Track Detecting Circuit; 3*4 Preamplifier And Laser Pcb 1*769*100*35; Laser Control; Focus And Radial Control Signals - Revox B225 Maintenance Instructions Manual

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B225
SECTION 3/8
3 . 3.5 Track detection circuit
The tracks which are skimmed over by the laser pick-up
are read through the interrupt input of the slave
processor (TRK-SIG). This signal is generated from the
signal available at IC6, pin 1 (proportional to
[ I1 + I2 + I3 + I4J ), the DO and the HFL signal. The signal DO
is
,!
L
,!
when no error location is detected on the CD.
The HFL signal is "H" when the HF signal is large enough.
The TRK signal is only "L" when the HE signal is too
small. For each skipped track, HFL is "L" if no error
location is detected on the CD (DO="L"). The radial-error
signal RE (IC9c, pin 8) informs whether the laser scanner
is positioned to the left or the right of the track. The
signal required for this purpose (RAD-POS) is generated
via IC3c, R12, R19, and R20 from the radial-error signal.
3.4 Preamplifier and laser PCB 1.769.100.35
This board comprises the following circuits:
- Laser control
- Signals for focus and radial control
- HF signal amplifier
3.4.1 Laser control
The laser diode is supplied via Q4. The intensity of
the laser light can be checked through the "monitor
photo diode" and adjusted with the trimmer potentiometer
R8CT.
The signal LAS-OFF (from PCB 1.769.310 / IC1) switches
the laser on or off. If the intensity of the laser
becomes too small, the current flowing through the
monitor photodiode decreases correspondingly. The voltage
across R84 also drops. The output of.opamp IC3a
consequently becomes more positive and Q4 more
conductive. The intensity of the laser is adjusted by
discharging some of the current through the monitor
photodiode via R80 and R81.
3.4.2 Focus and radial-control signals
The following signals are generated by this circuit;
- FOC-ERR
- RAD-ERR1
- RAD-ERR2
The currents of the four photodiodes (Al to A4) are
amplified in IC1. The corresponding error signals are
developed by the subsequent network. The focus error
signal is proportional to (11+14) - (12+13). 13 + 14
constitutes the•RAD-ERRl signal, II + 12 the RAD-ERR2
signal. The gain of the focus control loop can be
-^djiis/ted with R58, the balance with R41 (not included in
'all versions).
" '3-4.3 HF signal amplifier
The circuit comprising transistors Ql through Ql consti-
tutes the signal amplifier with bandpass characteristic.
Low-frequency (e.g. control signals of the servos)
and high-frequency disturbance signals are consequently
filtered out.
3.5 Decoder PCB 1.769.300
The decoder PCB contains the following circuits:
- Digital signal processing
- Digital sine-wave generator
- HF£ and DO detector