Cambridge Audio 840A - Power Supply PCB Description; Front Panel PCB Description

The Power Supply PCB
This carries:
Power supplies for everything except power amps
Mains on/o and inrush control switching relays
X2 capacitor
The mains-fail detect circuit
There are four power supply rails:
+15V Opamp supply
-15V Opamp supply
+9V Relay supply
+5V Microcontroller, logic and comparator supply
The power supply board also carries the mains switching relays that control the supplies to the power
ampliers. When the amplier goes from standby to ON, the inrush control relay RL4 closes rst with
R78 limiting the inrush current. After 3 seconds (while speaker-short testing is going on- see below) the
On/O relay RL3 closes. 100 msec later (to ensure no break in the supply) RL4 opens. When the
amplier goes from ON to standby, RL3 simply opens.
The front panel PCB
This carries:
The main PIC
The volume rotary encoder and control switches.
The LCD display
Shift registers for driving the relays
The RS232 interface for control and software updates
U1 is the main housekeeping microcontroller for the amplier; it is a 16F977A PIC running at 20 MHz. It
handles all the control functions of the amplier except for those on the A-BUS hub PCB, which are
dealt with by another PIC on that board.
PIC pins handling amplier conditions:
Pin Normal Active
Speaker short-circuit detect 7 LOW HIGH Active= short found
Phones detect 27 HIGH LOW Active= Phone jack in
Mains fail 28 LOW HIGH
DC oset protection 29 HIGH LOW
Overtemperature protection 30 HIGH LOW
Overload protection 33 HIGH LOW
Clip detection 34 HIGH LOW
The LCD display module interfaces to the front panel PCB through CN2. Display data from the PIC is
clocked serially into shift register U2 and sent to the LCD module in byte-wide parallel format. The
LCD module is powered from the +5V rail.
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