I had this cool looking Soviet Russian transistor radio Neywa 402 (made in 70s) on my shelf for a year. It was nice to look at and had a sentimental value but it didn't work. So I decided to give it a new life as a vintage bluetooth audio speaker.
Step 1: What You Need
A radio - You can make almost any radio to work with bluetooth but I think smaller is better.
A bluetooth speaker - I bought mine from ebay, they cost about 10 USD with shipping.
Step 2: Bluetooth speaker disassembly
It is quite simple. Just take out the screws from the bluetooth speaker, disconnect the two actual speaker wires and you will have a bluetooth board with battery.
Step 3: Radio disassembly
Just take out the screws, desolder the elements from the circuit board but leave one switch or button so you can use it for switching the bluetooth speaker battery ON and OFF later.
Step 4: Assembly
Solder the wires of the original speaker inside the radio to the bluetooth speaker contacts on the board. The bluetooth speaker came with a USB charging cable so connect that in to the connector on the bluetooth board (so you can recharge the device).
One extra possibility: It also came with a audio cable - cut it and but the jack in to the connector on the BL board and connect the three wires from it to the female jack (and attache the female jack to the radio enclosure). The good news is that now you can attach the radio to any device with a regular audio cable and the bad news is that in order to flip the switch from bluetooth mode to jack mode, you need to take the radio apart.
One of the original switches on the radio can be wired between the battery and the bluetooth board so it will work as a ON/OFF switch (light blue wires on the picture).
The charging cable of the USB can be placed discreetly to the radio battery bay and the letter radio enclosure will cover it nicely.