Tape deck A2DP conversion

I've had a dream since the day I learned about A2DP aka stereo Bluetooth. I've always wanted to be able to turn my car on, pull out my cell phone, and just play my music library from there. One device, zero wires, infinite satisfaction. After going through countless tape adapters, I got fed up, and made my dream come true.

First thing's first I had to figure out how to get my radio out of the dash, it usually involves sliding a shim of some sort into the sides of the radio to disengage locking clips. I saw four holes in the front of my radio, after a quick search I found it's as easy as making u shaped handles out of a wire hanger. A quick search should yield similar success for most radios.

Just a couple of torx screws later I was inside examining the assembly. I can't promise the same luck for everyone but, Ford was nice enough to make the tape unit almost entirely independent of the main unit. Three torx screws and two wire harnesses to un-clip from the main board and the tape player came right out.

I didn't toss the tape deck right away, since I'd be needing to decode it's secrets in order to emulate a tape insertion to trigger the input, which is usually reserved for the tape head. A close examination revealed a few more trigger switches on the mechanism than I had anticipated, so I moved on to the main board for some small victories.

A quick once over and it was clear that Ford did me another favor, the tape head in was marked "TAP IN" silk screened on the board. The white, black, and red wires coming from the tape head were simple enough to figure out for anyone who's messed around with a stereo connection or two. The larger wire harness was clearly my control and power harness for the motor.

Still avoiding the complex tape deck, I cracked open an old 12v to mini USB adapter that's wire had been cracking at the stress points. I'd been using it to charge my phone and also power the A2DP adapter in previous setups, so I was sure it would suit my needs. It was sealed shut pretty well so some gentle squeezes with vice grips were needed to crack it open. I removed the springs used for the positive and negative contacts and then the wire to prep it for insertion into the radio. I decided to leave the LED so I'd know when the adapter was powered on.

Once again a quick search worked wonders and I found the pin-out for the wire harnesses for my car. The ones I was interested in were the switched 12v and ground connections. I matched wire to pin, and pin to board contact. It's important to note that I chose the switched 12v to avoid the adapter draining power from my car battery while it's not running. The constant 12v is used for radio memory; stations, time, etc. The A2DP adapter also powers on and attempts to connect when power is supplied, which means when I start my car, the radio turns on and the adapter instantly tries to connect to my phone. Exactly my desired results.

Since you can't get anything done if you avoid challenges, it was time to gut the tape deck and start experimenting. Before I took it completely apart, I inserted the shell of my old tape adapter into the deck and found which switch was triggered. This one would be the trigger I needed to switch the radio into tape mode for the whole setup to work. The tape deck's guts contained a few daughter boards. The board that had the control wire harness also contained the switch that triggered a tape insertion and a smaller temporary push button that's purpose I had yet to discover.

From there a small ribbon cable led to the next board which contained a solenoid. I never figured out completely what the solenoid was for, I'm assuming it's involved in tape ejection as my tape deck didn't have a physical eject button that directly ejected the tape like most Walkman and stereos. This board was also where the motor was wired in. The first/wire harness board also branched off a couple wires to yet another board with yet another switch. This last board also had the wire harness that output from the tape head to the main board, and of course the tape head itself. The was also another smaller blue solenoid/magnetic trigger that led from wire harness board to the area of the tape player that contained the tape head. I'm assuming this trigger flipped the direction the tape deck spun, again electronically controlled.

After disconnecting everything from the tape deck I reconnected the wire harness and took everything back out to the car to plug in and experiment. (I recently moved and didn't feel like digging through boxes to find my 12v supply.) I was right about the tape trigger switch, but that was pretty obvious. The second switch on the tape head board told the main board which side of the deck the tape was playing. And though I didn't figure it out till I had an audio source, the small push button cut off audio if not pressed in. 

First thing I did was remove the solenoids, motor, and that pesky little push button. The first image I posted of the board shows why I didn't just jump into bypassing everything. The tape trigger switch has quite a few contacts, and I felt I needed to narrow things down little by little so I didn't get lost in the mess of extra components. After I spent some time with a multi-meter checking contacts and following traces, I narrowed down the two I needed to trigger the tape function. I soldered to the daughter board instead of the main board for testing purposes.

