Rebuilding an Old Transistor Radio into a Bluetooth Speaker

Question

I recently acquired an old transistor radio and I'm interested in transforming it into a Bluetooth speaker. I'm considering using a stereo amplifier kit that includes built-in Bluetooth, such as this one: Stereo Bluetooth Amplifier Board.

However, I'm unsure where to begin. I’ve attached a picture of the speaker, but I'm having trouble identifying the function of the various terminals and cables. If anyone could provide guidance on how to connect everything, I would greatly appreciate any tips or insights.

Additionally, since this is my first audio project, I'm a bit confused about using a stereo amplifier with just one speaker. Is it feasible to use a stereo amplifier in this case, or should I look for a mono option instead?

I appreciate your help!

Answer 1

You should verify that this loudspeaker is still working, by making a few tests.

Although three terminals are soldered, only two likely drive the speaker's coil, These are identified by spkr terminal + and spkr terminal-. The middle connection seems to be for another purpose, unlikely related to sound production...identified by ?frame?.

The box cavity should be sealed, if you want lower (bass) frequencies to be properly radiated from this "box". The upper area where the dial and controls were situated should be blanked off so that sound cannot escape through this path. The back of the box should be similarly blanked off.

Tests to do: Unsolder at least one of the spkr terminals. Momentarily attach a small battery (1.5V) between the two terminals and verify that you get a "click" sound, or a "thump" sound....only for a brief moment.
From the photo, it appears that what is visible of the black oval paper cone, it is still viable.

If you have an ohmmeter, measure resistance between spkr terminal+ and spkr terminal-. We normally expect something on the order of 10 ohms, perhaps half that. This resistance is what most audio drivers expect to drive. It is rare for such a loudspeaker to have higher internal resistance.

As for driving one speaker from a stereo amplifier, it is easiest to use only one of the two audio drivers, leaving the other open-circuit.