The encroachment of digital technology and embedded processing into common consumer items has been mentioned in articles about the Heathkit AJ-1510 (1972) and the Sherwood Micro/CPU-100 (1976) FM stereo tuners. Here we have an example of the continuation of that process, the Becker Grand Prix 612 AM/FM/Cassette Car Stereo, dating from the early 1980s.

Less than 10 years after the AJ-1510, it was possible to jam a complete microprocessor-controlled stereo, with phase-locked-looped tuning for both AM and FM, into a car radio form factor. As an early example of this degree of miniaturisation the Grand Prix 612 was a high-end item in its day. It was the standard offering for a radio in Mercedes-Benz automobiles for several years in the early 1980s.

In a notable break from convention, the volume and tone controls are also under digital control, the design completely avoids the use of rotary controls in favor of pushbuttons. The volume and auto-scan tuning 'bars' push both up and down for direction. The bass and treble pushbuttons function in a cyclical manner, pushing one of the buttons increments the respective level till the maximum, subsequent pushes then decrement the level till the minimum, and so on. The tone can be reset to flat by pushing both tone pushbuttons simultaneously.

The digital technology now allowed a clock to be included in the radio. A timer is also included, allowing one to set a time at which the radio will automatically turn on.

There were apparently two versions of the 612, an earlier version with two flat pushbuttons each for the volume and auto-scan, and a later version with the toggle bars for these controls, as seen on the unit here. In the later version a noise suppression IC was added in the signal path between the demodulator and multiplex decoder, the AM front end was redesigned, the tape drive electronics changed, and other alterations made in the electronics.

Internally, the microcontroller used appears to be a Mostek MK3870, going from the pinout. The 3870 is a descendant of the Fairchild F8 family. The firmware is mask-programmed into the microcontroller. Communication between the microcontroller and devices - the front panel pushbuttons, LCD display, tuning PLL, and audio control D-A converters - is accomplished with a clocked serial bus. The bus is composed of primarily 3 lines: data, clock and latch. This may be seen as a precursor to such schemes as the I2C and SPI busses.

A schematic for the later version is available upon request. (See email contact).


Top view with cover removed. This is the solder side of the MAIN board.

Bottom view with cover removed. Seen is the solder side of the FMAP board.

Rear view with connectors. The DIN jack must have the jumper plug plugged in as it connects the audio signal path.

The MAIN board rotates up after removal of two screws and carefully releasing the interconnect pins. The microcontroller is in the upper right corner of the MAIN board. The AM section is along the bottom of the MAIN board.
The FMAP board surrounds the cassette drive. The edge of the interconnect riser board - providing connections between the MAIN board, FMAP board and the cassette drive - is seen immediately behind the cassette drive. On the right the DAC and SYN board are seen on edge.

The front panel and cassette mechanism are removable as modules. The DAC, SYN and interconnect riser boards are unplugged here, leaving the FMAP board more visible. The FM section is along the rear of the FMAP board. On the left is the audio section with preamp and output amplifer ICs.
The power supply section is on the right of the FMAP board. This unit is non-standard in that an op-amp and associated circuitry forming a regulator have been removed and replaced with a simple 3-terminal regulator (TO-92 package) in the course of repairs.

The MAIN board with the shield removed from the microcontroller. This board is also waiting for a replacement TDA1046 AM Tuner IC.

The interconnect riser and rear view of the front panel. The front panel can be removed by loosening merely one screw on the right-hand chassis side.

The DAC board. This board contains the D-A converters for the volume, bass and treble. The volume is 5 bits, so 32 steps. The tone controls are 4 bits for +/- 8 steps each.

The SYN board. This is the PLL synthesiser for FM and AM.

Top view of the cassette mechanism.

Bottom view of the cassette mechanism.

  Unit Log
Becker Grand Prix 612
2015 Nov