Yes! There is a Fix for the
Noisy Mic Input | Dell Inspiron 700m

 
 

Late-breaking news, and I don't yet have the photos available to show the procedure... but you can fix this problem with a little skill and about 10 minutes of time.

Introduction

The Dell Inspiron 700m has a design flaw at the microphone input connector. This leads to severe noise when using a VoIP app such as Skype, Vonage, etc. The problem is a design defect, not a manufacturing fallout. So replacing motherboards and updating driver software won't do anything to fix the Dell Inspiron 700m noisy microphone.

You could go out and buy a USB headset. A USB headset (Logitech, Plantronics, etc) would fix the problem, because the noise is coming from the analog mic circuit. That circuit has nothing to do with USB audio so a USB headset is one possible solution.

Since I want to use the same headset as my Motorola cell phone, and not buy an extra USB headset, I decided to fix the problem. Here's the solution...
 
 

Root Cause

The microphone input circuit in the Dell Inspiron 700m has no filtering on the microphone phantom power injector. This was an oversight of the motherboard's designer. The Inspiron's raw 5V rail is driven directly to the mic power lead, which brings with it all the noise of the disk drive and other parts of the 700m motherboard.

This problem will affect any powered mic plugged into the Dell 700m.

Setup

To fix the 700m mic input, you'll need a good soldering station (Metcal, with fine tips). If you don't have the right equipment, a rework technician will be able to do this for you. It takes only 5 to 10 minutes, from start to finish.

You will need the following:

  • Metcal soldering station with fine tips
  • Tweezers
  • Resistor, 0603, 680 ohms (+/- 30% OK)
  • Capacitor, 1206, 47 uF (+/- 30% OK)
  • Capacitor, 1206, 1 uF (+/- 30% OK)
 
 

Procedure

  1. Turn off the Dell Inspiron 700m and remove the battery.
  2. Flip the machine over, belly-up.
  3. Remove the user access plate under the Microphone / Headphones jacks. This will expose the circuits on the bottom side of the mic input jack.
  4. Remove the series inductor on the mic phantom power lead (see photo). This is an 0603 device connected to raw power.
  5. Install the new 680 ohm resistor in place of the removed inductor.
  6. Install both new capacitors between the 680 ohm resistor and the mic ground shield. They will both be in parallel, from the 680 res to ground.
  7. All done!