ALPHA 5 CD SERVICE MANUAL
CIRCUIT
DESCRIPTI
ON Audio
Board
The audio board contains the digital filter, DAC, audio output stages and associated power supplies.
The required signals from the main board are passed to the audio board via the 18 way
connector. This connector carries DAAB, CLAB, WSAB, DEEM, OV, ATSB XSYS, EFAB and +5V
line.
The DAC requires 3 supply rails to function, +5V, -5V and -15V and also a series of averaging
capacitors, (C1-7,C101-107), for the bit switches in each channel of the DAC for good linearity
and low distortion. The +5V supply is filtered by RC network R216, C219, C220 and the -15V
supply L201 and C224, C225. The output of the DAC is fed to Z1 to convert the output current
to a voltage and provide gain. The second ic, Z2, with its associated components forms a low pass
2 pole filter, which combined with the first stage forms a 3 pole Bessel filter.
Transistor Q1 is used to switch in additional components if the de-emphasis flag DEEM goes high. This is
done automatically when a disc recorded with pre-emphasis is played.
The digital output (DOBM) is derived from pin 14 of the digital filter (Z206) and is formed by isolating
transformer T201 and associated circuitry.
The "Kill" line to mute the audio outputs, with transistors Q2, 102, comes in on the 4th pin of socket
SK202 which also carries the dc supplies for the audio board.
The audio board is supplied with smoothed +/- 25v D.C via a 4 way connector, SK202.
R201,C205 and R202,C206 provides additional filtering before the regulators. There are 4 regulators, +/-
15V and +/- 5V formed by adjustable regulators Z201,202 and Z203,204 respectively.
Mono Board
Photo diode signal & Radial Error Processors
The TDA8808(Z501) and TDA8809(Z502) ic's are the signal processors for the laser unit. The TDA8808
amplifies the photo-diode signals from the CDM-9 mechanism and processes the error signals for the
focus and radial control network. The TDA8809 provides control signals for the radial motor. These
signals are generated from radial error signals received from the TDA8808 and velocity control signals
from the control processor. Z503,(TCA0372) is a high power op-amp to drive the radial arm. Z604 is
used to drive the turntable motor with the use of the transistor output stage formed by Q604 and Q605.
Unlike earlier mechanisms the CDM-9 mechanism is aligned during manufacture and is not
adjustable.
Microcontroller & Decoder
The appropriate signals are passed to and from these ic's to the servo microprocessor, 68HC05C8,
(Z603) and the decoder (Z601) ic. The output from the decoder chip goes via an 18 way connector to
the audio board.
The meaning of the used signals on the servo microprocessor is as
follows: 2
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