Cambridge Audio 840A Service Manual page 58

The right power amplifier and its various protection systems operate identically to those on the left
amplifier PCB, except for the following differences:
Mains-fail detect system
The mains-fail detect system is based on comparator U1:B. It takes the MAINS FAIL IN signal from the
PSU board (see above) and compares it with a reference voltage of +310 mV set up by potential divider
R81,R80. When MAINS FAIL IN drops below this threshold, the output goes high. This signal, MAINS
FAIL, is sent to the PIC via CN1, pin 9.
A-BUS Hub Board
This board carries the following functions:
The XLR sockets for the balanced input mode of Input 1
Two A-bus outputs
Hub IR input
Hub microcontroller
Hub Power supply
IR in/out interface
The XLR sockets simply pass the Input 1 signals down to the input board via screened cables.
The A-bus outputs take an input selected by the hub input switcher, amplify it by 2.5 times, and send it
to the remote keypads. R59,R60 are DC drain resistors that minimise clicks shortly after power-up.
The output amplifiers U1,U2 work from a single +24V rail, and so are biased to sit at +12V. The bias
supply is derived from the +12V supply rail.
The hub amplifiers have their own ground (ABUS GND) to prevent ground loops, and this use of the
differential amplifier performs a change of reference so a clean signal reaches the hub amplifier seven if
the two grounds have noise voltage between them.
The hub microcontroller U5 is a PIC16F72. This performs the following tasks:
Control Hub input switcher from IR commands
Send input select data to main PIC
Control Hub input switcher to prevent thumps on 840A output
Control A-bus status line
IR commands to the A-bus keypads are sent to the hub so that the required input can be selected and
external equipment controlled. Thus, for example, a tuner station can be changed from the keypad; the
IR comes to the hub, is sent out again via the IR emitter outputs, and activates emitters pointing at the
tuner unit.
Modulated IR signals coming in from the keypads are diode-ORed together by D5, D6. The Zener
diodes D7, D8 reject low-level noise on the incoming IR lines. The combined IR signals are buffered by
emitter-follower Q3, and sent out via CN4, CN6 to drive external IR emitters. Incoming modulated IR
signals for input select are also sent to the Hub PIC U5; LED D2 is driven from Q3, and points at IR
receiver U4, the pair being housed in a small light-tight enclosure. The demodulated IR is then applied
to Pin 7 of Hub PIC U5, which sends appropriate commands to the hub input selector IC on the input
PCB, over the strobe, clock and data lines.
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