Sunday, 8 January 2012

Car Audio: Skoda MS402 AUX IN Retrofit

In August, I bought a Skoda MS402 car radio from a car boot fair, for the very reasonable price of £1. Since then it has been connected to a power supply by the side of my bed, getting used very rarely. Recently I purchased a new laptop, and as is common on laptops, the speakers are terrible, and so I needed to do something about it.

I wanted a method that could still allow the tape player to be used, and as such when playing from the input the tape motor is still spinning but is nearly silent.

After opening the radio, my first step was to search Google for any chip datasheets that could tell me where to easily input some audio. I found the Pre-Amp for the cassette deck (TEA0676T), and decided this would be a good place to connect some wires for an input socket.





If I solder wires to the output stage of the pre-amp, the signal will go to the main audio processor.
The pins don't have left or right designators on the datasheet, just 1 and 2.

This is the main audio processor, I could have soldered to this, however the pin spacing is minuscule, so I thought it would be best not to...

I then soldered the 2 audio input wires to the chip and soldered the other ends to a 3.5mm audio socket.
The ground cable from the audio socket is connected to the radio chassis.

Using generous amounts of hot glue, I stuck the audio socket into a hole I made in the radio chassis.
The socket from the outside of the case.
The display when playing from the audio input.
The back of the case with the case closed.

38 comments:

  1. Hi, I can appreciate this blog is four years old but I have a few questions. Did you refit the tape deck? I've got no need for a tape deck but wonder if there is any issue with leaving it out? When you apply a signal to the pre-amp can you then change source to tape? I plan to go a step further and fit a Bluetooth receiver instead of a socket.

    Any help would be appreciated

    Ryan

    ReplyDelete
  2. Hi Ryan,

    I did refit the tape deck, due to the fact that the source switching is done by a switch that's on the tape deck.
    Now, as I was using this on a desk, without the radio, I was able to activate the tape deck using a screwdriver and leave it, however, if you wanted to use this in a car, if you could find the switch and follow the wires back to the main board, and connect a latching SPST switch across it, there's no reason you couldn't remove the tape deck entirely. I reckon the switch would be best placed either behind the tape flap, or connected to the FF/RW button, so as to preserve the original look of the stereo.

    In addition, I suggest removing the TEA0676T entirely if it's going to be used with an external device, because there were some issues about voltages being present that were exposed to the device connected by AUX, which may or may not cause damage. This chip isn't responsible for device switching so could be easily removed without consequence.

    Also with regards to your bluetooth reciever, if it's the type that plugs into USB, there's a 5V regulator on the board, a 7805, that could be used to power it without having an external supply.

    ReplyDelete
    Replies
    1. Thanks the reply. The Bluetooth receiver is actually a hands free Bluetooth to FM transmitter. If there is a 5V regulator on board already, that's another piece I can leave out :)

      Do you know if the 7805 is permanently live, or a switched live via the combined volume control and on/off knob?

      If you remove the TEA0676T where do you feed the audio input? Would you then go via the main audio processor? I'm tempted to rely on the FM transmitter as the input, coupling the two antennas together. Not the purist's way but good enough.

      Thanks for the info. The radio is still used daily and its interesting to see how good a blank canvas it is.

      All the best

      Ryan

      Delete
    2. I never checked to see whether the 7805 is permanently live, I just remember it being there when I had a good look at the board in its entirety. In hope of good design, I would suspect it is switched, at least by ignition, to stop the slight draw that would occur otherwise. Also given the chunky size of the switch/volume control, I'd say it's fair to assume that it switches a fair amount of current, so hopefully that includes the 7805.

      I didn't quite make it clear in the original blog post, but as the TEA0676T is a preamp, it's not the input side that I fed the line level signals to, as they would be far too powerful in comparison to the tiny levels of a magnetic pickup. Rather, the left and right channel are connected to what's described in the data sheet as Out A and Out B, as each side of the tape head has it's own pickup, one channel for each direction. These tracks are line level and go from the preamp to the main audio processor.

      Whether this means stereo audio was actually played I'm doubtful, but I never noticed any lost vocals, and I'm usually pretty sharp at picking up when a channel is missing.

      Also, as far as bluetooth adapters go, I've got one of these http://ebay.eu/1OWtyUX, which at £3, I'd recommend to test out the whole setup before plugging anything more expensive :P, this has served me quite well, as fortunately I can hide it, and my aftermarket stereo under a convenient radio-flap so as to not ruin the aesthetics of the dashboard.

      Delete
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