Monday, September 22, 2008

poojalooba_cow: 2 Electronics: 0

Today I scored another point against that devious being, electricity, and without using a single bead of solder or hot-glue. Using only a screwdriver, a pair of pliers, a length of wire, a roll of electric tape, and an old headphone jack, I was able to rig my outdated car cassette tape player to play audio from an iPod or other mp3 player instead. Once again, I'll provide detailed (albeit kind of poor-quality) pictures of the process and the idea behind it.

The story begins with the sudden failure of my car stereo to even power on. I let it sit for a few months without anything changing, but on a whim when I went to get the catalytic converter's heat shield fixed, I asked the mechanics to yank the stereo for me as well.

When I got my car back, I inspected the stereo. There were no problems that I could see with it - the fuses were all intact and nothing was burned out on the circuit board. I removed the pins that prevented me from pulling the stereo out of its niche so that I could insert it and remove it at my pleasure this time (it also makes it vulnerable to stereo-jackers, but who wants a ten-year old stereo console that only plays cassette tapes?).

When I plugged it back in, it powered on just fine, and the radio worked perfectly. The tape player, however, was still messed up. I'm not a fan of tapes - being a more digital-age kind of guy - so I had no use for the tape deck anyway, but I had an idea. Maybe if I could bypass the reader-head on the tape player, and insert an audio feed from say, an iPod, then I would be able to listen to anything I wanted in my outdated old car, instead of being restricted to the radio (all they ever play on the radio is crap anyway. I'm not a teenybopper who enjoys David Archuleta's "Crush" every other song, so there's no appeal for me on the radio, except maybe NPR, which is great).

The inner workings of the stereo console:


There were a few problems. The tape deck's reader head would not engage unless it genuinely thought that there was a tape inside the machine, and the tape deck refused to recognize and play a tape, and it wouldn't eject the tape once you jammed one in there.

I took the tape player out of the main console and messed around with it, determined to overcome these problems. I discovered that the reason it wouldn't eject was because a spring had popped loose, and I put it back in place so that the spring-loaded eject action would work. I discovered that it wouldn't recognize a tape because of an issue with the reader-head section jamming and not fully engaging, because of a problem with the spring-loaded eject button (they're all interrelated. They have gears and little plastic pieces that affect parts clear across the unit, all over the place. It took forever to figure out what parts affected what actions). It wouldn't play a tape because the reader-head wouldn't fully engage and the system was receiving a messed-up audio stream and decided that there couldn't be a tape in with that output.

So, with a little jiggling and clicking of parts and a lot of dumb luck, I finally snapped the reader-head and the eject mechanism back into place and everything ran smoothly. I popped it back in the unit, and it played a tape just fine. The audio was all messed up, though. It sounded as if the sound was coming from underwater - it was all warbly and tinny-sounding. I figured the reader-head was old and messed up. This didn't matter too much, because if I had my way, the reader-head would never be used again.

The tape player section of the stereo console. The eject mechanism is the black plastic face on top of the unit, the reader-head is the square metal thing on the center right with the brown strip coming from it, and the two of them were causing jams underneath the unit on the bottom left (not visible):


The reader-head communicated with the main section of the stereo console through five wires encased in plastic that ran from the reader-head and plugged into a jack on the main control board. I figured if I could find out what signals these five wires sent, I could bypass them and add my own audio instead of the audio that the reader-head would normally pick up. Since the reader-head plugged directly into a jack, I didn't have to desolder anything or cut any connections, I simply unplugged the wires from the jack and popped my own wires in instead. Through a highly scientific process called "trial and error", I discovered that the top wire was the negative lead for all four other positive leads, the next down was left-channel A-side tape audio, the next down was right-channel A-side tape audio, the next was left-channel B-side tape audio, and the farthest down was right-channel B-side tape audio. I figured it'd be way too much pain to try and configure both sides of tape audio, so I put in a left and a right channel on the A-side of the tape (the two red wires in the picture) and twisted the two negative leads from those wires together and stuck them in the communal negative port (the twisted black wires in the picture).


The reader-head and the bypassed connection. The reader-head's cables are unplugged and the red and black wires are wires that send audio signals from an external source instead of the tape player's source audio:



In order for the main console to accept audio from the reader-head, it has to believe that there's a tape in the machine for it to play, otherwise nothing will happen. This is where the dummy tape comes in. I pulled all the tape out of an old cassette, so that it would be just an empty shell. That way, the tape player wouldn't ever come to the "end" of the tape and try to switch sides (because I had no B-side audio set up, remember. If it switched sides I wouldn't be able to hear anything from my external source). The dummy tape successfully "tricked" the console into thinking that it had a tape in (because technically it did have a tape in. It just wasn't playing anything), thus engaging the reader-head (which is disconnected; it actually engages the external audio source) and amplifying and playing the signal that comes from it.

The dummy tape engaging the reader-head. Since the reader-head is unplugged, the audio is actually coming from the external audio source plugged into the red and black wires in the reader-head's place:


So, as long as there's a dummy tape in the tape player, it will continue to play whatever signals are coming from the reader-head jack, which in this case is music from my iPod. I wrapped the wires through holes in the console frame so they wouldn't come loose or short-circuit each other, then I pulled them around to the front so they'd hang out of the console and be able to have a headphone jack attached to them:


Then I slid the unit back into its niche:



It hangs out about a centimeter from where it normally would fit, because the wires are thick and don't like to be bent three directions and squashed in order to fit, but that's fine because I can now pull the entire console out with greater ease. I might need to do this because it's an old tape player and if it ever has a problem, it'll refuse to play and might jam and then I'd have to open the console up and disengage the reader-head and engage the eject mechanism manually in order to get the dummy tape working again.

