-=EmpyreaL=-: Blaupunkt SC 202D (C) repair, AUX

Backlight is not working properly. Some lamps do not light at all, some from time to time. Few capacitors leaked near cassette mechanism. Pronounced hiss in speakers above half volume range. Amplifier becomes very hot and distorts sound on high volume when playing from cassette adapter. Need AUX to play music from phone, because adapters and modulators suck as hell.

First of all I figured out how to make AUX. I didn't want to remove cassette and connect to 4558 chip as lot's of people do, mess with ugly buttons, etc. I decided to make AUX instead of M radio, which functions through MC14051B Analog Multiplexer/Demultiplexer.

1 - no need to unsolder cassette mechanism, just put it gently on frame.

2 - no need to disconnect display. I disconnected because I shorted circuit and it glitches.

3 - pull up legs 2 and 15 on MC14051B to disconnect from M radio signal. Look in datasheet.

4 - solder R and L audio to 2 and 15 pins on MC14051B. I soldered left to 15 and right to 2. These pins should be disconnected from circuit board. Audio GND I soldered to 8 pin on MC14051B.

5 - short NF-R and FM-R to make AUX work - 13 and 12 pins - see Blaupunkt SC 202 diagram. Without shorting NF-R and FM-R M radio still hissing and phone don't send signal. I checked stereo with custom audio files - works as expected. The only problem - I don't understand how it works and why. FM works, just push the button and switch from M to U, cassette also should work, but I didn't tested. I think right way is to disable M radio on Radio Module. My logic was that MC14051B as channel switcher may do the trick but it doesn't feel right. Also I didn't afraid to kill it, because it's cheap and easy to solder. As I understood NF-R is Noise Filter Right and FM-R is right FM channel. If NF-R and FM-R shortage will produce some issues, I'll find another solution. Now it works perfectly. Car radio amplifier is not too hot. Audi 80 B4 8 speakers sound amazing.

I need to figure out whether I can make display work properly. Also I need socket for 3.5mm jack on the back of car radio and line-in cable, which will be hidden in cigarette lighter compartment. If you count time and costs, these repairs are stupid, because one can buy working old deck with AUX for 20 EUR. If you don't want to change car radio, repair is way to go.

DO EVERYTHING AT YOUR OWN RISK. DO NOT short VCC and VSS (GND) anywhere on circuit board, because it can damage other components even if shorted component can withstand shortages. Study datasheets carefully: Motorola MC14051B, Hitachi HD44790 (it's not HD44780 for ARDUINO), Motorola ZN408800CFN (MC68HC05B4 made for Blaupunkt).

I blew transistor 1Ks 2n between HD44790 and ZN408800CFN, and stupidly made Blaupunkt SC 202D (C) into Blaupunkt SC 202. Display shows all icons and do not change data. I changed 1Ks 2n (BC848B) to TZT 2N3904 (1AM, 3904, MMBT3904). Now display works well:

Type Designator: BC848B

SMD Transistor Code: 1K_1Kp_1Ks_8CB_G1K_SAB

Material of Transistor: Si

Polarity: NPN

Maximum Collector Power Dissipation (Pc): 0.2 W

Maximum Collector-Base Voltage |Vcb|: 30 V

Maximum Collector-Emitter Voltage |Vce|: 30 V

Maximum Emitter-Base Voltage |Veb|: 5 V

Maximum Collector Current |Ic max|: 0.1 A

Max. Operating Junction Temperature (Tj): 150 °C

Transition Frequency (ft): 300 MHz

Collector Capacitance (Cc): 6 pF

Forward Current Transfer Ratio (hFE), MIN: 290

Noise Figure, dB: -

Package: SOT23

I changed 4 capacitors - main 16V 2200mF and 16V 10mF, 16V 47mF, 16V 47mF near cassette buttons. According to LCR-T4 they are still OK, but both 47mF leaked... I didn't fix backlight, because I am fed up with all this. I dislike too much lights in car, so it's enough for me to have button light. Maybe later... Finally I put 3.5 mm socket and jack, added more isolation strip firmly fix cable with jack: