Guitar Electronics Repair
Volume & Tone Pots (Potentiometers)
A scratchy sounding pot could be a sign that dirt or corrosion is effecting the contacts inside of the pots casing.
Pots can be cleaned by spraying into the
open area of the pot below the soldering lugs.
In order to clean the pot you will need to spray contact cleaner into the opening of the case.
I prefer DeoxIT as it comes in both a spray, which adds pressure to "blow" junk out of the case, and a bottle, which provides precise application when trying to avoid a mess.
Now turn the knob repeatedly to allow the cleaner to thoroughly "scrub" at the contacts.
Badly worn or defective pots will remain scratchy or intermittent regardless of vigorous cleaning and should be replaced.
Common Replacement Pots
"Pot" stands for potentiometer. Volume and Tone pots are the same component but a capacitor is soldered to the ground lug of the tone pot. This cap prevents a certain amount of treble from grounding out.
Using The Right Potentiometer
- Linear and Audio
- Resistance OHM's (250K, 500K, 1meg etc.)
- Split shaft or solid shaft
- Long thread or short thread
- Size (Mini or Standard)
When replacing a volume or tone pot you need to know the specs of the pot you are replacing.
Tone vs. Volume Pot
Many manufacturer's use the same pot for both volume and tone. Others may use Audio Taper for volume and Linear Taper for tone.
Linear vs Audio
Linear pots are usually marked with a B or Lin (examples 250KB, B250K, 250K Lin). Audio taper pots are usually marked with an A or Aud (examples 500KA, A500K 500K Aud).
Using an OHM meter attach one lead to the outer soldering lug and one to the center lug. With the pot rotated to center the resistance will equal to 1/2 of the pots total resistance if it is a linear pot.
OHMS (Value/Resistance) 250K, 500K, 1MEG
Pots are given values according to their resistance which is measured in OHMS. Check the wiring diagram or original pot casing for value.
Checking A Pots Resistance - Using an OHM meter, attach the leads to outer soldering lugs. Rotate the pot fully to measure resistance.
Long Shaft vs. Short Shaft
Long shaft pots are often necessary when the pot is passed thru the actual top of the instrument. (ex. Les Paul). Short shaft pots are used when mounting directly to a thin pickguard. (ex. Strat)
Split Shaft vs. Solid Shaft
The type of shaft used on a pot dictates the types of knobs that can be used. Solid shaft pots are ideal for knobs which are secured by a set screw thru the side. Split shaft pots use knobs which press on.
Solid & Split Shaft Pots
A small brass sleeve can be used on a split shaft pot to accomodate
screw on knobs.
While you could use a mini pot in place of a standard size pot, these are most often seen on instruments with active electronics and cramped control cavities. .
Serve as a traditional rotating pot as well as a switch which is actuated when pulled up and pushed down.
Multi-function pots such as push/pull and stacked or concentric pots add an additional function to the pot. They can be used when limited space is available or no additional holes are desired.
Removing press on knobs with rag
Removing press on knobs with guitar picks for leverage
There are a number of different switches being using on electric guitars today:
- 3-Way, 4-way and 5-way Selector Switch
- Toggle Switches
- Push Pull Switches
- Slide Switches
- Multi-function pots
Worn switches can suffer from a variety of symptoms including popping and scratchy sound, sloppy movement and intermittent or total loss of connection. Because some of these can also be symptoms of a loose wire or corrosion the switch should be cleaned with contact cleaner and all wiring connections checked.
Replacing a bad switch can be as easy as removing the wire from the faulty component and soldering it to the same lug of the replacement part.
If the connections of your replacement part do not match the original switch make sure you are using the correct part. The black 5 way switch on the right is often found in import guitars. While I recommend upgrading when possible, some switches will not mount up properly due to differences in screw layout.
We have two common problems with output jacks, one is a broken wiring connection. (See below for connections). This happens easily once the jack becomes loose as it can twist just enough to pull the wires loose.
