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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 o 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 ampliers’ heatsink and is used to route power to the cooling fans and also to guide the 31 way
ribbon cable.
The VFD draws AC lament 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 lament with its required DC oset. Pins 4 and 5 connect to a relatively high voltage
transformer secondary which is half wave rectied 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 o 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 amplier 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 ampliers (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
tted 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 o. 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 o 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