Compaq iPAQ 3650 Modifications to
Support an External Microphone

Eric Woudenberg, Speechworks Int'l
eaw@speechworks.com

This work was funded in part under the
DARPA Communicator Project,
Contract N66001-00-1-8958

Summary

The iPAQ 3650 crams substantial computing power and a cool display into one tidy, handheld package. For speech recognition work however, the inability to connect a close talking (i.e. head-mounted) microphone to it is an unfortunate limitation.

These instructions give a procedure for permanently disconnecting the unit's built-in microphone and converting the existing stereo headphone jack into a combination mono-headphone / mono-microphone jack.

Caution

This work is not for the faint-hearted. You will be cutting and soldering on a delicate printed circuit board. Please proceed carefully.

Equipment

X-Acto knife with pointy tip.
Torx screwdriver, size T6 (I used a Radio Shack Torx Set).
Ohmmeter
Fine tipped soldering iron (I ended up purchasing a Hako Soldering Station to do the work. I'm not sure quite such expense is necessary, but once you look at the circuit board you will see that it requires quite a fine soldering tip.)

Procedure

Turn off the battery

Slide open the small hatch cover near the DC power connector and switch the battery off. This will erase all data in the unit, so make sure you have backed up anything important.
Open the unit
- Dale Coffing wrote detailed disassembly instructions that may also be helpful.
- We made a small movie of the iPAQ being opened (400KB).
1. Remove the stylus from the body of the unit.
2. Remove the 4 Torx screws on the back of the unit with a #6 Torx screwdriver.
3. Push a knife blade carefully into the crack between the two halves of the case and pry them apart (I use the can opener attachment on my Swiss Army Knife). Good starting spots are just below the RECORD button, and on the right side of the case opposite the RECORD button. Work your way down the crack gingerly, prying a little at a time.
Pull the back off

Carefully pull the back off. The battery flex-cable will generally pop out of its plug. If it doesn't, gently lift it out.
Locate and cut the circuit-board trace to the bottom-most headphone jack contact

Find the circuit board trace leading to the bottom-most of the 3 headphone jack contacts and carefully cut it so that the contact is disconnected from the circuit:

Check the resistance between the contact and the trace, it should be in excess of 1k ohms if you have successfully cut the trace:
Desolder the built-in Microphone

Carefully pull back the rubber boot holding the microphone to expose the terminals. Desolder the white and black wires (a third hand may be helpful here).
Solder disconnected microphone wire to headphone jack

Carefully solder the wire you just removed onto the headphone jack. The white wire (center conductor) goes to the bottom contact (the one whose trace was cut), the black wire (outer shield) goes to the top contact (ground).
Reassemble the iPAQ
Make sure that the stylus spring, stylus latch and battery switch door are all in their correct position. Holding the back over the unit, re-attach the battery connector plug:

Carefully reassemble the iPAQ halves, ensuring that their tops mate properly, and dealing with the expansion sleeve connector and bottom part last. Push and snap the pieces together, working your way down. At the bottom, you may find that the expansion connector and the plastic lip on the bottom of the back hang up as you try to push the halves together. I use the stylus to reach in through the socket cutout in the case and pull the expansion connector into position. Then I use my thumb to push and force the plastic lip on the bottom of the back into its correct position.
We made a small movie of the iPAQ being closed back up (2MB).
Test
A standard cell phone headset may be used, although the plug size is different. The iPAQ has a 1/8th" jack, cell phone headsets have a 3/32" plug. An adapter is available from Radio Shack to accomodate this difference, it adapts a 3/32" plug to a 1/8th" jack.
With the cell phone headset plugged in, you should be able to use the built-in recorder application to record and playback memos.

Problems

The first time I tried this, it did not work. I took the iPAQ apart, checked the wiring, and reassembled it. After that it worked fine.

I have noticed that after making this change, the external speaker is quite faint. I use the headset exclusively so this is not a problem for me. I suspect that the connection between the other side of the 3rd headphone jack contact and its associated circuit must be cut in order to totally isolate the microphone circuitry from the speaker circuitry.

User Comments

From Vit.

The headphone jack is conected to the two items: the AUDIO_OUT output and the internal speaker input. The speaker is connected to the AUDIO_OUT through the jack contacts - if there is anything plugged into the jack, the jack disconnects the speaker. I have understood that you have cut only the PCB traces jack<-->AUDIO_OUT. This means that the speaker is connected to the jack, but not the AUDIO_OUT. Thus, when there is nothing plugged into the jack, you got the speaker connected right to the MICROPHONE_IN. (Perhaps the speaker may then work as a internal microphone, but certainly not as a internal speaker.) If you have a plug pluged into the jack, then the speaker is diconnected completely. You would have to cut also the PCB traces speaker<-->jack and solder the wires speaker<-->AUDIO_OUT in order to have speaker properly working and to have the jack for the external mike as you wish.
The jack's contacts are loosing their buoyancy if heated. Therefore you need a quality soldering machine with a fine tip, and a sure hand to solder for a short time and only once. Anyway you are undertaking a risk that the contacts will loose their connecting ability (it may look like that sometimes the jack does not work, you touch it and it suddenly works - I have experienced this).