Arcam Alpha 7 Manual de servicio - Página 2

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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