There is nothing better than the sound of a scratchy record, or the hiss of an old cassette. Sadly so many of these old technologies are disappearing. They break and go unrepaired, or sadly more often they get replaced with newer technologies. Don't get me wrong, I'll take my 42 inch LED TV over a 17 inch CRT any day. That being said, why not find that old Boombox you that you used to listen to the sweet jams of Bon Jovi and Van Halen into something with new Bluetooth features.

This instructable will demonstrate two different variations on how to construct the Big 'ol Bluetooth Boombox.

Step 1: Parts and Tools Needed

In order to build this, we are going to need a few things.

Parts for Rev1

Any Boombox will do
USB Power Adaptor
Bluetooth Adaptor. I chose this one because it was inexpensive and ran off batteries.
Cassette Adaptor
Leaf Switch
Extra Wire

Additional Parts for Rev2

Various resistors and capacitors for testing
Heat Shrink Tubing
Schematic of your stereo

Tools for Rev1

Screwdriver
Soldering Iron
Solder
Desoldering Braid
Zip Ties
Hot Glue Gun
Can of Air

Additional Tools for Rev2

Dremel or Drill
Function Generator (I used a function generator app because I needed a function generator with Bluetooth capabilities)
Oscilloscope
Multimeter
Lighter
Sharpie

Step 2: Open the Box

Now the fun begins!

Time to remove knobs and open the box. I had to remove the volume, balance, and tuner knobs. Remember when removing screws, they could be hiding under stickers (not very likely for older models like this) or under battery covers like mine (see picture 3).

If you are ready to get started with this, go ahead and remove all screws holding the circuit board inside and let's get going!

The first order of business is to remove all that nasty dust from that last 25+ years. A can of air worked great for me with some dish detergent for some of the tough stuff.

Step 3: Power Your Bluetooth

While everything is out, it's time to get power to the Bluetooth Adaptor. Go ahead and crack it open and remove it. Solder wires onto the 120V input of the USB adaptor (see picture 6). Decide how far away the adaptor need to be from the 120V source inside the stereo, cut and solder wires to length (see picture 12).

Step 4: Bluetooth Adaptor and On/Off Switch

Pop open the adaptor and remove circuit board and battery (see picture 4). Unsolder the power switch (see picture 5) and save for later. Solder wires for the power switch (see picture 6). This is only requires a single pull single throw switch. This is why a leaf switch is ideal (see picture 11). Touch the wires together to make sure that everything is working as it should. Time to install the leaf switch (see picture 14).

The tape deck has a lever the moves over to touch the leaf switch to turn the stereo on. Secure the switch so that the lever will touch it as well.

Solder the power wires from the Bluetooth adaptor onto your leaf switch (see picture 16).

Now hot glue the switch in place (see pictures 17 & 18).

Step 5: Cassette Audio

The belt from the motor on my radio is worn out and needs to be replaced. In the mean time while I look for a replacement, I still wanted to get this done. The only reasonable response was to remove everything that causes any drag on the motor. After removing these 4 gears, it worked like a charm (see picture 4).

Put the cassette in place, plug everything in and hit play. Ensure that everything is working as it should (see picture 5).

Feed the audio chord through the cassette deck and zip tie together (see picture 6).

Step 6: Reassembly Rev 1

If this is as far as you are looking to go, reassemble and enjoy your new Big 'ol Bluetooth Boombox. If you are wanting improvements, such as cassette usability, read on!

Step 7: Schematics

I got my Service Manual from www.mikesmanuals.com. Mike ships all his manuals from Michigan in the US. If you have any questions you can even call him and he's more than happy to talk with you.

Once you have the Service Manual you will first look for where you need to tap into the circuit. Tap into the output of the preamp for the cassette deck (see picture 3).

Once you have found what you are looking for on the schematic, you will need to find it on the actual location in the circuit (see picture 6).

Step 8: Solder Time

Once the locations of the right and left channels are found, mark them with a Sharpie (see picture 2). Once marked, solder wires in place (see picture 3). I used red and white wires so I could tell the difference (though, more than likely you will never be able to tell if your right and left channels were swapped).

Locate a ground close to the Bluetooth adaptor and solder a wire there (see pictures 4 & 5).

Use hot glue to secure your solder joints.

Step 9: Component Selection and More Soldering

Once the wires have been soldered in place, it's time to determine what value of components are needed. There needs to be some value of resistive isolation coming from the adaptor, as well as DC isolation.

In order to test what values are needed, check what resistors are coming from your cassette preamp. Mine was 2.2KΩ. That is a great starting point (see picture 1).

Run your function generator through your bluetooth adaptor then through your cassette adaptor (I did this so that my signal would have the same amplitude coming from the Bluetooth adaptor).

Compare your output from your input on your oscilloscope. The amplitude of the two signals need to be as close as possible while having some sort of resistive isolation.

After testing I selected 22Ω resistors with 10 µF capacitors. I started with the 2.2KΩ as previously mentioned, however, that attenuated the signal far too much. I ended up dropping it down to the 22Ω as previously stated.

The normal output of my preamp is around 0.4v p-p (see picture 2). With my 22Ω resistor it only lowered it to around 0.35v p-p (see picture 3). After trying it out, it's more than loud enough for me.

The 10 µF capacitor will create a high pass filter. However if you notice that you are losing bass response, raise the value of the cap and you should be fine.

Once your components are selected, time to solder and heatshrink (see pictures 4, 5, & 6).

I used the 1/8" (3.5mm) jack from the cassette adaptor to solder all these together (see picture 8). If you are having issues with soldering these wires like I did, try burning off the internal insulation (it's usually colored) with a lighter. Works wonders!

Step 10: Motor Control

Since I plan on using this with the 5 "D" batteries, I want to make it as efficient as 1980's tech can be. It was suggested that there is no reason for the motor to run while using the bluetooth adaptor. So, in comes the switch.

I used the on off switch from the bluetooth adaptor. No need to buy one.

Switch placement is important. Since I didn't want to change the outward appearance, I chose to put my switch inside the battery compartment. I placed it between where the batteries would be so that it doesn't interfere with the batteries (see picture 1). Drill a hole big enough for the switch to move back and forth (see picture 3). If you use another switch, see the packaging for that device. I would suggest drilling this hole before you have all the circuitry back in the box like I did. I used a dremel tool, but a regular drill would work just as well.

Hot glue or epoxy everything in place (see picture 6).

Step 11: Final Testing and Reassembly Rev2

Test everything! Secure everything! Make sure that all is in order before assembling. After that, enjoy your new bluetooth speaker!

If you end up making one yourself, remember to post a picture in the comments. And if you liked this instructable, remember to vote for me in the Tech Contest.

If you liked this repurpose, take a look at some of my other projects that involve retro tech