For your pleasure - Bluetooth music streaming in a Xantia

[CitroenCrazy]

Apologies in advance, this is a very long post, but I found it difficult to get the level of detail right. I'm sure I won't have succeeded, too much for some, not enough for others, but let's try.

How to convert a Xantia radio cassette player to support music streaming via Bluetooth.

I first did this modification a few years ago and I’ve been meaning to write it up for ages, so today I took the opportunity to do another one and take some pictures at the same time.

First things first, everything in this post relates to a late model S2 Xantia with a Philips 22RC200/35 radio cassette player; the one that has a flap to hide it that nobody ever uses.

The original idea was to simply add a 3.5mm audio-in socket on the front of the radio, but since Bluetooth audio adapters are available from eBay etc for a couple of pounds, it made sense to go wireless instead.
Anyway, the principle of the modification is to remove the cassette mechanism and connect the Bluetooth adapter to the audio input associated with the tape.
Why bother ? Well; big improvement in functionality for less than a fiver – speaks for itself.

Top cover removed, this is what’s inside:

The cassette mechanism can be removed by undoing 4 x T8 screws and unplugging two connectors, one on the left for power /signalling and one on the right for the tape head.

Underneath, we can see the two ICs we’re interested in. The largest is a Philips SAA 7708, which is really the brains of the beast; this is the bit that selects the audio source and processes it for delivery to the power amplifier stage.
The other is the TEA 0676, which is a dedicated signal processor for cassette tape mechanisms. This takes the signal from the tape head, applies Dolby processing etc and passes the audio to the SAA 7708. The idea is therefore to isolate the TEA 0676 and intercept the connections to the main signal processor.

The other important requirement is to make the system think there is a cassette loaded, otherwise it will not allow tape to be selected as a source.
For this particular unit, it’s very easy, we can just short two pairs of wires in the ribbon cable, as shown.
Brown to blue
Orange to green

Incidentally, not all radio cassette units are as easy to fool was this one. The double DIN Clarion PU1562A unit fitted to other Xantias expects to see a square wave signal to persuade it the capstans are rotating.

However, before we do anything too destructive, let’s take a look at the Bluetooth adapter.

I’ve had this one in a drawer for a while. One you might buy today will be different, but the principle remains the same. This one, slightly unexpectedly, had a built-in battery,

I specifically did not want this, for reasons I’ll go into later, so I removed it completely. However, perhaps unsurprisingly, this made the audio crackly. The device was not happy without a battery, so, I replaced it with a capacitor. After trying various examples, I had to hand, I settled on a 100uF tantalum bead.

It’s worth stressing at this point that you need to test the Bluetooth adapter before you start hacking the radio. Make sure it will pair with your phone and the audio quality is good enough. It’s never going to be audiophile quality, but it would be unfortunate to butcher your radio and only then discover the audio isn’t of the standard you expect.

Once you’re happy with this aspect, it’s time to get serious.
The outputs from the TEA 0676 are on pins 1 and 16, so we need to cut the tracks connected to those pins.
Pin 16 is connected to the lower face of the pcb, so the bottom cover has to come off

I used a Dremel with a micro burr, but you could use a knife.

This next picture shows where I picked up the connections for left and right channels, and also where I cut the track from pin 1.

I picked up the common (signal ground) immediately above the IC

Powering the Bluetooth adapter
The Bluetooth adapter expects to be plugged into a USB port, so it needs 5v DC. I had a 12v to USB adapter that had fallen apart, so I used the board from that. This isn’t strictly necessary as 5v is already available on the main pcb, but I went ahead and used it anyway.

IMPORTANT. when I made the first version of this mod, I picked up a 12v source that was permanently live, but this was a mistake. (1) it meant I had to manually connect my phone in Bluetooth settings, each time I got in the car and (2) on a couple of occasions, the device locked up, and the only way of resetting it short of removing the radio was to disconnect the battery. Conclusion – it’s MUCH better to use a 12v source that’s live with ignition.

As it's configured now, as soon as the ignition switch is turned to position one, the Bluetooth adapter will switch on and the phone will automatically connect. (The phone treats it as a headphone).
(Hence why I removed the built in battery, as mentioned earlier).

I fished around until I found a suitable 12v source at the top of the board.

As you can see, I used chassis ground for the ex-USB adapter

Test and test again

So this is the assembly, fitted to the radio. Note the audio source connections soldered across the 3.5mm connector

Overview of completed setup. Note the cassette buttons glued into place. Yes, I’m a big fan of hot glue !

Just to be on the safe side, I cut a window in the top cover, to make sure the Bluetooth signal wasn’t completely shielded.

So, does it work ? Yes, very well.
The only snag that’s worth mentioning is that the main signal processing chip, the SAA 7708 is expecting an amplified and buffered signal from the tape drive. The output from the Bluetooth adapter is a little bit low.
With my iPhone on maximum volume, it all works fine. Obviously, you can turn the main volume up a bit to compensate, but the ideal is to have the cassette and the radio at more or less the same level, so you don’t get deafened when switching from tape to radio.

And there you have it. Sorry for such a long post. If there are bits I’ve glossed over, please feel free to ask in the comments.

Edit - version 2
Picture added at the end shows a much neater V2, which picks up a 5v supply from the main board

[GiveMeABreak]

Well I’ll give you 100 out if 10 for Percy Verance!

Thanks for the write up, I'm sure it will be of interest to members.