Introduction: Boom Box Aux in Mod

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

Picture of 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

Picture of 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

Picture of 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

Picture of 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!

Comments

alchemyandthunder (author)2014-05-15

Couldn't you jumper the contact closure for the play button to always play???

Sure. It didn't seem necessary to me.

Laral (author)mkasberg2016-09-07

I'm doing this mod now. Do you have to press both RECORD + PLAY to get the signal to the amp? I'm noticing there is a momentary switch on the chassis right next to where the two wires from the tape head are soldered to the board. I have to press both PLAY and RECORD to get a metal tongue to slide the switch into the closed(?) position. The switch has many poles so I'm guessing it connects the pre-amp outputs to the amp inputs. Since you are going directly from the pre-amp outputs, this is probably why you have to press at least PLAY, and I'm guessing also RECORD. The image shows the switch 'closed', with PLAY and RECORD pressed in, on the bottom.

Laral (author)Laral2016-09-07

OK, I forgot the reed switch I detached when I removed the tape transport assembly. It is closed when the PLAY button is pushed (or the PLAY/RECORD combo). And it has two wires going to a KIA6269P amplifier chip. I didn't yet trace the leads but it must switch the amp chip on and off when the TAPE switch is active. So I guess RECORD wouldn't be necessary. The switch could be wired closed so you wouldn't need to hit PLAY, just select TAPE.

pm0673 (author)2014-12-09

Hi, I have modified my own radio based on your instructable but I have a problem I hope you can help me out on. The radio has an AN7310n chip http://pdf1.alldatasheet.net/datasheet-pdf/view/12... Im hooking up a rechargeable bluetooth receiver http://www.ebay.co.uk/itm/Wireless-Bluetooth-Music...

The prob im hoping you can help me out with is this. When the transmitter is running of its battery all is fine. When i pick up 12v off the feed that i disconnected for the tape motor and run it through a voltage regulator to get my 5v the sound is very tinny and sounds like ground isnt connected. Any suggestions greatly appreciated. Oh one more thing when running off batteries and connecting the audio groung to ground on the IC i also got the tinny sound to resolve i connected ground to pin 2 NFB CH1. Any help greatly appreciated.

mkasberg (author)pm06732015-02-05

I'm not sure exactly what the best solution is with your components. I would try connecting different wires based on the data sheet you found and use trial and error to see what sounds best.

normalicy (author)2014-05-15

I've considered doing this for years, but never got around to it. One thing I was considering was to instead use the signal wires at the tape mechanism's location instead of locating the chip. Obviously, it would take some testing to see which two wires in the ribbon are the correct ones. Am I thinking right?

mkasberg (author)normalicy2014-05-17

I have read about other hacks that use the wires going to the tape head in the cassette deck and just splice a 3.5mm cable to that. Based on what I've read, it will probably work but you might have inferior sound quality because the signal from the tape head is typically weaker than the signal coming from your 3.5mm cable. When the signal from your spliced cable goes through the pre-amp, it will be more distorted. Because I chose to the wire to the pre-amp output, the signal does not go through the pre-amp and is therefore (hopefully) less distorted (not distorted at all?).

normalicy (author)mkasberg2014-05-18

That's kind of what I had feared. I may do some more research.

desidude52 (author)2014-05-16

In the past I've just shoved cassette adapters into the cassette bay. The wire does pinch a little bit but a very quick and easy work around to be able to plug in other devices.

mkasberg (author)desidude522014-05-17

Yeah, cassette adapters are great for that. I'm currently still using one in my car. This mod is cheaper, and produces at least marginally better sound quality.

bovine (author)2014-05-15

I think I would try to retrofit the cassette space to hold an i-pod, since it's now dead space and about the same size.

mkasberg (author)bovine2014-05-17

Cool idea. Maybe that will be my next project.

sfinley304 (author)2014-05-10

Great instructable. How is the sound quality?

mkasberg (author)sfinley3042014-05-11

To me, it sounds the same as if I played a CD from the same stereo. Based on some research I did before making this, I think the biggest factor in degrading sound quality in this type of mod is the cassette tape pre-amp. Because we attach the line-in after the pre-amp, the pre-amp does not distort the sound.

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