So what is a set-up? For starters its taking the instrument in the condition it is and making some notes about the current state of how it plays. By doing this you get a base line of what could be wrong.
Here are some general guidelines to let you know when your instrument needs a little help:
Chords sound out of tune: Intonation needs attention
Strings making noises when turning the tuning keys: Replace strings and inspect nut slots and saddle(s)/bridge condition
Seemingly stiff action when playing: Check string height and adjust via bridge, nut or truss adjustment
Constantly going out of tune: Tuning keys may be loose or intonation or a combination of gremlins
On Acoustics - if a "hump" appears below the bridge out of nowhere: Internal bracing may be loose or broken
Electrics- sound cutting out: check the wiring of the output jack or your cables or input on your amplifier
The list goes on and on. Troubleshooting is the name of the game.....If not "A" then "B" and so on and so forth until all the planets align and its back to business as usual for your instrument.
If you have questions on any weird thing that has you stumped - send me a message or give me a call and I'll do what I can to help. I have access to many many parts and accessories for practically any guitar or amplifier (past or present) out there.
And here is how to reach me (Keith the owner, repair tech, sales, marketing, accountant, advertising, web designer guy of Harbor Strings Instruments & Repairs)