We are going to modify an old boom box (AM/FM/CD/Tape) to add an aux in cable so we can connect an iPod or phone to it. I'm using a Koss HG835 boom box that I found at a thrift store for $15. When we're done, it will be able to play from the aux in cable, CD, and radio.
Some parts of these instructions are specifically for the Koss HG385, but the procedure will work on pretty much any boom box that has a cassette deck as long as you can find the pre-amp chip.
Step 1: Take It Apart
There are 6 screws on the back that attach the front panel to the back of the boom box. I needed a long 8" screwdriver to get to them since the screws were recessed so deep in the hole. Make sure the stereo is unplugged before you start working on it. When you pull it apart, you can disconnect some of the ribbon cables plugged in to various places so that you have more room to work.
Step 2: Add Aux in Cable
I used a 3.5mm stereo to RCA cable from Monoprice, but anything with a 3.5mm audio jack on the end will work. I cut the cable to about 2 ft and stripped the ends of the wire.
The tricky part here is figuring out where to connect the cable to the circuit board. We're planning on replacing the tape function of the stereo with our aux cable. The signal from the tape player is fed through a pre-amp to strengthen the signal before it goes into the regular amplifier. We want to connect our aux cable to the circuit between the pre-amp and the amplifier. The easiest way to do this is to solder the aux cable to the pre-amp output. On our board, the pre-amp is a little black chip labeled Toshiba TA8189N. I found the datasheet for the chip online by googling TA8189N.
We will solder the outer wire from each channel of our aux cable to a ground pin on the chip. I used pin 7. Then, we will solder the inner (signal) wire from our aux cable to pins 5 & 20. These are the pre-amp output pins for each channel, as I found on the data sheet. Thus, our aux cable is connected to the circuit between the pre-amp and amplifier for speaker output.
Since we don't want to actually play any tapes anymore, and I don't want any signals from the tape heads to interfere with the audio signal, I disconnected both tape head wires from the board.
Step 3: Hot Glue Cable
For extra strength (so we don't rip the soldered wires out), we will hot glue our aux cable to an empty part of the circuit board.
Step 4: Remove Tape Motor From Circuit
The tape play button has to be depressed for the boom box to play the signal from our aux cable, but we don't need the tape motor to actually spin since there is no tape. It creates extra noise, and uses power unnecessarily. Just cut one of the wires going to the motor.
Step 5: Reassemble Boom Box
We'll drill a hole in the back of the boom box for the aux cable to come out. Then screw everything back together.
Plug it in and test it out. The AM/FM/CD functions still work because we didn't mess with them. When you flip the front switch to the tape function, it will play from the aux cable (I had to have the play button depressed on the tape player for this to work).
With bluetooth phone speakers selling for ridiculous prices on Amazon, this is a nice way to get good sound from your phone or ipod without breaking the bank. Plus, you're keeping an old boom box out of a landfill by making it useful again!