The prong often looses
A second common problem we encounter is a poor mechanical connection. When inserting a cord into a jack, the click you feel is the tip of the cord seating against the metal prong on the end of the jack. With use this prong may spread outward and loose a bit of it's tension. A gentle bend of the prong may be just enough to create a solid connection, however, metal fatigue can dictate the need to replace.
On a mono output jack you will find two soldering lugs, one for the ground (frequently black), the other for the hot wire. The hot wire is often coming from the center lug of the volume pot. When viewing an output jack you can follow the lug's path ...the hot wire lug will lead to the prong, the ground lug will lead to the center of the jack.
Most often this is a problem on vintage instruments who have seen better days or those that have been improperly repaired. Loose wiring, cold solder joints, broken connections and burnt insulation can all cause problems.
By far the most common wiring problem is broken wiring at the jack. The input jack must be kept secure or it may begin to rotate when the cord is installed and removed. Eventually a loose jack may twist the wiring until the connection is broken.
Cold solder joints can be a bit of a mystery as hazy dull solder is not always apparent.
Check each wire for bare spots and missing insulation to insure it can not ground against another component.
The web is a great resource that can give you the basics on soldering but if you doubt your ability I would of course recommend letting a shop handle it for you.
When soldering parts on your instrument make sure to cover the body and protect the paint as solder loves to spit and pop. A hot blob of solder on the paint makes a nifty crater you may not want.
I have personally used Weller Soldering irons for all my soldering needs but regardless of brand I recommend choosing one with temperature control which can help prevent overheating delicate components. Other handy features are tip replacement and a secure iron resting place to avoid accidents.
Small alligator clips
serve as heat sinks.
Heat sinks (a.k.a. alligator clips) can be used to prevent heat from damaging vulnerable capacitors and other components. I will place a small alligator clip on the capacitors wires which will dissipate some of the heat that would normally pass straight to the capacitor.
A bare area on a wire can ground to other metal components and create shorts, be careful to avoid burning the insulation on other wires when soldering in cramped cavities.
Cold Solder Joints
When making solder joints to switches and pots, the lug and wire should be heated by the tip of the iron and the solder pressed (or flowed) onto the joint. In this manner you can avoid cold solder joints as both components are properly heated prior to the application of solder. Melting solder on the tip of the iron does not insure the actual components are being heated properly.
Electric guitar pickups can pickup frequencies from many sources. Most common interference is caused by florescent lighting, computer screens, power pack adapters and other sources.
Shielding lines the routed cavity as well as the back of the pickguard.
A "tab" is used to complete the connection from cavity to pickguard.
Shielding an instruments electronics cavity is one way to reduce interference and 60 cycle hum. Single coil pickups are notorious for humming and shielding may help to improve the situation. There are some pickups however that are just plain noisy and show little improvement regardless of effort (short of replacing them.)
Shielding is done by lining the instruments control cavity with special adhesive backed copper foil, though heavy duty aluminium foil sprayed with adhesive can also be used.
When overlapping multiple pieces of shielding a drop of solder is used to ensure continuity.
A "tab" of shielding is brought over the edge of the cavity which serves as a bridge to connect the shielded cavity with the shielded pickguard. I place this tab over a screw hole so the screw pulls the two pieces of cooper together.
Many web surfers contact me looking for a wiring diagram for an unusual / no name / import guitar after having no luck online. And sometimes you aren't going to find it, however, if you have an electric guitar that is similar to lets say a Strat ... it has 3 single coil pickups (and they are 2 wire pickups), one 5 way switch, 2 tone pots and 1 volume pot then you can simply use a Strat wiring diagram. It's often easiest to think of the instrument in terms of components not brand.
Sources of Wiring Diagrams
Electric Guitar Repairs
- Action / Set Up's
- Bracing (Archtop's)
- Bridge & Tremolo
- Buzzing - Noises
- Convert Rt. to Lt.
- Fret Replacement
- Fret Wire/Types
- Neck Problems
- Neck Set / Angle
- Part Glossary
- Strap Buttons
- String Changing
- String Choices / Effects
- Truss Rod
- Tuning Machines
- Tuning Troubles