Hacking a line-in socket into an 80s radio and cassette player

in Blog

A few years ago, my grandmother gave me one of the old radios she had lying around the house. My late grandfather’s hobby was participating in radio quizzes, which led him to win a lot of radios; I assume this was one of them. The radio was manufactured in the early 80s, so the guts are relatively easy to tinker with; everything is on single-layer PCBs.

Frontal photo of the radio before any alterations were made, showing buttons on top, speakers on either side of a cassette deck, slider buttons, a radio tuning dial across the top, and sound volume indicator lights
The radio, as it looked before opening it up

I realised that the audio quality of this old boy was really decent, so it would be a waste to just let it sit in a cupboard. I don’t generally listen to the radio that much, and I don’t exactly have a big collection of music on cassettes either (and if I did, I’d digitize it anyway). To really get some use out of it, the most logical thing would be to have a line-in, letting me hook up a phone or whichever external audio source.

So, I set to work, opening the machine up.

Photo of the radio laying down on its back, the front part of the case taken off. Revealed is the mechanism of the cassette player, various sliders, PCBs with various components on the left and right, and the transformer on the far left.
The front part of the case taken off, which removes the speakers
Angled photo of the backside of the radio, with the case removed; shows the soldered side of the PCBs, as well as the internal part of the radio antenna and some wiring
The innards taken out of the rear section of the case

I used a Korg Monotron as a relatively cheap audio source to test points where I could introduce the line-in to the system. If I made a wrong move, I could blow up my phone, so I didn’t exactly want to use that.

While testing, I didn’t want to risk electrocution by using the on-board transformer, so I wired up a bench power supply instead. I also had to use the headphone output to test audio output, as the speakers were glued to the case I took off – I had to disconnect them.

The PCB really didn’t have many useful markings on it, but I managed to locate the cassette player’s audio output. Experiments showed I’d have to press the play button – thereby setting in motion the rattling cassette mechanism – to get any sound through. Not an option.

Eventually, I spotted similar wiring coming from the radio PCB. For this to work, the radio has to be turned on, which, naturally, causes the radio to start playing. However, there are buttons to select the radio band you want to use (FM, AM, and a couple more obscure ones). I found that I could trick these buttons, allowing no band to be selected, that way stopping any radio audio from coming through. Bingo!

At this point I got the soldering iron out and de-soldered the three stereo wires coming from the radio output. I entwined them with some wires of my own and soldered them back in the correct holes. Remembering the correct holes was easy, as the manufacturer decided to indicate the wire colours on the board. With the help of crocodile clips I confirmed that this worked as expected.

Photo of the innards of the radio wired up to a bench power supply. A black, red and white wire hang down from the radio and are connected to an audio cable coming from a phone by crocodile clips
Testing the newly soldered wires with an audio source, through some crocodile clips

Then, it was a simple matter of cutting my new wires to a usable length and soldering them onto a 3.5mm stereo socket. I had to make sure I got my left and right wired correctly. At this point I was confident enough to use a more expensive audio source, so I used a simple free Android app, aptly named “Stereo Test”.

The 3.5mm socket is shown with red, white and black wires soldered to it. The wires are entwined to form a neat bundle.
Wires for the socket soldered and cropped to a useful length

Next up were some mechanical challenges: fixing the socket in the case, and putting everything back together.

I don’t have a drill in my kit at the moment, so I had to improvise a bit. With the aid of a nail, screw, screwdriver, and pocket knife, I dug a relatively neat round hole to fit the socket in the front panel of the case. I had to scrape a bunch of plastic to make sure the socket wasn’t too deeply buried in the panel, otherwise no jack would click in there properly.

The next challenge was keeping it in place. The socket is pretty tightly stuck into the case, but not quite tight enough to withstand repeatedly plugging in an audio jack. I didn’t have any hot glue or super glue to keep it in place, so I fixed it up with a very taut piece of string that presses the socket to the panel. It appears to do the trick, as I haven’t managed to push the socket back once.

Photo showing the socket inserted into the front-panel, held in place by taut string, knotted badly around what was available
Taut string knotting contraption keeping the socket firmly in place

Putting everything back together involved a lot of pushing around things to make them fit in place, but overall I didn’t experience any major hurdles in that.

Photo of the front of the radio, with audio cable plugged into the new line-in socket. A smartphone playing music is leaning against the radio as an example audio source
The finished hack, with some Digitally Imported playing as an example
Close-up photo of the socket in the front-panel, without an audio jack plugged in
Close-up of the newly installed socket

Works like a charm!

On a side note, I’d like to apologize for the terrible quality of some of the photographs. I took a few phone pictures throughout to show a friend, and those made it in here. I wasn’t focussed on getting good photography done, and only later decided to put together a write-up of the hack.