There is nothing more frustrating than the fault which comes and goes. Your guitar works perfectly all through the practice and warm up and then cuts out just as you do your big solo. How do the gremlins know when to get you? I can't anticipate every fault but some things go wrong more than others and I've fixed most things over the years (I am very old) so here's a guide to finding and fixing most of the common faults. This guide is aimed at someone who isn't scared to use a soldering iron or who has a mate who can help but it might also help save some money in the repair shop if you know a little about what is going on inside your gear. Any intermittent fault will be a so-and-so to find. Just as it will go wrong at all the most embarrassing moments it will work perfectly the moment you want to cure it. You need patience, be methodical, eliminating one thing at a time. Don't make any assumptions and keep an open mind, you should always consider the possibility that you have more than one fault.
You have three things to worry about: a guitar (mic, keyboard, whatever) an amp and the lead that connects them. Suspect the lead first, eight times out of ten it is the lead that fails. Because the fault is intermittent we are looking for a broken connection. Modern components don't fail very often and when they do it usually leads to complete failure of the system and not something that comes and goes. Even when components fail it is usually because the connecting wire fails where it joins the body of the component. The exception to this is in valve (tube) amps. Valves have a relatively short lifespan but when they fail they do not usually come back to life again. There are strands in the forums and columns on valves so I will leave valve failure out of this. I am assuming that you are not using any pedals or effects and that if you have active electronics in your guitar that you have checked the battery.
Finding the Fault
Always suspect the lead first. Every time you move the lead is wiggled and the cable pulls a little on the connection inside the plug. You can break any wire eventually just by wiggling it and in the end all leads break. If you cut out at a gig change your lead straight away (you do always carry a spare, don't you?) at the end of the gig mark the lead you were using with some tape or by tying a knot in it so you will know which one to fix. If you don't you will get it muddled with your good leads. If using another lead fixes the fault permanently then the lead is at fault. I'll tell you how to fix it later.
The next stage is to try the guitar with another amp, if the fault goes away permanently it's the amp, if not it's the guitar. If you don't have a spare amp then try a different guitar. If it is in either the amp or guitar suspect the jack sockets first as these fail quite often. Every time you push the plug in you are pushing dirt in and wearing away the contacts. Every time you wiggle the lead you wiggle the plug inside the socket and eventually the contacts fail. If the socket is failing then wiggling the lead near the socket usually makes the fault appear or makes the amp crackle. It is worth double checking the lead at this stage; it is easy to muddle these two faults.
The other symptom to think about is crackling. Does this happen when you are adjusting the controls? This is usually caused by a failing pot (potentiometer) which is what we call the twisty things on the other end of the volume and tone controls. If you get crackling when you turn the controls then it is the pot. Inside the pot you have a long track with a wiper that slides along as you twist the control, varying the resistance as you go. Constant turning of the controls will eventually wear the track and often dirt will get in and break the contact. The other common problem with pots happens when the nut which holds the pot in place comes undone. The whole thing twists inside your guitar or amp damaging the connections. If the knobtwists more than of a turn this is what you've done.
The last things which you can test from outside are the switches. Wiggle them and see if there is any crackling or cutting out when there shouldn't be. Again the fault will be dirt, wear or a broken connection.
I'm going in Captain
If you've eliminated all the possibilities so far then it may be time for the repair shop. If you want to go on then a word of caution: amplifiers can have very high voltages inside them and can kill you. Not all of these voltages go away when you turn the amplifier off. Transistor amps in particular store electricity in huge capacitors that can stay charged for hours. Valve (tube) amps have extremely high voltages, much higher than the mains and the DC voltages in transistor amps feel much worse than mains shocks. Never work on an amp that is plugged in to the mains. I'm going to assume that you won't try poking around inside the amp unless you are used to working with electrical equipment.
