My family and I like to listen to music when we're outside playing with the kids or swimming in our small above-ground pool. We had a couple of old CD/Tape/Radio Boomboxes but the CD players didn't work and the old analog radio tuner was often hard to lock in on a decent radio station. After reading some of the instructables here, I thought maybe I would try to modify an old tape boombox to hook up an old MP3 player. That way I could load the exact music we want to hear and the radio would be digital tuning with favorite stations saved.
So here goes...
Step 1: Open the Box
We start by opening up the old tape player. There are screws in the back of the player that have to be taken out to open it up. Different players will have screws in various places, but this one only had four screws, one in each corner. (If you take out all the obvious screws from your player and it still won't come apart, be sure to check in the battery compartment. Sometimes there might be a screw or two in there as well. Also, note that some times, as in my case, you will have to press Eject to open/release the tape door since, at least on mine, it latched a part of the cassette mechanism and ended up keeping the front half from coming completely loose.)
Once its opened up, you'll need to find and remove any screws that hold the cassette mechanism in. Also, locate the tape play/record head since we'll use the wires coming from it as the input from the MP3 player.
Step 2: Desolder Wires and Unscrew Cassette Mechanism
Here is the top side of the tape head where the wires *were* attached. I've already desoldered the four wires from it (positive and negative wires for the left and right channels).
To desolder the wires, just plug in the soldering iron and let it get fully heated up. Then just hold the tip of the iron to the solder pads while gently pulling on the wire. When the solder heats up enough to melt, the wire will come loose.
Follow the wires to where they attach to the electronics board in the player; it may be labeled there as to which is left and right.
I've also already desoldered the two wires from the tape player drive motor. We'll use those wires to power the MP3 player.
This picture also shows the headphones jack on the top of the player. We will "repurpose" that jack later on for use as the input.
You can also now remove any screws holding the cassette mechanism in so it can be removed.
Step 3: Remove the Cassette Player Mechanism
This picture shows the cassette mechanism laid over upside-down. Note that there is a "leaf switch" which actually turns the power on to the motor when you press any of the buttons on the player (play/rewind/fastforward, etc). We'll "repurpose" a switch on the old radio portion for use as a power switch for the MP3 player.
The grey and white wires in the lower-center of the picture are the ones that were attached to the tape player head. You can't see it here, but the board where they attached was labeled "L. Ch." and "R. Ch." for Left Channel and Right Channel.
With all the screws out and wires desoldered or cut, you can remove the cassette mechanism from the player.
Step 4: Remove the Electronics Board to Get to Its Back Side
Take out any screws that may be holding in the board and in my case, there were also three clips holding the bottom of the board in. I had to press in on these clips one at a time and gently pull the board up a bit to pass the clip. Once it is free from the case back, you can access the back of the board to make some changes and attach some wires.
Step 5: Prepare for the Input From the MP3 Player
In order to play an MP3 Player through the boombox, the plan is to hook the headphone output of the MP3 player to the tape head wires. One could just buy a 1/8 inch stereo phono plug from a place like Radio Shack and let that plug hang out of the boombox to hook to the MP3 player, but I didn't want to spend any more money on this project that I had to and didn't like the idea of having the wire hanging out and all, so I decided to re-use the headphones jack in the old boombox. The jack was soldered into the circuit board such that it poked through the top of the case so I didn't want to completely remove it and have to come up with a way to attach it.
To use the jack, I knew I needed to disconnect it from the rest of the audio circuit. To disconnect it with it still in place, I actually used a small screwdriver to scrape through the circuit traces so the headphone jack would no longer be really attached to anything. See the notes in the picture for other details.
Step 6: Setup a Power Switch for the MP3 Player
This switch was for changing between AM and FM on the boombox radio. Since we'll be using the radio on the MP3 player, we won't be needing it. I decided to "repurpose" the switch as a power switch for the MP3 Player. Again, I needed to disconnect it from the circuitboard, so I used the small screwdriver again and scraped a groove through the traces. I took the two white wires that were attached to the "leaf switch" (from step 3) and soldered them to the switch here. I did go ahead and bridge the traces to "hardwire" the onboard radio to FM, but I don't expect to be using the boombox's radio, only the one in the MP3 player.
Step 7: Hook It Up and Try It Out
Ok, I think we're all done, so we will now hook up the MP3 player and see how it works before putting it all back together.
Does it work? Will there be sound?
Hurray, it works!! There was a lot of distortion though and I had to turn the MP3 player all the way down to "1". It is still a bit strong with a little distortion though. It is probably because the headphones output of the player is still much stronger than the output that the tape head had.
I have an idea though. When I was figuring out the old tape player's headphone jack (which I "repurposed" as the input from the MP3 player), I saw that while the normal output of the tape player went straight to the speakers, the headphones output was routed through a couple of resistors (one per channel) to drop the strength of the output down to headphones level (to not blow the headphones...*or your ears*).
I'll try re-routing the MP3 input through those resistors and see if that helps. The second pane below shows them.
Step 8: Move the Input Wires to Resistors
Now we'll move the old tape head audio wires from the headphones jack over to those resistors in hopes of cutting down the strength of the signal.
Will it work? See the next step :)
Step 9: It Didn't Work, So Now Add Bigger Resistors
Ok the 150 Ohm wasn't enough; the sound is still distorting a bit. I'll try adding larger resistors in series. For testing I just soldered them in just sticking out until I find the right size. I first tried adding 470 ohm resistors. After trying it, it didn't seem to make much difference. Next I tried 3000 ohm resistors, but still hardly any change. Next I'll try 10k ohms.
Adding resistors didn't seem to be helping; then I got a suggestion from another great Instructables user, unknownuser2007 He suggested that instead of trying to use resistors to drop the level of the headphones output of the MP3 player, I could try bypassing the cassette head preamp IC like he did in his instructable. A great idea!! Thanks unknownuser2007!
The tape head wires (which I am using as the input from the MP3 player) are attached to the circuit board and traces ran from there to an LA3220 IC chip (see second pic). I looked up this chip on at a great Data Sheets website (see third pic) and found that it is an Equalizer Amplifier that is apparently acting as a preamp for the tape head.
There's another IC, an LA7769 (fourth/fifth pics) whose output goes straight to the speakers. This IC is apparently the main, higher power amplifier for the speakers.
"OK", I thought. "Great; its just what I need to amplify the headphones output of the MP3 player! I'll just move those input wires straight to the input of that IC".
Well, it worked, but then the sound was very loud all the time, even with the MP3 player's volume set at its lowest setting of "1"...bummer..so close!!
Well, one last idea....see next step
Step 10: Victory!!
I looked at the traces going to the volume control and they went through lots of capacitors and resistors and was very confusing.
I decided, what the heck, I'll just move the wires to the output side of the preamp, the LA3220 (pins 2 and 13), just for fun. Who knows, it just might help and if it blows something, too bad; I'm gettin' tired of fighting it.
Turns out, that was the trick! Now I have good volume control, no distortion; its great!
Hurray!! Thanks again unknownuser2007!
I can now plug the MP3 player, or any other device with a headphones output, into the MP3 Boombox and play it for all to hear. Yay!!
Now on to my next project where I will try to make a power supply to do away with the need for batteries for the MP3 player. Remember the power wires that originally went to the tape player motor? I'll try to do another Instructable for making a power supply to reduce that 7 volt motor power down to 1.5 volts for the MP3 player.
I hope this instructable might help anyone wanting to do something like this. I wanted to include my troubles I ran into along the way. Good luck!
In His name, HappyDad