I have my grandmother’s ca. 1975 table radio. Because of its size and wood cabinet it’s got a good, resonant sound but we’ve never used it because it’s just another radio. So I installed a Bluetooth receiver board into it. The little red circuit board on the right replaces all the discrete electronics you see inside the case.
That little board combines a Bluetooth receiver and Class D stereo amplifiers, powered by a wall wart I already had. The only active piece I reused was the speaker, though I have a dual-voice-coil stereo replacement on order. The switch mounted through the back (switching the speaker) will allow me to switch back to the radio if desired.