Step 1: Gathering Your Supplies.
replacement mic (make sure you get one that will improve the sound quality, condenser mics come in many different levels of quality and size and without careful consideration it is possible to degrade the sound quality rather than improve it)
tweezers or small needle nose pliers
Other helpful tools
files, drill, extra wire.
Step 2: Pulling Everything Apart
Step 3: Picking a Microphone.
Step 4: Switching the Mics
The reason for a positive and negative side to the mic is in how a condenser mic works. The microphone has a permanently charged side and a conductive diaphragm on the other side. As the diaphragm gets closer to and further away from the charged side, that charge pushes and pulls an electrical potential on the conductive portion of the diaphragm, turning sound waves into an electrical signal. Because the diaphragm is so small and can't move very far, the electrical potential this creates is very tiny, so most condenser mic capsules have what's called a FET in them, this stands for field effect transistor, and this transistor takes the miniscule signal created by the diaphragm and boosts that electrical potential to more usable levels. This FET need power though, so one pole of the microphone input will always be positive and the other negative, and the microphone has to be lined up so that the positive lead is powering the FET inside of the mic capsule.
The easiest way to determine which pole is positive and which is negative is to solder a pair of wires to the old mic terminal and lead them outside of the device, typically through the mic hole in the cover, so that way the device can be reassembled before being turned on. Once the cover is back on and the wires routed through, you can connect the other ends of the wires to your multimeter with some alligator clips. You'll have to turn your device on and set it to begin recording so that it sends power to the microphone leads, once you multimeter detects a voltage you can find out which lead is positive and which is negative. If the multimeter reads a positive voltage then the positive lead of the multimeter is connected to the positive lead of the microphone, if the multimeter reads negative then the opposite is true.
If you're feeling brave or in a rush like I was, you can try turning your device on while holding the leads directly to the PCB itself. I didn't have enough hands to get any pictures of this step but I would definitely recommend the wire method unless you're switching a really cheap budget camera.
Once you've determined the polarity, you can desolder the wires and connect your new microphone capsule in its place. Check the datasheet for your microphone capsule to find out which lead on the capsule is positive or negative. If you have trouble finding your datasheet or don't have a part number for the mic you're using, the lead connected to the metal case is typically the negative lead.
The process of attaching the new microphone will vary from one camera to another, but typically the mic is either held in place by solder to the board itself or is mounted to the case with wire leads to the board. My microphone capsule is simply soldered to the board so I held it in place with pliers while I soldered first one, then the other lead of the microphone.
In the case of my camera, there was a little extra work that needed to be done, the old mic fit in a groove in the top of the PCB, and the new mic was to big to fit in that groove. I tried placing the mic on top of that groove between the PCB and plastic case, but the mic was too thick, so I ended up using a file to widen the groove on top of the PCB. If you have to modify your PCB like this, just be careful and check your board for any traces near where you will be filing. Even if you leave enough copped trace without cutting the connection you could created a short circuit if the conductive metal case touches the newly exposed copper trace. depending upon the style of camera you may need to either modify the PCB or the case if your mic is a different size than the original. After I do a bit of experimentation with this and other cameras I'll add a few more steps for improving the sound and before/after recording of sound, both in the vehicle and in the closest thing I have to an anechoic chamber (padded storage room). Until then, I'd love to hear the experiences of anyone else who's tried a similar mod or any suggestions or requests for certain mods to include since I'd like to expand this instructable over time.