So here goes...
Step 1: Open the box
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
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
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
Step 5: Prepare for the input from the MP3 Player
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
Step 7: Hook it up and try it out
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
Will it work? See the next step :)
Step 9: It didn't work, so now add bigger resistors
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 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