Replace tape block with a bluetooth receiver in a car radio

[den]

Hi everyone,

I am trying to "improve" my original car radio (Blaupunkt, 2001, installed in an Audi A6 as Chorus radio) by adding Bluetooth. Bluetooth receiver is just a small device with a loudspeaker, which can play music, that's all. The radio has only 3 inputs - Radio, CD changer and Tape (no Aux). I decided to replace tape with Bluetooth. Important thing - I have full schematics of this radio (attached screenshot of the tape preamp, full document here). But still cannot get it to work normally.

What I did up now is - connected Bluetooth receiver to a 5 V rail in the radio and connected loudspeaker outputs to tape COM and input (either L_NORM or R_NORM) through capacitors (you can see there is 4.3 V bias voltage there, so I guess I need capacitors). It kind of works - I can hear music playing, but very quietly, even at full volume. And also - it works actually with only COM connected to one of the loudspeaker outputs, the other input does not matter.

Any ideas/suggestions? Why does it work the way it does and how to get it going correctly?

Thanks in advance,
Den

[IanMacdonald]

The inputs will be very sensitive at ~ 1mV and have a massive bass boost/treble cut. The sensitivity could by overcome just be using an attenuator (with good quality metal film resistors for low noise) but the 'equalisation' would be better handled by defeating it.

https://www.iasa-web.org/tc04/magnetic-tapes-replay-equalisation

I suspect the RC networks near the chip inputs are the equalisation, but without chip details it's a bit hard to say what would be needed to give a level response and less sensitivity. 

Other issue is that the tape mechanism has Enable and Data lines. it's likely that it signals to the processor that a tape is inserted and running, in which case audio is switched over. Without the right signals the output may simply be ignored. This might just be a logic level,  or a pulse train. Hard to say as the manual doesn't give much detail on this.

[CJay]

Post a ,link to the full schematic but I suspect the operation of the tape deck is being monitored and the radio mutes unless it can see signals from teh deck to say it's moving and in operation.

Quick and dirty proof would be to put in a tape with the L_Norm and R_Norm from the tape deck disconnected so you can feed in your signal.

[BrianHG]

I mute out, or cut the tape head preamp.  & usually feed my auxiliary audio either mixed. or switched A/B through the line level at the volume knob.  Though, this was with an old mechanical volume radio.

No preamp hiss and flat audio and if I was using the switch, I can leave the radio wherever it was, like an FM radio & quickly switch on and off my auxiliary.  Say you BT receiver has an LED for when passing sound, use that guy for the auto switching...

[den]

To make it clear - I actually connected the tape block to the first connector to ensure that everything is enabled. Audioprocessor gets a correct initialization from master MC, selecting input 2, which is tape (tested with scope). The problem is - there IS sound, but it is too quiet, even with max volume.
I will take a look at the posted link tomorrow, but it actually looks like I am lacking knowledge (apart from other things) of how actually tape stuff work.
And I still don't get why it works with one signal wire only.

[BrianHG]

Why are you trying to force feed the tape pre-amp, which requires a balanced inductive input of a tape head, which also switches back and forth and has a mute instead of feeding your sound through C1426 for right, and C1406 for Left.

Just follow the output of the tape pre-amp you'll find those 2 caps on the schematic.
Lift them up and feed into each cap standard line level audio.  The other side of the 1uf cap should be feeding the AF-Controller IC TDA7342.

GND on you BT module goes to GND...

[BrianHG]

Have you tried putting in a cassette while playing?
Run the tape in reverse direction so the play heads switch...

[Audioguru]

I suspect that you are shorting the Blutooth outputs to the tape player preamp outputs. Shorting outputs together does not work, you must disconnect the tape player outputs and replace them with the Bluetooth outputs if they still work after being shorted.

[BrianHG]

I suspect that you are shorting the Blutooth outputs to the tape player preamp outputs. Shorting outputs together does not work, you must disconnect the tape player outputs and replace them with the Bluetooth outputs if they still work after being shorted.

He is not doing that.  He is shorting the BT to the tape head inputs...
3 posts above, I directed him to the correct caps to remove and feed in the BT module's audio signal to the preamp-volume/tone input selector IC for the 'tape' channel.  It is up to him if he wants to try it out as it would be the correct way to go about this.  The tape mechanism, tape head preamp, and, audio source selector input switch preamp with volume control IC are under MCU software control and the latter has a software controlled mute.  If the MCU doesn't disable the mute, and the low volume he hears is audio leaking through one of the preamp ICs, he must pass that hurdle first before getting proper sound.

[den]

I actually tried several things, feeding to the tape inputs is the only thing which somehow worked. Don't know why, but feeding sound directly to Audio-MC didn't work (but I fed 1 line only). Still, I will try it again, thanks for the advice.

@Audioguru - no, I did not short the inputs, tape head is disconnected, so that is not the case.

I also find it interesting, that it's possible that sound is still there, but quiet, even if something is muted. Didn't know this was possible. Will check that again if neccessary.

For the time being - thank you all for advices, will try them and then give feedback (maybe this takes longer, Saturday is work day this week...)

[den]

Soooo, have tested some things today and actually got it louder this time. The only problem now - very noisy. Completely disconnected tape traces and feeding directly to Audio MCU inputs. The reason it was quiet before is the same noise. When switching to tape Master MCU gave command to Audio MCU to go into Soft Mute. I thought it was something like mute (no sound), but when increasing volume it does it slowly (or the other way around). But as far as I understood from a brief search - it's actually a noise rejection feature (makes sense for tape input). Since I have tons of noise - it was reducing that and simultaneously also music volume was reduced. Using Arduino (Audio MCU talks using I2C) I just disabled that for now.
Tried to disconnect the running tape block (two el. motors) - no effect. The noise comes from a bluetooth device itself. It is not heard in a small loudspeaker, but apparently when amplified - can be heard very good. Did not take screenshots from scope yet, but one of the components is a approx. 200 kHz sine wave, which sits on top of the sound wave. This I removed pretty easily with a simple RC low-pass. But there is something else - some strange peaks in the form of triangles with 3 kHz frequency, so no filtering this time.
Any ideas why this noise is there and how to deal with it?