Bose head unit repair turned to mod / hack....Suggestions. - Page 1
Forum section: electronics / projects

author: rmacintosh
author_rank: Regular Contributor
timestamp: March 30, 2017, 09:13:00 pm
content: About a year ago the 6-CD changer in my Nissan died....and by died I really mean jammed and would no longer respond. After a bunch of research and many hours of dismantling the changer and trying to get the mechanism re-home-ed I gave up on that idea. It was just too much of a mechanical nightmare, it was not built with service in mind. Making adjustments meant taking the entire thing apart, and testing that adjustment meant reassembling it again. It became a huge time sink and just wasn't worth the effort with the headway I was gaining each time.This lead me to search for a replacement CD changer module to swap into my head unit, but that was too expensive and I eventually found that this is an extremely common issue with this head unit and will likely happen again. From part of that research I realized that a few earlier models of this head unit had a din connector populated on the back that is an AUX IN port for whatever you choose. Turns out my stereo didn't have that port present but it did have the hole in the chassis where this connector would normally be mounted.Picture of said, not present on mine, DIN connector:After diving down the rabbit hole I found that the main board of the head unit has an unpopulated connector that the missing DIN connector would plug to. This seemed like a good idea at the time but after tracing where some of the pins go I found there were a number of passives not populated and a few transistors. This may not be a dealbreaker as I found that the 'brains' driving these pins were present, namely a micro and a pre-amplifier chip for the audio signals.The DIN connector has 8 pins, which I believe only 7 are used:COMB. ONAUX. INL+L-R+R-GNDSo my ultimate goal here is to hack this such that I can plug a standard 3.5mm audio jack for Aux in functionality on my stereo.I was able to trace out these lines and drew out the schematic for the AUX IN interface. None of the components are populated except for the green highlighted parts:One problem I have run into is what values to use for the unpopulated parts. I can probably get away with general purpose NPN PNP xistors, but for the caps and what seems to be balance resistors for the audio L/R +- lines I am not sure. I haven't done much work with audio before.My other question is how to convert my audio in (eg phone headphone jack audio - Left, Right, Gnd) to this differential audio input with L+ L- R+ R-?Full Size:http://tronicslab.com/wp-content/uploads/2017/03/IMG_0999-edit.jpgFull Size:http://tronicslab.com/wp-content/uploads/2017/03/IMG_0969-edit.jpgFull Size:http://tronicslab.com/wp-content/uploads/2017/03/IMG_0968-edit.jpg

author: moffy
author_rank: Super Contributor
timestamp: March 30, 2017, 10:54:14 pm
content: C233, C234 etc would probably be 100pf ceramic, looks like they are there to stop RF. C331, C332 etc would be of the order 10uf non polar electrolytic or tantalum(if there is such a thing). They will determine your low frequency roll off in combination with the input impedance. R236, R237 would be the same as R336. R791 say 10k. Transistors PNP(BC557), NPN(BC547). I'll let others fill in the other blanks.

author: SL4P
author_rank: Super Contributor
timestamp: March 30, 2017, 11:47:31 pm
content: After doing your hardware mods, I'd be more concerned whether the current software/firmware implements the extra inputs for your enjoyment.

author: rmacintosh
author_rank: Regular Contributor
timestamp: March 31, 2017, 12:25:27 am
content: I hear you.I had made the assumption after tracing out the circuit that they perhaps didn't perform any software modifications as evidenced by two of the few remaining resistors RA607 and R745. Those would hold the uC input pin high...assuming active low for the AUX IN enable line.Really, before doing the actual build I can quickly pull R745 and ground that pin to see if the stereo switches to AUX IN input.I may just do that after dinner.

author: rmacintosh
author_rank: Regular Contributor
timestamp: March 31, 2017, 02:30:13 am
content: Looks all good. The micro will switch to AUX input if I bridge the unpopulated R630 footprint to ground. If i switch over to FM/AM theres no way to get back to AUX so it definitely seems it is looking for a -ve edge to switch around audio inputs.

author: CJay
author_rank: Super Contributor
timestamp: March 31, 2017, 07:00:31 am
content: Interesting hack, i lovethis sort of thing, unmasking features and working out how to enable them.If the six disk CD mechanism is in the head unit can you not just find the DAC output and tap in there though?

