Resurrection of a Bose SoundDock iPod dock

I'd like to begin this blog by sharing something both useful and nerdy. Something about music and science and perhaps environmentally conscious at the same time...

Well, I was given a broken Bose SoundDock a while back.  It went "shhhhhhhhhhhhhhhhhhhhhhhhh"-(silence) when turned on.  The "sh" noise lasted only a few seconds.  No music, no sound once it fell silent.  Given the option to fix it and keep it or throw it away, I figured it might be worth fixing this beast (priced at ~$230, but worth nothing as delivered).

In this first post, I'll tell you about how I turned this trash into treasure.

Patents

Incidentally, while I was reading the label to figure out what this Bose product was officially named, I couldn't help noticing U.S. PAT. NOS 7,277,765 and D514090 on the bottom.  Thanks to Google's great Patent Search, I could have a good laugh at the bullshit contained in these documents.

The second one, "Sounds system for portable music player," is a design patent and makes just a brief claim:

The ornamental design for a sound system for portable music player, substantially as shown and described.

It contains several figures which I suppose secure Bose's design IP, including FIG. 1 shown above.  Such short claims do not seem unusual for design patents, such as those for this popular beverage maker, hideous but somehow unique grand piano designs 1 and 2 (good one Zeiser, Mr. Von Rohl will be really upset he didn't think of that!), and the curved body of cheese made of offset slices of cheese... yeah.

The first patent "Interactive sound reproducing" contains a little more content.  Its abstract describes something about an audio system that attaches to a computer...

ABSTRACT

An audio system attachable to a computer includes a sound reproduction device for producing audible sound from audio signals. The sound reproduction device includes a radio tuner and a powered speaker. The audio system further includes a connector for connecting the sound reproduction device with a computer. The computer provides audio signals from a plurality of sources, the sources including a computer CD player, digitally encoded computer files stored on the computer, and a computer network connected to the computer. The sound reproduction device further includes control buttons for controlling at least one of the computer CD player, the digitally encoded computer files and the computer network.

Let's see if this makes sense with what I know about the SoundDock:

• "attachable to a computer" > you can stick your iPod onto its connector, I guess.
• "producing audible sound" > not anymore!
• "includes a radio tuner" > not that I'm capable of finding.  You?
• "powered speaker" > check.
• "connector" > iPod connector, yes.
• "audio signals from a plurality of sources" > yes but not the CD source (thanks for killing the CD, Apple).
• "control buttons for controlling at least one of the computer..." > it seems to me like there are only volume buttons.

Well, those patent claims are dubious... but so is the functionality of their product!  Ok ok, on with fixing it...

Diagnosis

The first step when trying to fix electronics is to open them up and check for things that look broken.  Pretty obvious, but you should be aware of this: electronics run on smoke -- they will continue to operate normally until they have released all of their smoke.

The telltale signs of smoke leakage were obvious on the main board in the bottom of the unit:

The affected IC was made by International Rectifier and probably does some power reg function or other.  At any rate, the part number was totally burned off and rendered unreadable.

My guess was that the big computer-y looking chip with the bajillion pins on it did something smart and complicated and that I would have very little hope of fixing this board.  This computer-y chip probably talked to the iPod and allowed the control of its volume output or something relatively useless with regard to basic audio amplification functionality.  I thought that somewhere deep down in this SoundDock, there would have to be some simple amplifier whose workings I could understand and whose analog input I could just highjack.

Pulling off the front screen and unscrewing the heat-sink panel that lies between the two speakers, I found the board with the power amplifier on it.

The amplifier is a Philips TDA8922 2x 25W Class D power amplifier.  (Class D amplifiers achieve high efficiency using a neat pulse width modulation scheme)

Consulting the Data Sheet for the TDA8922, I noticed that pin 6 was a "MODE" pin used to select standby, mute or operating modes.  This is a common feature in Class D integrated circuit amplifiers since it allows them to be turned on/off by some control voltage to save on standby power.  I looked for the existence of a "MODE" pin because the "shhhhhhhhhhhhhh"-(silence) behaviour tipped me off: the power amplifier was clearly not broken (it made a "shhhh" sound), but something might be turning it off -- perhaps the smokey computer-y circuit discussed above.

