Automatic Gain Control (Agc) Circuit; Offset Control Circuit - Revox B225 Maintenance Instructions Manual

K E I / O X
B225
SECTION 3/5
3.3.3 Automatic gain control (AGC) circuit
The bandwidth of the radial-control circuit is stabilized
by this circuit. A 650 Hz sine-wave signal is generated
in the circuit IClOc, IClld, C22, C23
#
C24, C25, R85
r
R86, R87
?
R90, and R91. As the gain increases or
decreases the phase relation of the supplied signal
becomes also larger or smaller with respect to the
returning signal* The two signals are compared in the
phase detector (ICll) and a new signal is developed.
The latter is proportional to the phage and also to the
gain. This signal controls the factor k. The gain is laid,
out for a " phase shift between the signals -of -.135° The
650 Hz.sine-wave signal is therefore fed into the control
loop at pin 13 of IC9d through a 45° phase shifter (R84,
C21). The phase shift is 90° at the input of the phase
detector. The 650 Hz sine-wave signal is eliminated from
the control circuit through a bandpass (IClOd, R82
r
R83, C19, C20) and converted in IClla to a square-wave
signal. This square-wave signal is input to the phase
comparator (ICllc, pin 8). The phase reference is
supplied by pin 9 of ICllc.
3.3.4 Offset control circuit
The asymmetry of the reflected laser beam is corrected
by this circuit to ensure that the laser beam is kept
exactly on the center of the track. The difference
between minimum and maximum reflection is the greatest
(presence of pit = minimum reflection, between pits
= maximum reflection) when the laser beam follows exactly
the center of the track.
Through the 650 Hz sine-wave signal the moving arm
oscillates with an amplitude of 0.05 jim.
If the laser beam is on the track center, all half-waves
of the filtered out signal are folded to the same
(positive) side. If the laser beam is to the right of the
track center, the signal is in~phase with the 650 Hz
oscillations of the moving arm, or antiphase if the beam
is on the left of the track center.
The voltage across R63 is proportional to the reflected
light intensity (11 + 12 + I3 + I4). The voltage is amplified
through IC6a, R30
f
R31
r
and R64
#
and the 650 Hz component
is filtered out in a bandpass (R32, R33, R34, R35, R37,
Cll, C12, and IC7b). The filtered signal is inverted
through lC7a
r
R36, and R38. The former and the
noninverted signal are taken to switch C of IC5. The
output of this switch (IC5, pin 4) is integrated through
IC7d and C.30 and determines the factor d (Q5 and
transistor' F of IC8). The status of switch C (IC5)
is controlled by the 650 Hz signal in the control loop
. -vara-the bandpass R42
f
R43
r
R44 , R45, C13, C14 , D4,
" and IC7c.