I have an Info 4000 car radio, which was often installed in a Mazda RX7. In the picture, there is a button labeled “DK” at the bottom right. I’m wondering if this button has anything to do with the DIN socket on the back and whether I can connect a USB decoder there.

 

I think what you mean by USB connection is that you want to plug your phone into a USB connection that is an aux input to that radio? Also I don't know about this specific head unit in your photo, this is just speculation.

I would be amazed if that DIN connection is an input, as back in that era the concept of an aux input did not exist. My guess is that it's an audio output for an external amp and probably has a 12 volt trigger and stereo signal output, maybe even faded for a 4-channel external amp.

​​​​​​ However, if that DIN socket is intended as an equalizer loop, then you might be able to hijack the input in order to route an external into the head unit, but I think that's a long shot. My reasoning is that if it were an equalize loop there would be a loopback connector inserted into the head unit for use without an equalizer.

You need the pin out diagram for that head unit to be certain of anything.

My current solution to maintain factory look but also have some convenience is to use a Bluetooth adapter connected to a cassette adapter. The audio quality is not great but it's enough for me right now. If it were a daily driver I would consider one of the aftermarket "classic" radios with all the modern features and I would bypass the OE Clarion amps that seem to be fairly frequency limited. That head unit of yours looks pretty rough so that might be a good plan for you anyway.

 

As stated you need to get the schematics for that head unit to understand what can be done with that DIN plug. I know for the series 1 cassette/radio deck you could make a DIN plug to audio jack that when plugged into the extra DIN on the back of the radio, it cut out the internal feed and takes sound from the audio jack. I did this to get my IPOD to play directly into my OEM radio. Works great. Not BT but all analog.

Is this a head unit from Europe? I don't think I've seen this model before.

 

This is just a regular USB stick with music stored on it.
The device from the main question is a spare parts donor.
The pictured unit is exactly the same as the one currently installed in my RX-7.


 

There's no way that head unit will play music files directly from a USB stick.

To play your own music files, you need something like a phone that will play them, and then you have to somehow get the audio signal from that device into your head unit's analog signal path. In the past people have used FM broadcasters and cassette adapters. Both of those solutions can be had with Bluetooth capability. As noted, the sound quality is just okay.

If you can hijack the signal path through that DIN socket then that would sound better, but again you need the pin out to understand what's really going on there.

Since you have a sacrificial head unit, or at least one you can take apart and look inside easily, then you might be able to gain some insight as to what that DIN socket does.

Are you in the UK?


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I want to connect a USB decoder and I’m wondering whether the DIN socket has anything to do with the DK button. So that when I press the DK button, the music from the USB will play.

I’ve had my RX7 since around 2010, and I live in Germany, near the Harz mountains.

 

Based upon the fact that you live in Germany, and the German printing on the back of the deck, this would be a European Spec radio & deck. As TG Farrel stated, on the separate US cassette deck, you could take a AT style plug and modify it with a mini stereo plug and play music over the stereo. An AT style plug can be found but they are very old school.

The sound through the old stereo would be very poor, and if the stereo is bad, it will not sound good at all.

I used the AT plug, but my stereo is shot now. I am upgrading to a Bluetooth Amp and new speakers and more importantly new wire for the speakers. The old wiring is just to marginal for the power that a modern amp will put out. As part of the upgrade I had my 50 amp alternator upgraded to 65 amp. You will want to do that also.

I am going to install the amp in one of the storage bins behind the seats.

The old stereo will be left in the dash as is, I had enough of cutting dashes for stereo installs in the 80's.