Holding an oscilloscope to pin 6 showed that the voltage was around 5V while "shhhhh" came out, and then it dropped to 0V.  The MODE pin controls three regimes of operation:

• Standby 0V - 0.8 V 
• Mute 2.2 - 3 V
• On 4.2 - 5.5 V (maximum voltage tolerated by MODE pin = 5.5V)

The strategy I had in mind for fixing the SoundDock by this point in the diagnosis was

1. Remove the connection to MODE pin 6 and allow it to be controlled manually by a switch (i.e. force the amplifier into the operating mode)
2. Figure out where the analog (music) voltages are in the circuit board and make them accessible by a connector.

Fixing it!

Mode Pin

I checked if forcing the MODE pin to be "on" helped by constructing this voltage divider to provide 4.9 V to pin 6.

The input impedance of pin 6 seems to be on the order of 5.5 kOhm = 5.5 V (max input voltage) / 1 mA (max input current).  The Thévenin Equivalent resistance of the above circuit is 0.36 kOhm (if I calculated it properly), so it should be able to supply enough juice to drive pin 6.

Here's what the resistor divider test apparatus looked like.

The red wire connects to pin 6 (the leg of pin 6 was ripped up with the help of a soldering iron and some tweezers so as to be disconnected from the original printed circuit board signal).  The resistors connect to +18 and 0V pads that I happened to find nearby (see description of J601 later on).

With the resistor divider in place to force the amplifier to be "on", I touched audio input pins 4/5 and 8/9 with some tweezers -- sure enough the amplifier clicked and buzzed, so it worked.

Finally, I transplanted this resistor divider near the power entry on the main board and took the red wire 4.9 V output and connected it via a switch (shown later on) to pin 6.  (if you want details on how to do this, let me know and I'll take it apart again and take extra photos... I'd feel honoured if you were interested in it)

Audio connection

The Philips TDA8922 has analog inputs on pins 4/5 (+/- in) and 8/9.  5 and 9 seem to be referenced to the circuit ground, so effectively, we are looking at connecting audio signal to pins 4 and 8.

These audio signals travel from the main board (with the fried chip) to the power amp board (seemingly working) through a flexible flat cable.  The connector on the main board is J601, for which I have mapped out the following connections:

After identifying locations on the main board corresponding to the L audio, R audio, and ground signals shown above, I soldered some wires to them in order to inject some audio from a 3.5 mm headphone jack to be added to the front panel.  Here, blue = LEFT, red = RIGHT, black = GROUND.  Note how I cut the printed circuit board traces for the original L and R signals coming from the computer-y chip... follow from the point at which the wires are soldered and move slightly to the right - the traces are cut using a sharp knife.

The Result

Here's a view from the bottom of the modified SoundDock:

On the left, you can see a switch which manually sends 4.9 V to the MODE pin of the Philips TDA8922 to put it in "on" mode.  This is useful so that the speaker system can be turned off when plugging/unplugging things from the audio input to avoid loud clicks.

On the right, I have installed a 3.5 mm headphone jack which brings the left, right, ground (BLUE, RED, BLACK wires) to the places you saw above to inject audio signals into the circuit board.

The front panel of the BOSE unit had just enough room under the base to accommodate the new switch and jack... almost like it was built for this in mind ...

Final Notes

It's been working perfectly well for the last 4 months.  It's worth noting that the original iPod docking connector still works for charging, but since I disconnected the audio connection from the computer-y chip, it can't play audio through that connector anymore.  Another issue is that there is no volume control - you must be able to set the volume on the device you're connecting to it (a minor hassle).

One could consider wiring up the analog L and R audio pins from the iPod connector to the same place I showed above for the 3.5 mm jack.  This would allow for full music playing functionality via the attached iPod / iPhone - but you'd still have to set the volume on the device itself.

So far, I've found the 3.5 mm jack to make this unit more useful than it was originally designed to be:  I can plug in a laptop or iPad or other analog audio source now!

In the future, I might venture into adding a volume control knob on the front panel.

For now, I hope this helps you save a few tons of CO₂ emissions by giving new life to that decent sounding but completely broken SoundDock you might have lying around.