112 lines
4.4 KiB
Markdown
112 lines
4.4 KiB
Markdown
---
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title: "Make Your Ears Bern"
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date: 2018-01-11
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lastmod: 2019-01-16
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categories: ["Blog"]
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tags: ["electronics"]
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---
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A colleague offered a pair of Bern Bluetooth drop-in headphones to me fore free,
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with the catch being: _I had to fix them_
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<!--more-->
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# Don't Turn It On, Take It Apart!
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Past mistakes have taught me to be gentle and patient when it comes to taking
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things apart. This was no exception either. After looking over the unit on each
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side, I figured the only way *in* was lifting the mesh cover off. So I went at
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it, carefully, with a pair of tweezers. I worked my way around the edge and
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wedged the mesh upwards.
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{{% admonition info %}}
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{{% /admonition %}}
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# Okay, Maybe Turn It On
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Now that the problematic speaker side was successfully opened without any
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damage, it was time to investigate what was wrong.
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I played a song via smartphone on the speakers. The result was as expected: _the
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right speaker put out no sound._ I checked the known-good left speaker using my
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**Rigol 1074Z** oscilloscope. This may not have been entirely necessary, but I
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wanted to find out what to expect when troubleshooting the right channel.
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{{% admonition info "Left Speaker" %}}
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{{% /admonition %}}
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{{% admonition info "Right Speaker" %}}
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{{% /admonition %}}
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Knowing what to expect on the oscilloscope, I hooked up the probe to the right,
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problematic, speaker. The result was much different, indicating either noise or
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an open circuit. It may be worth mentioning that the right speaker was
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disconnected at this point in time to ease the troubleshooting process.
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# Where Did It All Go Wrong?
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Lucky for me the PCB pads were labeled -- even better `SPKL+` (_left_) and
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`SPKR+` (_right_) were easy to find.
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{{% admonition info %}}
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{{% /admonition %}}
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Outside of the bluetooth board hidden under the piece of tape, there's not a
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whole lot going on in the circuit. It was my guess that the visible surface
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mount QFN chip was most likely the op-amp used for the speakers. A quick Google
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search of `AIWI TI` (_as shown in the photograph_) resulted
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in [the following datasheet](http://www.ti.com/lit/ds/symlink/tpa6132a2.pdf)
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which verified that to be the case.
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<center></center>
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**Bingo!** Now knowing the pinout, I could use my trusty multimeter (_a Fluke
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115_) to test continuity of the circuit from the known-good and the now
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known-bad speaker traces back to the `OUTL` and `OUTR` outputs of the amplifier.
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{{% admonition info %}}
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{{% /admonition %}}
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Removing the board from the housing required a bit of finesse. I didn't want to
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bother desoldering the left speaker connections to make removal easier. So, with
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a bit of gentle back and forth I was able to get it the PCB out and inspect
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traces on the bottom side.
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# Something's Not Quite Right...
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Continuity from `SPKL+` to the QFN pin was good, yet `SPKR+` to the op-amp
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showed open circuit. Visibly, everything on the PCB looked fine. There were no
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apparent signs of damaged or lifted traces, nor bad soldered wires or
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pins. Somehow the trace shortly after the chip was damaged in a way that
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resulted in an open circuit at the point of the right speaker's solder pad.
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After a few minutes of scratching my head and repeatedly going over the
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datasheet to check for any misunderstandings on my part, I realized the cause of
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the issue didn't matter so much. The objective was to fix the unit. I simply
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needed to re-establish the connection for `SPKR+` to the chip.
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Using the 3.5mm mini-jack's solder pads, I found continuity to be true from the
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chips left and right outputs to the conveniently accessible solder pads. _A
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bodge wire was in order_..
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{{% admonition info %}}
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{{% /admonition %}}
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# All's Well That Ends Well
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Again, using my trusty Fluke 115, I verified continuity from the chip's `OUTR`
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pin to `SPKR+`. Lo and behold it was now closed-circuit! I was very happy to see
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the expected waveform from the known-good left channel now also appearing on the
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right channel.
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{{% admonition info %}}
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{{% /admonition %}}
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At this point I quickly re-soldered the wires to the speaker and enjoyed music
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now coming into both ears!
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