Take the inspection plate off the back of the guitar. Put the screws somewhere safe (sticking them to an old magnet is good). You should see a little flat metal cylinder under each knob with three connectors sticking out of each, these are the pots. There are also little cylinders with coloured rings on them (resistors) and other electric components which can be a number of shapes and sizes (capacitors). There will be lots of wires connecting these all up. If your quitar is active there will be a neat little printed circuit board covered with mysterious things.
The first thing you are going to do is look very carefully at each component one at a time. Do they look damaged, have they come loose are they touching something they shouldn't. Most faults are visible but you need to look carefully.
Now plug the guitar into an amp which is turned down low. Gently wiggle each component and wire one at a time to see if you can make the fault happen. If it does then you've found your fault. If the fault becomes permanent at this stage then don't panic because now you know exactly where the problem is.
Still not found the fault? The next trick to try is freezing spray which many repair shops use to find faulty components. Spray it onto one component at a time making it shrink suddenly showing up any dry joints or broken components. Listen while you do this to see if the fault occurs. You can buy a spray for about 5 or less in the UK.
By now you should have located the fault, if not go back and start again. If you still haven't found it then I'm sorry but you've been really unlucky and you have a problem which is beyond this guide.
You should have found one of the following problems:
A broken or dry joint
A noisy or broken pot, plug or switch
A failed component
You should by now know exactly where it is but don't forget there may be other faults to look for later.
Dry Joints And How To Fix Them
This is the name for a joint which has not been soldered properly. It may look good but inside there is something stopping the joint from making a connection. I've written the guide for someone who knows how to solder but here's a reminder of how to do it just in case:
Separate the joint by melting the solder
Remove any excess solder with a solder sucker or de-soldering braid
Clean the separated halves by scraping or with emery paper if you can see any dirt on them
Now 'tin' the two halves of the joint by melting some fresh solder on each.
Hold the two halves together and join them with a little extra solder making sure that the solder on both halves has melted. Make sure the joint doesn't move until it has set.
The fresh solder contains flux which will etch away a lot of the impurities and chemically clean the joint. Just applying the soldering iron to a dry joint could seal in the muck which caused the original problem and means your repair may only be temporary.
Cleaning Switches, Sockets And Pots
Amps and gear generally often work in difficult environments, cigarette smoke used to be a killer and I've spilled more than one drink on an amp (it was dark, what can I say) stuff gets inside switches and controls and pots get dirty and noisy. Fortunately you can buy spray cans of cleaning fluid specially made for switches based on petroleum or iso-propyl alcohol.
Try spraying switch cleaner in the jack socket or inside the pots or switches then plug /unplug the jack a few times or give each of your controls a wiggle. See if this has cleared the fault after the cleaner has evaporated away. If not then try a couple more times. I've found that this has about a 70% success rate but that components don't always last very long once you start having to do this. Think of it as an early warning.
The other reason that pots switches and sockets fail is that the contacts inside them are usually made of nickel plated steel and overtime they corrode and or the spring weakens and they lose contact. Replace them, I have tried cleaning them and bending the springs but it is never very successful. It may get you through a gig but that's all you should expect.
Electronic components like resistors and capacitors are really cheap, usually only a few pence (cents) and rarely cost more than a set of strings, so replace them. If you need special manufacturer's parts then look in the stickies in the forums for suppliers. UG is a great resource so learn your way around it. Before replacing parts I suggest very strongly that you make a sketch of how it all looks before you pull anything apart. I always think I will remember and then find that I've left the job for a week giving me plenty of time to forget what I'd planned. Replace like for like, google resistor codes or capacitor identification to help you identify what you are looking at and make sure electrolytic capacitors are the right way round. UG gear building will help with most problems, look in the stickies.
I hope this guide will get you over the hump of knowing where to start. You must be handy or you wouldn't have read this far and you have nothing to lose by just looking. The worst that can happen if you keep out of the amp is that you have to pay a repair bill you would have to have paid anyway. Oh. and expect a little teasing from the repair shop. Most of the gear I repair has very minor problems and can be sorted with a spray cleaner or a soldering iron plus a little intelligence.