Success! Switch it on, and I get my tape player, switch it off and I get the radio back. Not that I ever use the radio, but it's good to have options. I soldered the USB connector onto the 12v adapter and then soldered a hefty 1/8th jack cable I had used to repair the tape adapter previously. So, I had the tape trigger switch hooked up, the 12v adapter wired up and plugged into the A2DP adapter, and a 1/8th audio cable that when straight from the A2DP module into the wire harness for the tape head input on the main board. It was ready for action. I plugged the radio in, turned my car on, the radio popped on and the A2DP module fired up and connected to my phone, I switched the stereo into tape mode... and nothing, no audio. I guess it was time to learn something.

What I didn't know is the problem I had is a common one in car audio, I'd never cracked open a car stereo before so it hadn't occurred to me. I noticed some noise on the line, except when I plugged the 1/8th jack into the A2DP module. The noise also disappeared when I touched the ground on the 1/8th jack to the body of the radio. I suspected a ground loop was the culprit for my lack of audio. I've worked with audio equipment and audio hacks before, so I'd heard the term around, but I hadn't really struggled with the problem before so I hopped over to the Hack-A-Day forums and asked for some insight.

Enter the 1:1 Isolation Transformer. The Hack-A-Day crowd pointed me towards this bad boy. It essentially uses inductive coupling to isolate an AC source from the device. Oft used in audio setups to thwart pesky ground loops. After discovering that the only Radio Shacks that still carried this device were out in the boonies, I realized that the tape heads from the deck and my old tape adapter do exactly the same thing and could be rigged into an isolation transformer. I tested the idea, and it worked great. I saw one problem with the setup though. The tape heads need to be PERFECTLY aligned for proper stereo separation and volume. I thought about the time and effort it would take to to make them a permanent fixture, and then opted to for the thirty five minute drive to pickup the last two transformers (on for each audio channel) from one of those distant Radio Shacks.

The isolation transformers were not without their own set of issues though. First, they picked up a lot of noise from the radio internals, my cell phone, the 12v adapter, and possibly the A2DP module as well. I wrapped the coils in electrical tape to insulate them, then wrapped them again in aluminum foil and again with a layer of electrical tape. I grounded the makeshift shielding to the body of the stereo with a couple wires and one of the screws intended to hold in the tape deck. That helped clean up a lot of the outside noise. There was still one last problem involving switching the tape input from a tape to my phone's Bluetooth which is amplified. The phone had to be kept at a very low volume to avoid distortion because of the tape deck's pre-amp. After experimenting with some resistor values, I settled on the largest value I had 1M ohm. As you can see, I cleaned up my setup a bit. I removed the daughter board completely and wired the switch straight into the main board. The switch fit nicely where the FF and RW buttons used to be. 

Final thoughts...

I do plan on cleaning up the front plate a bit, adding a nicer switch for the tape activation. Maybe change the 12v adapter LED to green and bring it to the front plate with the switch.

I wish I had bigger resistors, but I didn't want to add some in series. Something to go back to when I clean up the front plate.

I wish I had a different A2DP module. Although I like the mostly automated setup I have. It'd be nice to have an A2DP module that also supports AVRCP, or the Bluetooth Remote Control Profile. Track controls could be added to the front panel. The first A2DP module I owned, which was so small I actually lost it, worked well and supported AVRCP. It was quite reasonably priced, but would require a little more modification since it was battery powered only.

I'd rather have cracked my A2DP module open for tighter integration, but I can still take it to the next car or stereo should I need to. I opted not to crack it open for it's lack of screws and capacitive touch surface being the only button.

Making my own 12v to 5v adapter would have been easy enough if I didn't have one already bound for the junk yard. See 5v regulators. 

I'm really satisfied and quite proud of this simple hack, which only cost $7 since I already owned everything except for the isolation transformers.