Introduction: Adding a Line in to a Boombox With a Tape Player

About: I usually end up doing an instructable because I have to figure out how to do something myself. I just get pictures during the process, and if it works out, BOOM, an instructable!

** As with all the instructables, you take your item / health / whatever into your own hands when attempting ! Be mindful of high voltages on the main power board, the hot soldering iron, etc. Being careful and patient will bring you success. **

This is my first instructable, but I've been doing this sort of thing for a long time !

I know about the cassette adapters, but all the ones that I have tried cause too much background noise.

With this in mind, I decided that I wanted to add a line in to an old boombox that has seen better days, but still sounds good.

I found this baby left for dead on a job site, so I took it and cleaned it up (goof off plastic safe formula), fixed the cd lid, and now I want to add a line in so that I can use it with my mp3 player.

The reason is simple. Now I use an FM transmitter, plugged into a cigarette lighter adapter (which is plugged into the wall), and I hook my mp3 player to the FM transmitter (with an adapter from the 3/32" jack to 1/8") and then tune the radio on the boombox to the transmitter's station.

Besides being bulky, it picks up a lot of static and interference.

The white boombox in the picture was going to get the line in install, but after taking it apart I didn't want to chance breaking it to find the signal from the FM board (or even the CD player).

Step 1: Disassemble

Sony (and many other brands) makes it easy to take their products apart by stamping an arrow by the screws that you need to remove to get the unit apart.

This boombox is model # CFD-S36

I took out all the screws indicated, and that got the front chunk (which holds the speakers) off.

Then I followed the next set of arrows and the unit split into 2 more parts.

Step 2: Locate the Wires From the Tape Head, and See How the Mechanisim Works.

The wires coming from the tape head were easy to spot on this boombox. At first, I thought that since the player had a power button I wouldn't need to press play to get the sound to go through the system, but I found a leaf switch under the board that has to make contact before sound will play through the system.

From there I was thinking about hooking up a toggle switch for the leaf switch, and a toggle switch to cut power to the tape player motor. In many cases the motor causes background noise.

After shopping at Radio Shack for an hour I revised my plan to just a single toggle to cut power to the motor. Pushing play will take care of the leaf switch !

The smallest toggle I could find was a micromini toggle switch, part #275-624 at $2.99
I decided to use a closed circuit 1/8" stereo phone jack, part # 274-246 at $2.99

I'm not sure if feeding a signal back into the tape head will cause any damage, but it may use up some of the signal that I want to be amplified, so the closed circuit switch is the way to go.

The tape head is connected until you plug something into the jack, then the only thing connected is what you are plugging in.

Step 3: Drilling the Holes and Mounting the Hardware.

After looking for a good place to put the toggle switch and phone jack, I decided to go under the carry handle (when it is down). Since this player comes apart in three main chunks, putting them anywhere else could cause a problem if I have to take it apart again.

I located a place that would have clearance inside, and under the handle, then drilled the holes.

This plastic was really thick, and the jacks wouldn't go all the way through, so I used a larger bit to do a type of countersink on each hole, making the hole larger on the topside, and thinning the plastic so that I could fasten the jack and the toggle with the supplied hardware.

Step 4: Hooking Up the Hardware

After both items were mounted I unplugged the connector that goes to the leaf switch and to the tape player motor. I separated the motor wires, cut the red wire and hooked it up to the toggle switch.

The next step is to de-solder the wires coming from the tape head and connect them to the phone jack, then connect the wires from the phone jack back to the board that the tape head wires were soldered to.

The tip of the line in jack is the Left positive, the next band down is the Right positive, and the rest of the jack is the ground.

The picture of the back of the phone jack box shows the diagram, and I have the area on the right labeled as follows:

1 = Ground
2 = Left Board
3 = Left (head)
4 = Right (head)
5 = Right Board

I soldered all the wires in place and tested the system:

Step 5: It Doesn't Work.... Maybe... Wait.. Success !!

The sound is terrible ! It's all broken and fuzzy with distortion.

I tried turning the volume on the mp3 player to 1 and it was still distorted.
Next I tried another mp3 player - with the same results.

I checked the wiring again and it was all fine.

Then I started to comb the boards for a line in or L, R marking. All attempts at getting a signal through sounded distorted.

I didn't want to add a resistor to each channel because of possible sound quality issues, so I decided to check the IC on the tape player board.

I went here:

and discovered that the IC was a system preamp for the tape deck.

The IC is a Toshiba TA2068N - here's a link to a PDF datasheet (2019 update!)

I got the datasheet and found 2 pins that gave a really good sound for input.
(pins 18/Right and 20/Left)

It was a tough solder either way, but I chose to tap the wires on the top of the IC.
It went pretty well and the wires are pretty stable. If I get a chance to get a hot glue gun I'll add some to make sure the wires stay put.

I left the toggle for the tape player motor, since it does kill some noise.

After that I put the box back together and plugged my mp3 player into it. It sounds great !

It ended up that the closed circuit phone jack didn't get to be fully used (nothing is connected when there is nothing plugged into it). That's OK, though, because it was the only type Radio Shack had at the time !