A faulty input jack on a guitar amplifier renders it nonfunctional. The signal from the guitar is either unable to pass through the jack or distorted significantly. There is little need to attempt to actually repair the faulty jack. The amount of time, effort and materials needed to diagnose and repair the issue far exceeds the cost of a new jack input. This is one of the few amp issues that can easily be repaired by guitarists without extensive knowledge of the workings of a guitar amp.
Plug in the soldering iron. It normally takes a few minutes before it is hot enough to be used.
Remove the panel to access the electronics inside the guitar amp. This is not necessary on open-back amps.
Unscrew the nut holding the jack in place to remove it. Two wires should be connected to it. The wire connected to the prong touching the metal ring on the jack is the hot wire. The wire connected to the prong touching the plastic ring is the ground wire.
Melt the solder holding the wires to the faulty jack by touching each with the tip of the soldering iron. Discard faulty jack.
Touch the tip of the soldering iron to the fresh solder to melt a drop of it on the tip.
Touch the end of the hot wire to the prong on the replacement jack connected to the metal ring.
Touch the drop of molten solder to where the wire and jack prong are touching. The solder hardens almost instantly once the heat from the soldering iron is removed. This holds the wire in place to the input jack.
Repeat Steps 5-7 to connect the ground wire to the prong touching the plastic ring.
Insert the end of the replacement jack into the hole vacated by the faulty jack. Screw the nut over the end to hold it in place.
Replace the panel on the guitar amp if you needed to remove it in Step 2.
Only use solder marked for use with electronics for fixing the input jack. Other types of solder can corrode or damage electronics. The tip of the soldering iron retains heat for at least several minutes after being unplugged. Do not touch the tip without waiting at least 15 minutes to give it time to cool down.