Cambridge Audio 840A Service Manual page 59

Information on which Hub input is selected (or if no input is selected) is sent from the Hub PIC to the
main PIC by the two lines HUB CLOCK OUT and HUB DATA OUT. This is then displayed on the
LCD. The active 840A input is denoted by a round marker; the active Hub input is denoted by a circle
around it.
The Hub PIC controls the A-bus status line so that it is high when the 840A is powered and low when it
is not. It also implements the "all off" feature; when this command is received from one keypad the Hub
PIC ensures that both are turned off.
The Hub PIC can be reprogrammed by the main PIC. This is done by the PGM, MCLR HUB,
PROGRAM_DATA, and PROGRAM_CLOCK lines that come from the front panel PCB via CN15.
IR in/out interface
The other IR system on the Hub PCB is wholly concerned with control of the 840A amplifier itself, and
has no connection with the Hub IR system.
Modulated IR data (as an electrical signal) enters via 3.5mm jack CN9, and drives LED D1, which
illuminates IR receiver U6, the pair being housed in a small light-tight enclosure. This demodulates the IR
and passes it on to U7:A, which ORs it with the data from the unmodulated IR input, and sends the IR
IN signal to the main PIC via CN15. When it reaches the front panel PCB (see above) it is further OR-
ed with the IR data from the receiver on the front of the 840A unit.
The unmodulated IR signals enter via phono connector CN10:A, and drive optocoupler U8; this isolates
the grounds and prevents ground loops. The optocoupler output drives Q4 through R47. Q4 collector,
if Q6 is off, provides the IR signal to U7:A as mentioned above, and also drives Q5 via R53, to re-invert
the signal and preserve its phase. It is then OR-ed with the PIC TX signal by U7:B, and buffered by
U7:C and U7:D to provide enough drive for the IR output. R50 protects the output and C11,R49 isolate
the IR output ground.
IR commands can also be generated by the 840A itself. These come from the main PIC as the signal PIC
TX on CN15, and are combined into the IR output as described above. When such commands are being
sent, it is important to disable any external IR commands from being sent through as they will combine
to give garbled data. This is done by the BLOCK signal from the main PIC, coming in via CN15. When
this is high it turns on Q6 and prevents external IR commands from reaching Q5.
The Rectifier PCB
This board mounts on the main heat-sink of each amplifier module. It consists of the rectifier itself,
three suppression capacitors to reduce RF noise created by rectifier commutation, and two fuses in the
incoming AC lines from the transformer.
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Operating Instructions Summary
This is a very brief guide to enable all the controls and menus of the 840A to be accessed if the full
manual is not available. (see www.cambridge-audio.comfor a downloadable user manual).
To adjust the balance:
Briefly press the Mode button. The balance display appears and can be changed using the volume knob.
To return to volume mode, give the Mode button another quick press. The 840A will automatically
return to volume mode after 5 seconds.
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