The season is changing, old guitars are coming out of cupboards, humidity is rising and jack socket are oxidising.
It's pretty simple really, if you've got a crackly jack it may just need cleaning.
Incidentally, it's an "output" jack, not an "input" jack - just a pet hate of mine.
You can see the jack in this Les Paul is looking a tad fluffy. This one isn't very bad but it's worth a clean anyway as part of a set up.
I use 600 grade wet'n'dry paper used dry.
I simply tear a piece off, roll it up and clean the jack out with it. A squirt with some contact cleaner can help. It's good to keep a piece in your guitar case in case your jack goes crackly at a gig.
You can see a fair bit of dirt can come off even this relatively clean jack. Just think what that was doing to your earth connection.
If a jack is really bad and the customer is gigging I usually just replace it. It doesn't cost much and it's something you just can't do without.
I am no longer repairing guitars - I now work full time making pickups