Arcam AVR400 service manual

It communicates with the Input Board via CN101 and a 31 way ribbon cable. Other connectors comprise CN94 which connects to two secondaries of the main power transformer and the hard wired BN93 which connects up the daughter board.

There are two associated break off boards. One houses the front panel mounted single pole mains switch plus its suppression capacitor C901 and a connector BN502. The second is mounted on the power amplifiers’ heatsink and is used to route power to the cooling fans and also to guide the 31 way ribbon cable.

The VFD draws AC filament power from a centre tapped winding of the power transformer connected to pins 1, 2, and 3 of CN94. The centre tap connects to ground via the zener diodes D901/2 and C903 to provide the filament with its required DC offset. Pins 4 and 5 connect to a relatively high voltage transformer secondary which is half wave rectified by D916 and smoothed by C907 and C960. The zener diodes D903 and D904 in series with R906 generate +40V which is then coupled to the emitter follower Q901 to provide a nominal regulated 40V HT power rail for the VFD.

The VFD’s internal driver IC and external data bμFfer IC901 run from the main +5V supply generated in the Power Supply board and routed onwards through the Input Board. The drive signals (data, clock, chip select and reset) come directly from the system microprocessor via CN101.

The 12 front panel switches are arranged in 2 blocks of 6 with resistive divider chains connected to two ADC inputs on the system micro. These have 10K pull-up resistors at the system μP end to complete the potential divider chains. The pnp switching transistors Q906 and Q907 turn on the 3V3 supply from the system μP via another 10K pull-up resistor at the μP end to provide interrupt control.

The power status LED D905 is a tri-colour type. The green side indicates power on and the red side standby. A high signal on the LED net turns on Q902 and Q903. This turns off the npn Q913 to disable the red LED and turns on the pnp Q912 to enable power to the green LED. The reverse is true when the LED line is low. Pulling the STB(LED) line low when the LED line is also low powers both LEDs and gives a yellow light to show when the unit is booting up. Note that the power supply is STBY+5V to allow the red LED to operate in standby mode.

RC901 is a Kodenshi KSM603TH5B encapsulated infra red receiver for processing commands from an external IR remote control. It is designed to work with the 36-38kHz carrier frequencies associated with the Philips RC5 protocol. Note that it operates from the ST+5V rail to enable the AVR400 to be woken up. It does not demodulate the IR – this is done by the system microprocessor.

The headphones amplifier IC902 drives external headphones directly via the 330μF series capacitors C935 and C936. IC902 has a gain of about 4, meaning that the headphones output will be about 4V rms when the volume control is set to clip the L and R main power amplifiers (equivalent to about 120WPC into 8 ohms). IC902 is a JRC NJM5556AL capable of driving 7V rms into150 ohm loads and about +/- 100mA peak current into lower impedances. Mute transistors Q904/905 in series with 100R resistors are fitted in front of its input to minimize switching transients and it is powered from the +/-15V supplies generated on the Power Supply Board.

The L and R headphones outputs go via BN93 to the Headphones Board, after passing through relay RL902 which is normally off. A positive voltage from the μP at the emitter of pnp Q911 turns on Q911 and generates the same positive voltage at the base of the npn transistor Q909. Because its emitter is connected to the -15V rail this pulls the relay on.

The FRONT AUX and MIC inputs come from the Headphones Board and through the microphone relay RL901. When the relay is off the AUX line level L and R signals are switched through to the unity gain bμFfer IC904 and then on to the Input Board via pins 3 and 1 of CON101. When the relay is closed (in