[Need help with] Car stereo hacking - Adding an AUX input

[Gleix]

So I'm curious if anybody here would have any suggestions for this idea of mine...

I'm looking for an easy solution to add an aux input to my car stereo.  I've hacked FM transmitters but still can't stand less than CD quality.  I'd love to be able to add a little 3.55mm jack to the dash so I can just plug in my mp3 player.

http://www.the12volt.com/installbay/stereodetail/1896.html

The stereo wiring diagram is there, but I'm not sure that would really help at all.

The idea I have is to remove the the car stereo, take a look inside, and figure out where the CD player output goes to on the circuit board, and take those points and wire them directly to a 3.55mm jack (with a resistor in line).  Would I be able to burn a CD with nothing but silence, but choose to play the CD and have the audio input from my mp3 player come out instead?

Or, if I looked up the datasheets for the amplifier chip in the stereo, could I just wire the jack up to the inputs?

Any advice is welcome.  I don't wanna buy an aftermarket system, just too expensive and I'm happy with what I've got.  Just want AUX support...

[Gleix]

Here's a large clear picture of the main circuit board on the CD player itself. The little connector with the red wires and one blue one goes to the main PCB for the stereo itself.



Can anybody gather some information from that picture? I see some interesting markings, like "OUT" "L CH. CD ON" and others, but haven't the slightest what to gather from some of these. I haven't yet found an amp IC or anything on the other PCB's.  I expect somebody like gordon to show up here and give me some pointers? 

Any guidance or advice would be appreciated. 

Points of interest...?:

<- Tried connecting both the "IN" and "OUT" boxes to a jack, using the "1" points as L and Right and the other points which are traced together as ground to no avail. I have no clue what I'm doing really, does this area look helpful?

[Gordonjcp]

I've done it with cassette player stereos - the (otherwise extremely good) factory stereos that came with Citroën XMs (I've had five of them) used to lose one channel of the tape preamp chip.  Since this handled tape/radio switching, Dolby decoding and tape de-emphasis and was entirely unobtainium I modded a couple of the stereos to have a radio/aux switch on the glued-down tape flap, and a 3.5mm jack.

You could try using a CD burned with silence as you suggest, and probe around the pins of the amp and switching chips.  Now what do I always suggest when you're hacking an unfamiliar machine?  Google all the chips!  Get a good look at what's on the board, tap it into Google and see if any of the datasheets suggest anything interesting.  There will probably be some sort of microcontroller that will be house-coded but the rest should probably be fairly standard parts, especially in the RF and analogue path.

[Gleix]

That's the thing! I'd been doing that as soon as I opened it up but everything seems to be a bit useless and I can't find any interesting chips that seem usable.  They're all either not on google, specific to this machine like you pointed out, or just something I can't use.  I'll keep digging around - There's got to be an answer. 

I looked in my dad's old audi stock radio and right off to the side there was a little 8 pin amp chip.  I was hoping it'd be as easy as that

[Gleix]

Trying hard to look through this huge blob of tan epoxy I saw an 8 pin chip... I very very carefully tried slicing away as much of the epoxy as I could, and got to the top of the chip..

m828
014
38617 

The problem now is getting around all this gunk and figuring a way to test the input pins... I'm sure removing all this epoxy AND soldering onto this surface mount component will be a task. I hope it works. I'll report back.

-----------------------------------

EDIT: UGH! How frustrating! I haven't ruined or broken anything, but despite all the datasheets I've found on this chip I'm getting NOTHING out of it. The output pins to a speaker = nothing. The input pins = nothing. Even wiring in ground connectly doesn't help.  Seems like finding this chip might have been useless.

I know that there are complpetely separate outputs on the back pin connector of the stereo itself... There's a separate wire that goes to the Left and Right of all 4 speakers in the car.  I figure there's a chip in there somewhere that splits up the audio channels? Gordon like you said, I guess I should keep looking for a switching chip.. I'm having absolutely no luck yet though.  I uncovered another chip covered in epoxy but the top of it got rubbed off in the process so I've no clue what IC it is :[

[Gleix]

I sent a message right to Gordon to ask if he thinks he could help, but I figured I'd also put this here if anybody else thinks they could help me out?

I found one of these hidden behind a big metal plate:

http://www.mouser.com/Search/ProductDetail.aspx?qs=9PdvrqCL7oCiMT3aBz4E6Q%3d%3d

Here's the datasheet: http://www.st.com/stonline/books/pdf/docs/8160.pdf

There's also a "Delco 23136" next to it which I believe may be of interest but I can't find a cross reference or any information on it.

I'm, when it comes to getting technical with electronics, quite unknowledgable.  It looks promising based on the pin assignments on the datasheet, but I've no clue what I should try and what to avoid if I was to attempt anything involving this chip.

If it sounds like an idea, I'm going to, once I get the chance later, try hooking up an aux jack to pin 13 which is S. GND and either input 1, 2, 3, or 4. First I'll try connecting a speaker to the output pins (like 9,7) and see if any sound comes out, and if it's empty, the radio, or the CD player.  I really don't know what I'm doing as I have no technical knowledge, but it seems to make sense to me.

That datasheet has so much information, I love the standard test application schematic. Helps out a lot.

[noystoise]

so your trying to add a stereo input to your existing car stereo? is the stereo a an am/fm receiver? or is that cd drive the whole thing? i dont have a lot of experience with car stereos, but the ones i have messed with usually have an aux in all ready to go right in the receiver. if i were you i would check the model of the car stereo and see if you cant find a manual for it online. the input might be right in front of you...

[Gleix]

It's a 1999 pontiac sunfire GT. The stock stereo has an FM/AM stereo and a CD player.

Also.. a breakthrough!

Well, this chip obviously can do something, though it lacks a lot in what I was looking for.

I connected my 3.55mm jack, playing music to ground and the inputs 1, 2, 3, and 4. These each went to separate speakers.  I then connected it to pin 10, which is labelled SVR on the data sheet and music comes through every speaker from my mp3 player.  The music comes through whether I'm using the CD player or Radio function on the stereo.

Thing is, I can't control the volume on the stereo's dash itself, or use any of the features (I can't choose balance, fading, use the EQ, volume knob, etc...) and the music lacks some mid tones (though I may just need to change capacitors). Along with that, it's not true stereo. The music is balanced from left to right, but instead of sounds that would normally be on the left speaker, they're simply quieter so you get an echo rather than a panning effect.

Any suggestions on where I would find a different connection that would allow me to use the dash functions (like volume) when using the AUX input? pin 10/SVR doesn't trance back to anywhere, just the chip itself.

Thanks in advance..

[SineHacker]

man I would have smashed the stereo up with a hammer by now... and put that chip in the microwave

hope that helps...

[Gleix]

It does - I just set the microwave on the "popcorn" setting and the stereo is now in pieces all over the floor

[SineHacker]

It does - I just set the microwave on the "popcorn" setting and the stereo is now in pieces all over the floor 

[Gleix]

I came across a forum online that helped a lot.  Don't know why I didn't think about just checking the ribbon cable that comes from the CD player before for audio - cables 1 and 3 are the left and right channels. It was a matter of splicing in a jack right there and... boom! PERFECT stereo sound! It's wired up so that all the normal functions still work but when a 3.55 plug is inserted in the jack it re-routes the sound from the CD player unit to the jack so.. no need for "silent" CDs, it just doesn't let any CD audio through.  It sounds great, too and .. now I regret buying that $30 CD wallet that carries 246 CDs

There was a nice little empty spot on the unit the just stick the jack right on.