The M-Audio FastTrack is a microphone preamplifer with a guitar/line input, an analog-to-digital converter (ADC) and a USB interface to the host computer.
My friend's M-Audio FastTrack had developed a fault that made the microphone channel pick up hum. Now, the microphone input on a preamp like this is balanced, which means that it is designed to minimise hum. This works by feeding the microphone signals to a differential amplifier, so that any hum that is picked up will cancel out, leaving just the audio signal to be amplified further.
In this case, one side of the differential amplifier had failed, leaving the other side to amplify the microphone signal and the hum. When testing, one side of the XLR input connector (pin 3) would pick up hum when I put my finger near it, but the other side (pin 2) did not. This led me to suspect that the operational amplifier chip (op-amp) had failed.
The unit can be opened up by unscrewing five small screws from the underside of the case. It then opens up into the two case halves and the PCB with front and rear panels attached. All the connectors and controls are soldered to the PCB.
We obtained a replacement op-amp chip, LMV722, from Farnell, order code 175-4976. It's an 8-pin surface-mount chip, but not too difficult to solder with a fine tip, good lighting and magnification.
Photos of the chip removal and replacement are in this Flickr set.
Reassembly is the reverse of disassembly. Note that the case halves will only fit the PCB one way round; the fifth bolt is not in the middle, but closer to one side.