This should work on late nineties NON Bose HU’s…
Open you stereo and look on the motherboard for a TQFP32 chip with the following print ZPLA774 and a ST logo on it. If you got it keep reading.
This chip is the audio processor of your HU. The actual part number is TDA7342EQ2N, here is the diagram.
How it works:
The chip has two stereo inputs per channel plus one mono input and one quasi-differential input. The sound signal coming from the radio, CD changer and tape player is selected based on the input from the “SOURCE” button (this is the mixer module of the chip). The selected signal is directed in the volume control module and bass/treble adjustments. Then the signal is sent out to the 4 outputs (2 x rear/front). All adjustments are done based on the control signal received from the HU processor (on I2C communication bus).
Enough with science, now what do you need:
- Soldering Iron and Soldering Wire;
- Ohm meter;
- Flat head screwdriver;
- Long nose pliers;
- Steady hand;
- 3.5’’ - 3 conductors female jack connector with two normally closed switches connected to the contacts themselves (i.e. should have 5 pins not 3);
- Low signal shielded wires (like the one from your headphones or RCA cables, preferable 4-wire cable with braided shield).
1. Remove your stereo from the dashboard (procedure varies from model to model);
2. Remove the upper stereo panel cover;
3. Remove the tape player mechanism by removing the 3 screws and the two connectors;
4. Locate the audio processor: a TQFP32 format chip (i.e. square with 8 pins on each side), about Ľ of an inch big, usually covered in silicon with the ST logo and ZPLA744 print. On Seville HU’s is located in the front/left side of the board and on Catera in the middle.
5. Clear the silicon covering the chip and the surrounding area (use a knife and be very careful);
6. Locate the pin pairs 2, 3 and 15, 16 (use the pins description page 6 in the diagram). If between the pin pairs is a capacitor (100 nF on Seville HU’s) then disconnect the line between pin 3 and capacitor (same with pin 16) by scratching it with the knife. Be very careful this is a very delicate operation. Use the meter to make sure that you have no more continuity between the capacitors and the corresponding pins (3 and 16). On Catera, just scratch the circuit-connecting pin 15 and 16. Pin 2 and 3 connection is under the chip, so in this case desolder one of the pins (be very careful not to bend the pins, they break very easily).
7. Locate a ground connection close to the audio processor (using the meter) and now solder the braided shield on the ground (if you have more wires connect all the grounds together). Then connect one wire to pin 3, one wire to pin 16 (direct connection) and the other two wires on pins 2 and 15 (on Seville connect the wires before the capacitors: serial connection and not directly on pin 2 and 15).
8. Use the meter to identify the pins for left and right channel on the 3.5’’ jack female connector. Usually the outer most pins are the outputs and the inner pins are the inputs (ground is the one in the middle). Connect wire from pin 3 to the Right channel input pin of the connector. Connect the wire from pin 16 to the Left channel input pin of the connector. Connect the wire coming from pin 2 (on Seville from the capacitor connected on pin 2) to the connector’s right output pin and the one from pin 15 to the connector’s left output pin.
9. Test with the meter for continuity. When no male connector is inserted in the female connector, you should have continuity between the wires for the left and right channel. When a male connector is inserted you should have no continuity between but wires coming from the female connector outputs should have continuity with the corresponding channel core wire from the input cable. On Catera due to the fact that you have no capacitor in serial connection with pins 2 and 15, you might have the input signal level to high (resulting in distortions). In this case you have to insert 2 100nF capacitor in serial connection on the core wires from your input cable (one for each channel). Seville already has the capacitors so the signal level is perfect (no need to add the capacitors).
10. Is your choice where to place the input connector (stereo fascia, glove compartment, etc.). I will not insist on this aspect.
11. Follow step 1 to 5 (in reverse order) to put everything back and ENJOY.
Follow the diagram, is much more comprehensive than 100 pages of tutorial.
When you insert a male connector (let’s say coming from the Ipod) the mixer output signal is interrupted and you will hear on the speakers the signal from your Ipod. You have full control over the volume, bass and treble (including the steering will controls). The quality is the best you can get as good as the one of the CD changer adapters only that you will keep the option of having a CD changer.
Warning! If you don’t have experience with electronic devices this is NOT a job for you.