If you want to listen to an old single-speaker monaural radio through stereo headphones, you'll need to install a headphone jack to do so. Choose either a 1/8- or 1/4-inch headphone jack and decide if you want the speaker to continue playing or not when you plug in the headphones, which dictates the selection of either an open or closed circuit headphone jack. After these decisions, the process is relatively straightforward.
Unplug the radio and open the back panel to access the circuit boards. Look for a schematic diagram on the back of the panel. Find the amplifier section and look for the signal input and speaker output. If you do not see a diagram, find where the speaker wires originate on the circuit board. This is the amplifier.
Drill a hole for the size jack you are installing on the side of the radio as near to the amplifier as possible. Measure and cut enough wire to run from a few inches outside the hole to the amplifier board. Strip the ends of the wire and solder the two leads to the positive and negative extensions marked on the open circuit headphone jack. If they are not marked, the outer pole is always negative. If you are using a closed circuit jack where the speaker disconnects when the headphone is plugged in, there is a pair of unmarked leads on the side of the jack. Solder a wire from the positive speaker output on the amplifier to either lead on the jack and another wire from the other lead on the jack to the positive speaker pole. Cut and discard the original connecting wire from the amplifier to the speaker.
Mount the phone jack in the drilled hole; use the counter boring tool to widen the opening if necessary.
Solder one end of the 2k-ohm resistor to the positive lead wire coming from the headphone jack. Solder the other end of the resistor to the positive speaker pole on the amplifier. If the amplifier is putting out more than 25 watts RMS (root mean square) power, solder the 4-, 8- or 16-ohm resistor from the positive pole of the headphone jack to the negative pole to properly "load" the amplifier output. This 4-, 8- or 16-ohm impedance measurement and the amplifier wattage should be marked on the rear of the speaker and on the schematic diagram on the rear cover panel of the radio.
Replace the back panel. Plug in the radio and plug in the headphones. You will need a stereo to mono adapter if you are using stereo headphones. Turn on the radio and listen to the headphones. If you mounted a closed circuit jack, unplug the headphone plug with the adapter and note if the radio volume returns.
- If impedance and wattage are not marked on the rear of the speaker or on the schematic diagram on the rear cover of the radio and you do not know the speaker impedance or amplifier wattage, you can substitute an 8-ohm, 3- to 5-watt resistor for a 4- or 16-ohm resistor wired from the positive to the negative pole of the headphone jack if you think the amp may be putting out more than 25 watts. All components for this project can be found at your local electronics store.
- Do not try to work in a tight space with a soldering tool. Remove the entire circuit board from the radio; solder the connections and replace the board. Be careful not to drip solder onto the circuit board.
- 1/8- or 1/4-inch mono headphone jack
- 4-, 8- or 16-ohm resistor matching the speaker impedance (optional)
- Drill with matching 1/8- or 1/4-inch bit.
- Counterboring tool
- 2k-ohm resistor
- 18-gauge wiring
- Soldering iron and electronic flux solder
- Needle nose pliers
- Standard pliers
- Stereo-to-mono plug adapter
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