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Michaël Duerinckx

 

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

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.

25 comments

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  4. [...] [Michael] just sent us this nice example of some good ol’ fashioned radio hacking. [...]

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  7. ColdTurkey says:

    Interesting hack! The string holding the jack in place is quite amusing, hopefully a hot melt gun(and drill!) is next on the list to keep this classic hardware working. It’s a shame you couldn’t keep the radio functionality (just for the sake of original hardware still doing what it should) but at least it can be easily returned.

    I wonder, could you use 2 diodes with a low Vf in reverse bias to safely probe the circuit with an iPhone or similar?

    • Mich says:

      Thanks! Yeah, a hot glue gun or similar is definitely something I should get my hands on, as well as a drill. Where I currently live I don’t have much space to put these things out of the way though. Here’s hoping I can move pretty soon, so I can set a more reasonable workspace.

      The radio actually still works mighty fine! When I want to use the aux input though, I just trick the band selector buttons into not selecting any of the bands, thereby not feeding any radio audio into the amplifier. I’ve found, however, that even if the radio is playing, and I then plug in an audio source into the aux socket, the radio just gets silenced, and my external audio source takes over.

      So, I haven’t had to compromise any of the original functionality for this hack. Otherwise I don’t think I would’ve gone through with it.

      As for diodes for probing: I think it’d still be complicated, as I’d never be too sure about the voltage orientation (is that a term?) at the points I’m trying. Not to speak of AC signals in there.

      Thanks for the comment. :-)

      • ColdTurkey says:

        Ah, I assumed you had removed radio functionality as you said after you had found the correct traces “At this point I got the soldering iron out and de-soldered the three stereo wires coming from the radio output”. Glad to hear I just mis-read it!

        What I meant with the diodes was one forward and one reverse so they conduct an ac signal together (might sound a bit distorted) without letting anything nasty through but thinking about it I don’t think it would. Never mind, just a thought.

        Anyway, nice hack!

  8. Thanos Baxevanis says:

    On the back of the case there is a 8 pin din socket, typical of consumer audio of this era, that was purposed to audio in / out. Did you check this?

    Nice to see adapting old beasts though, as they are manufactured as simple as possible for cost reduction.

    Thanks

    • Mich says:

      This was also mention on the HaD write-up. I did try connecting my audio source to those pins, but was unable to get any audio through the amplifier that way. My guess is that it’s mostly there for recording external audio sources to cassette.

      I must admit that I haven’t tried changing the switch position next to it while testing that part, so maybe that’s something important I’ve overlooked. I may just pick up an adapter for the port and see if I can exploit it that way.

      Thanks for the comment!

      • ElektroNeko says:

        The trick to use the DIN socket is to connect the sound source to it and then press record, play and pause on the cassette player while having a cassette with the security tabs intact (or sticky tape covering the holes). That way you should get sound through that input.

        The input was used for recording on the cassette deck, not for connecting stuff to play sound through the speakers. There’s sound coming from the speakers only because you could monitor whether the sound clipped or not.

        There are many many boomboxes and ghettoblasters with real either real aux in connections or phono in for a vinyl player.

      • Adrian says:

        Often you can just press Record on its own. It won’t actually record, as Record+Play is needed for that. But it does allow the external signal through to the speakers.

  9. question says:

    “…I didn’t want to risk electrocution by using the on-board transformer, so I wired up a bench power supply instead.”
    How did you do that?
    Just plug the cables from your bench power supply to the secondary winding of the transformer?

  10. drandel says:

    Wow!! That’s a great idea! Now, here’s the question … can you record what’s playing on the iPhone to cassette tape??? ;-)

  11. [...] Hacking a line-in socket into an 80s radio and cassette player | Michaël Duerinckx via Hack a Day [...]

  12. [...] Hacking a line-in socket into an 80s radio and cassette player [Michaël Duerinckx via Hack a Day] [...]

  13. Since I haven’t listened to radio in ~25 years now(been building a humongous collection of FLACs), I would have simply left the radio part disco’d. Leaving the radio in will almost certainly have some effect on the audio, more or less but some. I did this thing to one of mine a while back.
    I’ve always had a simple principle, any job worth doing should buy you a tool. In this case, I’d probably go for the hot-glue gizmo. I use mine often, it’s a great investment, and cheap. Every job buys a tool, so you’ll develop a very nice toolbox, tools you -know- you’ll use.
    Good project, thanks for posting it.

    • Mich says:

      I still do use the radio every now and then, so I’d rather not break it. Besides, as mentioned, when I trick it into not selecting any radio band, there’s no audio coming from the radio part anyway.

      “Any job worth doing should buy you a tool” – that’s actually a good idea. For some jobs, I just want to have them done in one day though, which makes buying a new tool for the job unrealistic. In this case I did most of the work on a Saturday evening, when all the shops would’ve already closed.

      I’m definitely looking to but a Dremel kit or similar soonish though, as well as a hot-glue gizmo.

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