author: rmacintosh
author_rank: Regular Contributor
timestamp: March 31, 2017, 04:40:40 pm
content: My head unit has the 6-cd changer built in as a stand-alone module that connects via a ribbon cable to the main board.I have the module removed as I wasn't able to repair it, but this is the pinout for that ribbon cable.I have seen mods where people have tapped into their CD changer by cutting the L/R ch traces on the changer PCB and tapping in their AUX IN at that point. What turned me off that is that it required a CD playing on repeat in the changer for that audio channel to be selected and not be interrupted at the end of the disc when changing to the next disc. That and I wasn't able to fix the changer.I could probably still tap in at that point without the CD module but I may need to fake a few more signals (RESET, CD.ON, P.ON, MUTE). SIM, SOM, and SCKM is only transferring track position time etc... and inputs for changing tracks/cds that I wouldn't need to bother with. The added benefit might be that it has a more familiar audio interface Lch Rch Sgnd as opposed to the differential audio input lines I am dealing with on the AUX input side.Choices choices...If I use the aux in I still have the option of fixing or replacing the cd changer and getting that functionality back.I doubt i will, im still bitter about what the jam did to one of my CD's.

author: rmacintosh
author_rank: Regular Contributor
timestamp: April 02, 2017, 06:27:38 am
content: So i've made a change of plans with this project.I decided to go with a RN-52 bluetooth module from Microchip to make my AUX in of my stereo a bluetooth audio streaming channel.http://ww1.microchip.com/downloads/en/DeviceDoc/rn-52-ds-1.0r.pdfThis module has the added benefit of having a differential audio output which will mesh nicely with my AUX in front end of my stereo. That being said, I still have to complete the circuit on my head unit to get anything to work. A bit of research and taking moffy's suggestions I have come up with the below so far...The COMB. ON line is maybe indended as some sort of wake up or power on signal for whatever was to be driving the AUX in input, once i find out what drives that output high i may be able to use it to my advantage.I've already determined that my stereo switches to AUX when I tie the AUX IN line high so I could use one of the I/O of the RN-52 to drive that line when it is connected to an audio source (phone) and ready to stream. I can figure out those missing passives pretty well.Im left with the 3-resistor network in the middle of the differential audio input circuit. Researching led me to find that it is a attenuator / attenuator pad configured as a "Balanced L-Pad"http://www.aes.org/par/a/QuoteQuoteNot sure what my driving and load impedances would be yet. the image above suggest values for <300ohm input and >20k output impedances. Then again, its presence may not be for impedance matching but rather simply attenuation.

author: rmacintosh
author_rank: Regular Contributor
timestamp: April 02, 2017, 06:41:12 pm
content: oops,failed to add the image where I said "this is what I have come up with so far"My only confusion is on that R630 resistor. I don't see a need to populate it, if I do it would need to be a fairly large value.I'm going to build this up once I get the RN-52

author: rmacintosh
author_rank: Regular Contributor
timestamp: April 08, 2017, 11:28:59 pm
content: Quick update on progress.....I built up a test circuit with the RN-52 and populated all the parts on the head unit as in the previous post. Tested on the bench and results seemed positive, could connect to Bluetooth with my phone and start streaming music.I was able to switch to AUX input tying AUX-IN high. I'll have to wire up a button in the final install and provide a short pulse to AUX-IN as that pin is looking for a +ve edge to switch inputs.I tried a test in my car next and initially I thought it didn't work but I simply had it too quiet to hear. Everything is working!!!!Just not quite as intended.I had my phone volume at full and my head unit volume was at max (32) and it was only at a comfortable listening volume. Normally if I had volume at 20 I'd be blasting my ears out. Either my signal is weak, I'll have to look at the data sheet and firmware to see if I can boost the output, that or maybe the u pad attenuator is doing too much .Getting closer, starting to hate driving in silence

author: moffy
author_rank: Super Contributor
timestamp: April 11, 2017, 03:02:25 am
content: Congrats!It's probably the signal level is too low, the input attenuator is only about 10db.