Bad input jacks are one of the most common problems with electric guitars. The jack is held in place with a single nut at the base of the guitar. Once this nut starts to come loose, the input will start to jiggle. This jiggling can break the two solder connections on the interior of the guitar and prevent transmission of sound from the pickups to the amplifier.
Humming or Buzzing
If, when you insert your cable lead into the input, there is a deep humming tone or loud buzzing sound, it may be an indication that the input ground wire may have come loose. The buzzing or humming is similar to when you touch the end of the lead to a metal object. There is no completed circuit so a feedback loop is created which is the cause of the sound. Resoldering the connection should fix the problem.
If you are not getting a humming or buzzing sound but instead are suffering from a crackle while playing, it is a sign that the positive connection has come loose. If you are still able to generate sounds from the strings, the connection is not completely broken. The crackling comes from a lack of signal, or dead point, in the electrical cord. This dead point causes a crossover from the negative to positive soldered connections and delivers a surge to the amplifier creating the crackle. A quick solder will fix this problem.
A no sound problem indicates a full break in the positive connection on the interior of the guitar. The negative connection is still secure or there would be a humming or buzzing sound. The lack of positive input will prevent any transmission from running through the wire because there is no complete circuit. Soldering may fix this problem but the jack may have burnt out and may need to be replaced.
Check the Cord
Before running out and getting a replacement jack, first check that the jack is the problem. All of the symptoms that can be attributed to a faulty jack can also be indications of a bad cord. Connect a different cord to the guitar and see if the problems persist. If they don't, then the problem is with the cord, not the jack.