This is the end product - what it looks like in my car and how it looks when it's in use. The four wires hang down and are connected to a cut-apart headphone jack that sends the right and left channels from my iPod to the right and left channels of the tape player's reader-head jack:


There is a little bit of static feedback - my guess is that it's from the sensitive amplification circuit picking up the electric "hum" of the motors running the tape player gears - but if I turn on the Dolby Noise Reduction System on the console, it effectively reduces the static to a point where it's not even noticeable (and when you're listening to music it's barely noticeable at all even without the noise reduction, the only time it's audible is between tracks). The iPod volume must also be turned almost all the way down in order for good sound quality to be preserved. Apparently after the signals are received from the reader-head, they go through some pretty hefty amplification. You can crank the console volume as much as you want, but you have to keep the iPod/external source volume down or the gain becomes too great and things sound really muddy. There's one more peculiarity: For some reason, the bass is amplified a lot more than the treble. On just normal equalizer settings, the bass is loud enough that it shakes the whole car while the treble is much quieter. Fans of hip-hop and rap might not object to this, but as a person who wants to rock out and hear the guitar chords and the singing, I'm a little put off by this. Luckily, the console has an equalizer, and if I turn the bass all the way down and the treble up a little, it sounds almost normal. If I activate my iPod's equalizer as well, and set it on the "Bass Reduction" setting, the sound is completely normal and balanced and the bass is not too heavy or overbearing. It's just right.

All in all, this project was a resounding success. I can now listen to whatever I desire whenever I drive anywhere, instead of being confined to silence or crummy radio junk. It works a lot better than one of those FM radio iPod transmitters or even one of those faux-tapes that plays your iPod through a tape (mine uses a tape also, but it's just a dummy and the real audio is a direct line in, so it's better sound fidelity). It's [just barely] not quite as good as a professionally-configured line-in port where all the circuits are built to handle an iPod/mp3 player audio signal, because I'm patching into the signal from an entirely different sort of media. The ultimate redeeming quality of this project is, of course, that it cost me absolutely NOTHING to make, and I even learned a thing or two along the way!

14 comments:

  1. I caught this off of make:blog...and I must say, "well done!"

    This reminds me a lot of my old Chrysler, where I mounted a set of "surround-sound" computer speakers to boost the existing sound system to acceptable traffic-rocking levels.

    Though, this is far and above better than what I did. A+!

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  2. cool project. i always wondered if that was possible, i guess it is. thanks for taking the time to work it out.

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  3. Pictures have exceeded their bandwidth. Can you upload them to another server?

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  4. You should buy an audio in plug and then mount it to your dash, just to make it a little cleaner. Great work though.

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  5. Wow. That one comment "pictures have exceeded their bandwidth" is seriously the most awesome comment I've ever seen directed at me. Today alone I've received over 2,000 hits on my blog, quintupling my previous total number of hits after an entire year of not-quite-so-diligent blogging.

    Thanks for the feedback, everyone. It was awesome to hear other peoples' opinions on this and even more awesome to see it posted on other, far more respected blogs. I'll keep the projects coming, I'm a pretty avid electronics-tinkerer.

    Also, as soon as Blogger starts working again (it's currently down) I'll make sure I upload the images to a better server than my own free-host domain. I never imagined I'd exceed bandwidth limits on this!

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  6. Very well done, my good sir...makes me want to tear apart my car and do the same. haha...awesome work, really =D

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  7. Great idea, but the execution is ghetto. Take some pride in your ride. For less then $2 you can make a nice built in jack and hide all the wires. Also, I would look for the sensor that tells it there's a tape in the deck and jumper it. Then disconnect the whole tape mechanism so you don't to listen to it spin. BTW, the static may be a ground issue. Try different ground points in the radio to see if it clears up.

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  8. Such a sweet job! The wires hanging out are pretty pretty ghetto. The only thing I could suggest is that I've seen an instructable where they make an iPod input for an old tape based boombox, and had the same kinda overgain problem. The fix they came up with was by using a combination of resistors, and by putting the input after the preamp. They got some decent audio quality after that. (http://www.instructables.com/id/SJ8MHGVF7AVCD5B/ this one)
    Keep up the hax!

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  9. You do know that radio has audio in on the back connectors, right? You need a small computer to enable it (see Sparkfun, about $15), but the sound quality is MUCH better.

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  10. Absolutely Genius!!!
    Excellent Stuff!

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  11. hello i want to do that to my aura cd head unit dut i just dont know whch wires i shud hook up my i pod to i know the wires but i just cant find them could u help i want to burn a blank cd then put it into fool my cd headunit its playing but it aint please let me know my e mail is brendanquigg@hotmail.com thanks

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  12. Thank you for your idea. It helped me a lot and happy to share the improvements that I found. When I removed the cassette module and disconnected the wires, I saw R-out, L-out and Sign-gnd printed on the pcb. I soldered the wires I salvaged from a broken earphone on those knowing that right is red, blue is left and the two uncolored (copper) wires on the Sign-gnd. I tested the sound and found the cassette playing noisy. I disconnected the cassette motor by removing the solder connecting it. Eureka!!! I found the sound much much better and cleaner ie. by by-passing the cassette module circuitry and tapping on the pre-amp (I reckon). My son and I were very ecstatic with the outcome. When he wants to play the radio, all he has to do is eject the cassette and whizzing sound of the cassette is totally gone when he use his i-pod.

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  13. Oops I forgot to mention above that I disconnected the five wires(being auto reverse) coming from the cassette by unsoldering them. This will let me put it back should I change my mind in the future which is unlikely. I thought I could have just snipped them with scissors but I want it done clean.

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  14. erratum ...coming from the cassette head by unsoldering them.

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