Marantz SD4000 - Bias and Erase Oscillator Circuits

shown in the table below.
TAPE SELECTOR SPECIAL/ j
R109, R107 [R109, R105| R109
R111
3.3/4 1pg [R110 R108 [R110, R106 eo ae
3.5 LED METER CIRCUIT
1. The signal amplified and rectified by Q601 and Q602
is applied to the LED drive 1C(Q651).
2. The LED drive IC(Q651 makes LEDs light during
playback or recording operation.
3. During fast-forward or rewind operation, 0463 is OFF,
so that the regulating transistor (Q642) in the LED
power supply is OFF.
4. Since Q642 is OFF, noise signals are prevented from
lighting LEDs.
DOLBY IC
OUTPUT
@ 601
. 9651
R610 aao2 > 65
9603,
604
3.6 SKIP CIRCUIT FUNCTION
PLUNGER
ORIVE
CIRCUIT
SCHMITT
TRIGGER
In the above diagram, HOO1 represents the recording head.
During the skip operation, the cassette tape is set in CUE
or REVIEW mode, so the gap in the playback head must
be inthe center of the cassette; otherwise, the head out-
put varies.
In SD4000, a combination head (HOO1) is employed,
which is mounted and tilted toward the right so that the
recording head gap is aligned with the center of the cas-
sette. Thus, the recording head is used as a playback head
for the skip operation. During the skip operation, the
playback output of the recording head is fed to the skip
circuit, while the output of the playback head is fed to
the playback amplifier as usual. In the recording mode,
the recording head functions as a normal recording head
through S901. R906 is a resistor to measure the bias
current,
Principles of Circuit Operation:
During the skip operation, the playback output of the
recording head passes through the flat amplifier (Q913,
Q914)
and
is
amplified
by
Q451~0453
(this
circuit
is
a
narrow band amplifier and protects the skip circuit from
misoperation due to playback noise).
The output of 0453 is rectified to DC voltage which turns
on O481 and 0482 off. At this time, C482 is charged
through R485 and 0483.
When the input ceases at the blank space between pro-
grams, 0481 turns off and 0482 on, while C482 is dis-
charged through Q484, 0485, R486 and Q482. The
current discharged flows into the base of Q484 and hence
Q484 turns on. The current from C482 also flows into
the plunger drive circuit via 0484.
C482 is shorted by Q486 to prevent skip operation
during normal playback. Q486 turns off when FF/REW
button is pressed. Conversely, when the FF/REW button
is released at the end of skip operation, Q486 turns on
_ and thus the output of Q482 remains in the ‘0’ level.
3.7 BIAS OSCILLATOR CIRCUIT/ERASE OSCILLATOR CIR-
CUIT
The SD4000 has a bias oscillator circuit and an erase
oscillator circuit to provide optimum bias and erase cur-
rent for the use of meta! tapes.
@ The bias oscillator circuit is driven by a regulated amp-
lifier power supply.
@ The erase oscillator circuit is driven by the motor
power supply.
The bias current for any particular type of tape is selecta-
ble by the circuit shown below (selection of bias current
is independent of tape speed).
Q302
BIAS CURRE NT AOJUST
METAL POSITION ONLY
°,
of S03 op stos $901; REC/PLAY SWITCH (REC)
ae eee zs $302: TAPE SELECTOR (NLORMAL)
BIAS CURRENT AOJUST. S303. TAPE SELECTOF(SPECIAL/CrO2)
NORMAL, SPECIAL/CrO3, FeCr $304: TAPE SELECTOR(FeCr)
Referring to the above diagram, the bias current is ad-
justed by Q302 which changes the operating voltage of
the BIAS OSC circuit at each tape selector switch posi-
tion. In the “Metal” position, the operating voltage is
set by R304 and R303, while its bias current isd etermin-
ed by R912 (R913).
@ Erase current is increased only at the ‘Metal’ position
by shorting the series resistor (R916) of theoscillator
circuit power supply with S305 (Metal).
DC 12V
L901
$901: REC/PLAY, REC
$308 S305: TAPE SELECTOR M ETAL)