Start with a good cable tester. but use De Oxit on the cable first. If the cable is good spray every possible contact with De Oxit (AKA) contact spray. Follow the directions on the can and repeat several times. My bass player swore that his speaker was blown. This spray applied and worked into and onto all cable plugs and all input jacks (instrument and speaker cable) did the trick. As I was doing this, he was insisting that it wouldn’t work. He was absolutely dumbfounded when it worked like a charm. Be liberal with it and repeat if necessary. I used it once on my guitar input and it didn’t work. I brought to my guitar tech. He did the same thing I did with the deoxit 4 times in a row and it’s been working ever since. You spray it on, plug, unplug, twist etc and repeat 3-4 times if needed. Works wonders on volume and tone pots to. Mixing boards amps etc.