Arcam ALPHA 7 Service Manual

ALPHA 7/8/9 CD SERVICE MANUAL ALPHA 7/8/9 CD SERVICE MANUAL
Arcam Alpha 7/8/9 CD Service Manual
Issue 1 Serial No. 0001 - (Paul Newton July 96)
Circuit Description
The following notes describe the operation of the
Alpha 7/8/9 CD players. They include the circuitry on
the main, DAC and display PCBs but not that on the
servo PCB attached to the mechanism.
Power Supplies
The mains input is fitted with inductors L2,3,6,7 to
filter electromagnetic interference coming out of the
player.
Mains transformer T201 has three secondary
windings supplying two separate power supplies and
an AC supply for the display filament:
Analogue Supply
Bridge rectifier D4,5,6,7 and reservoir capacitors
C209-C212 produce unregulated split rails of about
+/-22V.
Z204 and Z205 provide regulated outputs of +18V
and -18V respectively that power the opamps in the
audio output stage.
Since there is no power switch on this secondary, the
analogue supplies are always connected.
Digital Supply
Switch SW201-A disconnects the digital secondary
winding when the 'Power' switch is out.
An unregulated supply of about 11V is produced by
the bridge rectifier D8,9,10,11 and reservoir
capacitor C36.
This is then regulated to provide three +5 volt
supplies (Z208 general logic supply, Z207 DAC
supply, Z203 system clock generator supply) and a
+7.3v supply for themechanism and motor drive
circuits(Z206).
A -30v supply is formed by Z209 and associated
components to power the display grid.
Muting
When the unit is turned off SW201-A discharges
capacitor C29 quickly. This along with the AMUTE
signal from the microprocessor, controls the output
mute transistors Q1,101 via level shifter Q3.
Clock Generator
The system clock is generated by a discrete oscillator
based around Q2 and 16.9344MHz oscillator X1. The
output of Q2 is amplified and buffered by inverters
Z202-E & D and sent to the clock input of the DAC
Z201.
DAC & Audio Outputs (Alpha 7 only)
The DAC is a Delta-Sigma design from Burr Brown,
the PCM1710U.
The serial digital data from the decoder Z206 is input
to pins 1 to 3. The system clock is input to pin 5.
The DAC is powered entirely from one 5V supply,
although it is split and decoupled between the
analogue and digital sections of the device.
The analogue outputs from each channel are
processed identically.Three poles of filtering are
provided by the active filter based around Z1-A. Z1-B
buffers the audio output and sets the output level.
Muting is provided by turning on Q1 and Q101.
DAC & Audio Outputs (Alpha 8 only)
This is a similar circuit as the one used on the Arcam
Alpha 6 DAC PCB but designed for Sony format
input data and a better digital filter, the SM5843AP1,
Z14.The digital filter is used with a SM5864AP 20-bit
DAC chip, Z12, (described here as a PWM noise
shaper to avoid confusion with the 1-bit DAC Latch
which follows it). The output from this is re-latched by
Z7, Z8.
Digital FilterA PIC micro on the mother board
controls the filter with the signals RB0 to RB7. The
SM5843 filter has internal pull-up resistors on its
inputs resulting in RB4 & RB5 being logic 1.
The SM5843APT requires several control lines from
the PIC...
RB7 FSEL2, Selects sampling rate for de-emphasis.
Static low for 44.1K.
RB6 FSEL1, Selects sampling rate for de-emphasis.
Static low for 44.1K.
RB5 IW2N, Selects input word length. Static high for
16 bits.
RB4 MDT, Serial data input for volume control. Burst
at switch-on for setting to full.
RB3 RSTN, Reset line. Short pulse low at switch on,
then static high.
RB2 MLEN, Serial data latch input for volume
control.
RB1 MCK, Serial clock input for volume control.
RB0 Not connected.
No Dither On Silence
The digital filter adds 'dither' to the output signal.
This improves the low-level performance of the DAC.
The PWM filter normally mutes when it detects
digital silence to prevent any unwanted noise and
idle-tones on the audio outputs when no music is
playing. A slight click is audible when this happens.
Unfortunately, the added dither prevents this mute
from operating because it holds it open. Q11 detects
digital silence on the data input to the filter. When
silence is detected, the dither function is switched off