 When all of the instruments and equipment are set up and soundchecked, if there are problems — crackles, hum, no signal from the guitar, no sound from an amplifier — the tech may have responsibility for troubleshooting to determine the cause or causes.Common problems include damaged patch cords, ground loops in connection between instruments and amplifiers, weak batteries in effects boxes or on-board preamps, bad vacuum tubes in tube amplifiers or overdrive effects, broken electrical connectors or solder joints, speaker voice coils damaged from the previous concert, or equipment damaged during transport.Tuning problems may come from old or dirty strings, damaged or worn machine heads or frets, or mis-adjusted bridges.