[R129 SL] FW: Get a input on a becker radio

[Jan Kuylaars]

If you have a original Mercedes antenna, they  have most of the time a connector for the coax cable. On this connector you can connect a duplexfilter for the car phone. If you got some, or the housing with connectors,you can easily change it in to a coupler.in practice should a voltage-divider network from three resistors from 100 ohm do the job. the load impedance is then 100/2+100 = 150 ohms. (carantenna) You can connect the transmitter output from the accessories true the coupler directly to the antenna-input of the Becker. Also it is possible to make a inductive coupling. Just simply turn a wire what is connected on the transmitter output cable a few times around the antenna bar from the Becker, and sealed it, or coupling it with a capacitor from 100nF When the Becker has a normal antenna inputplug,  you can put the two coaxkabels together to one plug. When you want to do it right , put the resistors between, or make a inductive coupling with the wire from the accessories  turned around the inner wire of the carradio if there is enough room in the plug or use the 100 nF capacitor .

When you want to make a 500mV stereo input on the Becker, the way to find such a thing without a oscilloscope or drawings is to look for the volumecontrol-potentiometer. These controllable output most of the time goo's to the end amplifier. Normally  one end of the potentiometer is negative or earth, the other side comes from the tuner/cassette/CD preamplifier/tonecontrol, and the middle one is the control output. You can connect the input directly to  the hot side (not the earth) of the potentiometer so you can use that knob for control . But you don't wish to hear two signals together, so you must put a double a-b switch or relay between the potentiometer and the two signals ,so you can choose which input. The switch is the easiest way, if you have space but if you want to make it automatically , you can power the relay by two transistors BC547 npn. The collectors from both  transistors gets the 12 volts by a fuse . The emitter feeds the relay what’s on the other side on earth. The base is connected to the 
Emitter from the first transistor. The base from the first transistor is feed by the positive output from a  rectifier block.(or four diodes in bridge.) The negative from this blok lay's on earth on the negative side of the relay. So the two alternative current inputs are used to pickup the signal from the accessory .one on the hot wire and one on the shield negative.for safety reasons it is recommend to put an electrolytic capacitor from 10 uF in series with both the inputs.
Then you are sure there could not come a DC current to your amplifier.
Explanation;
When there is no input from the accessory , everything is in the rest position , and the relay is in NC.(normally closed) and switched the output from the radio preamplifier to the hot point of the volumecontrol . The radio works normal . When there is music, the rectifier bridge makes from the AC a DC current and opens the first transistor. The second transistor (darlington connected) gets a small current from the first and opens a amplified current to the relay . The relay opens and switched the hot volumecontrol point to the extra accessory input. The amplifier now gets a signal from the accessory and play's that sound.
I made it several time's in the past  but then with extra potentiometers so I can trigger the switchpoint. I put the police radio on the extra input and have normally music, but when there came an police emergency call it switched only for that call and then go back for music. Ofcourse it also will work with CB- telephone or other receiving equipment.
with complements
Jan Kuylaars
Technical manager  RIAN BV


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