Symptom
When attaching an external microphone to the ThinkPad, it does not work. If the connector is removed part way out, then it appears to work.
 
Affected configuration
Any ThinkPad where one is attaching an external microphone to use instead of the built in microphone .

NOTE: Not all models have built in microphones. Refer to your User's Guide for your particular ThinkPad.
 
Solution
If you are using a condensor microphone, then it must provide own power, either through the use of batteries inside of the microphone, or through the use of an AC adapter. Pulling the connector part way out, the external microphone is no longer disabling the internal microphone which gives the appearance of the external microphone working.
 
Additional information
Following is an explanation of the ThinkPad microphone connector:
 
1. Some ThinkPads use a combined microphone or line input connector. Basically, the microphone jack is a 1/8" (3.5mm) stereo mini-jack, but differs from "standard" jacks in two ways:
 
a. There is a second contact that touches the ground ring when a microphone is connected. The purpose of the second contact is to ground out the internal microphone when an external microphone is plugged. This second contact should be of no consequence for external microphones and their connectors.
 
b. There is a metal ring on the outside of the connector (visible from outside the ThinkPad). This connects to +5V through a 2.7 kOhm resistor.

2. When using a condensor microphone, the microphone must either access the +5V ring or provide own bias voltage to work properly. Andrea Electronics (www.andreaelectronics.com) sells an "Auxiliary Power" device (that uses 2 AAA batteries) that works well with ThinkPads. Andrea also makes another device called the ThinkPad Adapter Model T-100), which allows the use of an external microphone without carrying an additional battery.
 
3. The microphone gain can be adjusted on ThinkPads with Mwave DSP by editing the MWAVE.INI file. The recording MIC gain is set by the parameter:
 
     RMICGAIN=430
 
The number corresponds to tenths of dB and can be increased from default by about 100 (10 dB).
 
4. Of course, the audio mixer control should be set for microphone on, line input off and set to maximum level. This will boost the gain of the input.