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ALPHA 7/8/9 CD SERVICE MANUAL
ALPHA 7/8/9 CD SERVICE MANUAL
allowing the PWM filter's mute to operate as normal.
Mute Relay
The mute relay is controlled from the PIC micro via
RA0 after switch-on. In Standby mode, XRST is low
which interrupts the relay via Q9. RA0 is low when
the CD player comes out of standby mode and goes
high after a short settling delay.
The power rail, RLYPWR, is un-regulated and has a
small reservoir capacitor (on the mother board) so
that the relay is released very soon after the mains is
disconnected from the CD player before the other
power supply rails collapse.
PWM noise shaper
To reduce EMC, the clock signal going into this chip,
Z12, is very weak on the XTI pin (23) pin, taken
directly off Q5. This is amplified so that there should
be a larger signal on the CKO pin (26).
The PWM output on pins 11, 13, 16 and 18 can be
resolved into an audio waveform on a scope simply
by measuring them through a 100K resistor. The
same is true for the PWM signals through the latch
and the level shifter.
Latch
The PWM data from Z12 is re-latched using 74HC74
Sony Format Di g i t al
Audi o
Control from PIC mi c ro
(Z211) on mai n pcb
Cl o ck
Block Diagram of Alpha 8 DAC PCB
SM5843AP1
SM5864 PWM
Digital Filter,
Noi s e Shaper, Z12
Z14
16.9344MHz Low-Ji t ter
System Cl o ck, X1, Q5 & Z13
latches, Z7 and Z8.The power supply for these chips
is the DAC voltage reference and the output drives
the analogue filter stage directly. The latches reduce
any edge-timing jitter induced on the PWM signal
coming from Z12.
Analogue filter
The first half of the analogue filter, Z3 and Z4 is
balanced. The signal through one side of the op-amp,
pin 1, should be phase inverted with respect to the
other, pin 7. These signals should be viewed on a
scope through a 100K resistor since there may still
be quite a lot of RF noise on the signal.
Z1 and Z2 filter the audio signal further and also
convert the balanced signal into a single ended
signal. Z18 acts as a DC servo to reduce the output
offset voltage. This has a very large time constant so
an offset as high as 50mV is possible when the unit
is first plugged in. This reduces to less than 3mV
after 10 minutes and stays low if the unit is switched
off and on with the Standby button.
74HC74
Anal o gue Fi l t er,
Latches,
Z1, Z2, Z3, Z4
Z7 & Z8
Audi o out
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