Sony XR-CA400 Car Stereo with faulty TUX-A20

[netmike]

Hi Guys,

This is my first post here. I am a novice hobbyist just enjoying electronics and trying to learn a few bits and pieces along the way. I got stuck with my latest project and thought I would seek some help here.

I have a Sony XR-CA400 with what seems to be a faulty TUX-A20 tuner unit. There's no audio at all when in radio mode, although both tape and cd works great.

A bit more detail: The voltages to the A20 are ok (+8 and +5). I don't have means to decode SDA and SCL but they show up on the scope. What is missing is the MPX that seems to "hold" the actual audio. Also no RDS.

This makes sense because the radio does not find anything when seeking. It seems to be "missing" the antenna. I've traced the path of the antenna a fair bit on the A20 board but I do not understand how this piece of the circuit works, making it a bit difficult. Same thing for both FM and AM. The A20 is not explained in the service manual for the radio and there's no schematics for it.

I know this is a long shot but have anyone a clue of what could be wrong? Otherwise, where can I get I new A20 board? Or the schematics for it?

Here is a link to the Service manual: http://elektrotanya.com/sony_xr-ca400,ca420x.pdf/download.html

[HalfSpace]

Hi netmike,

On the linked service manual on page 17 the tuner unit is listed as FU1 FAE347-A13 is it the same circuit as I don’t see an A20 anywhere?

We need to check if the local oscillator is working for the radio.

In AM radio mode test pin 11 (AM/FM-OSC ) on the tuner unit with your scope. It should have an oscillator signal somewhere between 500KHz to 2MHz at around a 1 volt peak to peak. With the scope still connected do a manual tune and see if the oscillator frequency changers.

HalfSpace 

[netmike]

Hi HalfSpace,

Thank you very much for your response.

It seems I got a bit carried away when posting and just googled for the service manual and linked it. It turns out that the one I linked is for XR-CA400, CA420X but my radio is a XR-CA400 (!) and should use the manual for XR-CA400, CA410... Go figure! :-) Anyway, here is the link to the one that correspond to my radio: http://www.eserviceinfo.com/downloadsm/63532/Sony_XR-CA400-CA410.html

The PCB indicates that my tuner is an tux-A20. Unfortunately there is no OSC pin on this module... There is an IC labeled: "CO2, W884, OC"(got power) , and an other one named: Sony 8-759-653-23 (got power), an oscillator at 20.5 MHz (working). The rest are passives.

I include a photo of the tux-A20 that I found online.

BR
/Mike

[HalfSpace]

Hi netmike,

Attached below is the data sheet for the TEA6849H used in the tuner unit. On page 40 and 41 is an application diagram which should be a very similar circuit to the tuner unit.

The local oscillator is on pins 49 and 50 but an oscilloscope probe connected to ether pin may stop the oscillator working due to the probe capacitance. Try it anyway and see if it is oscillating.

The Vtune signal controls the local oscillator frequency. While measuring the DC voltage on Vtune (at pin 40), do a manual tune and see if the voltage changes. If it’s changing level and holds the voltage when you stop manual tuning then the local oscillator is working.

HalfSpace 

[netmike]

Hi HalfSpace,

It is REALLY difficult to determine if there's some oscillation or not on PIN 49 and 50. There's definitely nothing much. Pin 42 have about 126 mV even while doing manual searching. The datasheet talks a maximum voltage of 8.5v for Vtune.

Everything is smaller than tiny, making probing a bit of a chance...

BR
/Mike

[HalfSpace]

Hi netmike,

Pin 42 is the Vtune pin but could be some form of PWM signal which may not read well on a multimeter. Try measuring on pin 40 or after the loop filter (the 3 caps and 3 resistors connected to pin 42 and 43) this will be a smooth DC voltage.

Also see if you get a 75KHz signal from pin 45 (FREF)

If there is no change in the Vtune voltage the TEA6849H may not be receiving the I2C commands to control it. Poor solder joints can be a common fault with modules like these.

HalfSpace