JVC KD-A66 Service Manual

9. Upon completion of tuning the equalizer and the tape
sensitivity (item 8), the tape is rewound to return to
the position as described in item 1. Then, the flickering
READY LED lights continuously.
The above-mentioned operation is shown in Fig. 6.
The basic operation is as previously described. In addition,
the microcomputer is provided for the functioning of the
tape deck.
Upon application of power, the microcomputer is auto-
matically reset, thus resulting in a temporary stoppage of all
output.
SERVICE switch
This switch is used to adjust the tape deck.
The microcomputer with 7 adjustment program checks the
generation of the test signals, the variation of the bias cur-
rent, the variation of the equalization level, the variation of
the tape sensitivity, the operation of the electronic mecha-
nism, and the like.
When the SERVICE switch is at the OFF position, the
microcomputer confirms that the B.E.S.T. system is func-
tioning. At this time, the microcomputer emits those auto
set values when the B.E.S.T. system completes operation
and emits the preset values even if an error occurs during
the operation. Thus, even when the auto setting of the res-
pective values is impossible, normal recording is possible
since the preset values are emitted.
Subsequently, the microcomputer waits for the START
switch to be pressed. When this switch is pressed, the micro-
computer sets bias current, equalization level and tape sen-
sitivity values.
In this process, when auto setting is impossible, the micro-
computer returns to the bias setting mode to re-tune. At
this time, when an error recurs, the red PRESET LED flick-
ers and the preset values are emitted.
In addition, irrespective of presence or absence of error, the
tape is rewound to its starting position. As a result of this,
the tape is effectively used. In normal auto setting of the
respective values, the rewinding time is about 25 sec.
Operating Principle of Respective Sections
KD-A66A/B/C/E/J/U
1. Setting of bias current
Fig. 7 represents the bias current varying circuit em-
ployed in the KD-A66.
The bias oscillation circuit consisting of transistors
X701 and X702 supplies current to the erase head and
also activates the bias amplifier consisting of transistors
X703 and X704. As the supply voltage for this bias
amplifier is changed in 8 steps by a control signal from
the microcomputer, the bias current is changed in 8
stpes.
When the binary control signal from pins 2, 3 and 4 of
microcomputer (ICKO1) is applied to IC701 which
includes 8 electronic ON/OFF switches of 8 circuits,
any one circuit alone is turned on. At this point, the
base current passes across a specific one of transistors
X709 — X716 turning it on.
Concerning transistor X708 which controls the supply
voltage for the bias amplifier, its base voltage is deter-
mined by a divided voltage by R728 and the associated
collector resistance of X709 to X716.
The bias oscillation circuit, which is turned on/off by
the BMUTE signal from pin 304 on the computer
circuit board, is also turned on/off by the NSTOP
signal (the control signal of the brake and solenoid)
from pin 512 on the same board when stopping the
bias oscillator circuit operation during the PAUSE
mode.
+VCCRI
TO HEAD
Equalization level
change-over signal
Tape sensitivity
change-over signal
Fig. 8
2. Setting of equalization level and tape sensitivity Fig. 8
represents the recording amplifier section employed in
the KD-A66.
The input signal is attenuated by resistor RO and any
of resistors R1 — R16 and entered into the specific
transistor. At this time, a tape sensitivity select signal
emitted from the microcomputer causes the respective
resistor in the group of R1 — R16 to be grounded, thus
determining the attenuation level. The select signal is
transmitted in the form of 4 binary bits and converted
into decimal bits. In addition, a CMOS IC incorporat-
ing analog switches is used in the select switch unit to
protect the audio signal against distortion. This select
switch unit permits the change in recording level of +4
dB.
No. 4193

JVC KD-A66 Specifications

General

Brand	JVC
Model	KD-A66
Category	Cassette Player
Language	English