Please note that I have already inspected how the input jack and its cable's connection is; tight and sturdy. The solder on its contacts seemed to be a little poorly done, by whoever touched it last. I cleaned them and redid it myself to be positive the connections were good. Another thing to keep in mind is that changing the DSP control between 'bypass' and any active setting has no affect on the sound.

Also, I've gone through the main board several times with my multimeter and haven't been able to find any anomalies in components. The speaker wires themselves have been used to test the speaker on a separate, working amplifier, so I know they are in working condition.

At this point, I don't really know what to look for. Any tips/suggestions would be greatly appreciated!

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Did you try a know working instrument? It's kind a matter of elimination.

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Yes, I’ve tried several different working instruments as well as cables.. Thank you for your comment

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If all hardware connected to the instrument is good, and the output is working as you said, a signal tracer can help find the point the signal is lost. If there is a burned smell, that is a good starting point for a signal tracer. Be sure to stay away from power supply. A plastic probe can be used to poke around on the printed circuit board. With amp on, instrument connected and near speaker, probing around on the board with a plastic probe can help find a cold solder joint or a crack in the board. As probe closes a broken connection, you will hear feedback. Cold joints and cracked print is hard to see and you might need to apply medium pressure with your [plastic only] probe to close the connection. If you find the point of signal loss, a magnifying glass might help to check for a cracked board. Large items soldered to a board are a likely place to find a crack or broken solder joint. You can also shine a light at one side of the board and look from the other side to spot cracks .

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you could try checking for bad opamp/buffer chips near the input jacks.

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