Intro: Replace a Volume Potentiometer in a Modern Guitar Amp
I did this repair at TechShop Detroit. All of the tools used can be found at the shop, including the bada*# electronics bench. The website for TechShop can be found here:
This Fender guitar amplifier was made around 2007. It is a modern guitar amp with a Printed Circuit Board, just like a computer (yuck...)
Anyway, a very ambitious rockstar friend of mine got excited and turned the volume pot past it's maximum and it broke. Upon examination, I realized that it just needed to have the pot replaced. I visited several electronics websites and ordered the wrong part several times. Finally, I just caved and ordered the official Fender replacement part ($17.00 on ebay!!! DANG!)
For this instructable, the supplies needed were:
Screw driver (phillips)
Screw driver (flat-head)
Weller Soldering Iron
Step 1: DO NOT TRY TO FIX YOUR OWN AMP UNLESS YOU KNOW WHAT YOU ARE DOING!!!
THE INSIDE OF A GUITAR AMP IS A DANGEROUS AND POTENTIALLY LETHAL PLACE
DO NOT EVER ATTEMPT TO REPAIR YOUR OWN AMP WITHOUT CONSULTING SOMEONE WHO KNOWS WHAT THEY ARE DOING.
That said, Lets continue...
Step 2: Expose the Guts
The first thing to do in this project is to open the back of the amp and expose the problem. I won't bore you with the details because this will be different for every amp.
Step 3: Access the Motherboard From the Back
I had to remove the screws holding down the motherboard to expose the underside. This is where the soldering is that holds all of the components onto the board.
The real issue with working on a modern amp is that like this fine example, all of the components are soldered right to the board. This means that to get at the underside of the board, all of the knobs and jacks must be unfastened from the chassis in order to move the board.
So, Off come the knobs, washers, everything holding the board to the chassis.
I used a tupperware dish to keep all the bits in so that I don't lose anything. It's disastrous to lose one little screw and not be able to put the amp back together!
Step 4: Remove the Old Part
Using a solder sucker is very simple. It has a spring loaded plunger that sucks air (or in this case, solder) when you push the button on the side. It takes the place of desoldering braid. The braid would have worked just fine, I just didn't have any at the time.
Using the soldering iron, I heated the joints where the pot was attached to the board, and sucked the solder out with the sucker.
Step 5: Insert the New Part and Solder
The old Volume pot was a Logarithmic taper 250k potentiometer. It took a while for me to find the correct audio pot replacement, and I eventually had to just buy the fender part instead of trying to source the cheaper generic part. In the future, I will just go with the actual fender part first because I wasted a bunch of time looking for a cheap pot.
I used the soldering iron to insert the new part. Make sure to heat the thing you are soldering (not the solder itself) so that you get a good joint.
AUDIO NERD ALERT
Quick bit about pots:
They are primarily either linear taper or logarithmic taper. This means that the rate in which they increase resistance is either linear, or exponential.
Logarithmic pots are used where volume control is necessary. This is because of a complex audio property involving decibels and I don't pretend to fully understand it, however the basics are that human ears percieve audio logarithmically.
Linear taper pots are often used in non-volume applications where a steady decrease in power is needed. Think light dimmer.
Since this was a volume pot, I went with Logarithmic taper.
Step 6: Put It Back Together and Test
Everything went back together smoothly. The new pot works great.