Akai 204 DSS Service Manual

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SERVICE MAHIIAl

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AKA] SURROUND STEREO TAPE DECK

MODEL EDED-SS

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SURROUND STEREO TAPE DECK
MODEL 202D-SS

SECTION 1 SERVICE MANUAL ........................ 2
SECTION 2 PARTS LIST ............................ 33
SECTION 3 SCHEMATIC DIAGRAM ................... 60

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1. STOP MODE (Refer to Schematic 1)

At stop mode, the various system control (hereinafter
referred to as SYS.CON. transistors are maintained
as follows: TR1(ON), TR2(0FF),TR3(OFF), and
TR4(ON). Accordingly, the reverse relay connected
to the collector of TR2 is OFF, and capstan motor
revolutions are in forward direction. Also as TR4
is ON and TR4 collector voltage is roughly zero,
and thus Diode Dl connected to the collector of
TR4 (Playback Amp. RC. Board) is turned OFF,
the Playback Amp. Mute Circuit is also turned OFF.
Both the supply and take-up reel motors are equipped
with a Micro Switch SW4 which is activated to
OFF and ON by the operation of the brake lever,
and which at Stop Mode are turned OFF. Con-
sequently, no voltage is supplied to either reel motor
(both reel motors are connected in series).

When at stop mode, switching by meansof P.B./REC
Switch SW-7 does not take place because the base of
SYS.CON.TR2 is grounded and maintained at OFF
condition. That is to say, capstan motor revolutions
will not reverse even when a REV Button is depressed.
When the tension lever is raised, Tension Switch
SW9 is turned OFF. Thus, when no tape is
loaded, this switch is ON. Also, because Tension
Switch SW-9 is installed in parallel with P.B./REC
Switch SW-7, when SW-7 is at stop mode, if a tape is
loaded, but not threaded through the Tension Lever
and playback is effected, reverse mode cannot be
effected when a REV Button is depressed. And, of
course, reverse mode will not be effected when the
sensing foil passes the sensing poles.

2. FORWARD PLAYBACK MODE
(Refer to Schematic 2)

When the PBJREC Switch is at PLAY position, the
various SYS.CON.transistors are maintained as follows:
TR1(ON), TR2(OFB, TR3(0FF), and TR4(ON),
i.e., functionally the same as at stop mode. The
difference is that the base of TR2 which was
grounded by P.B./REC Switch SW-7 is now open.
That is to say, when TRl is turned OFF and TRl
collector voltage increases, bias is supplied to TR2,
and TR2 is turned 0N.

Accordingly, while TRl is (ON) and TR2 (OFF)
(FWD Playback mode), capstan motor revolutions
are in forward direction. Also Micro Switch SW-4
is ON, and voltage is supplied to both reel motors.

At FWD playback mode, an R2(10W, 2509) resistor
is in parallel with left reel motor winding. That is to
say, the left reel motor resistance differs from the
right reel motor resistance due to the R2(10W,
2509) resistor which is parallel with the left reel
motor as shown in Fig. 8. In short, the voltage
supplied to the left reel motor is low and FWD
playback mode is maintained.

3. FROM FWD TO REV PLAYBACK, AND
REV PLAYBACK MODE
(Refer to Schematic 2, 3)

When the sensing tape passes the reverse sensing
poles or when the REV Button is depressed, the
base of TRl is biased at zero electrical potential
and TRl is instantly turned OFF, TRl collector
voltage increases and bias is applied to the base of
TR2, TR2 is turned ON, and Reverse Relay RLl is
turned ON. In this TR] and TR2 circuit, TR] is
maintained at OFF and TR2 at ON by means of
Diode D3(IN34A). (Refer to Schematic Diagram-3)

By means of the reverse relay function, current
flows from the reverse direction to the capstan
motor sub coil, and reverse revolutions begin. Also,
at FWD playback time, Resistor R2(10W, 2500)
connected in parallel with the left reel motor becomes
parallel with the right reel motor. (Refer to Fig. 9)

At this condition, reverse playback begins and is
maintained, and the capstan motor revolutions begin
reversing from forward direction. For normalized
and stable revolutions, flywheel inertia at time of
forward revolutions and flywheel mass when reverse
revolutions begin is applied so that stabilized tape
speed is not immediate, for this reason the signal
output circuit must be muted until capstan motor
speed is stabilized. (Refer to MUTING CIRCUIT
OPERATION explanation)

Solenoid Plunger SL-l also operates to switch Reverse
Switch SW-l. In other words, the PB. Amp. l.C.
Board output is switched by means of Reverse
Switch SW-l. (Refer to Fig. 10)

4. FROM REVERSE T0 FORWARD

PLAYBACK (Refer to Schematic 3)

When the sensing tape passes the FWD sensing
pole or when the FWD Button is depressed and
the base of TR2 becomes biased at zero electrical
potential, TR2 is instantly turned OFF, and reverse
relay RL~1 is also turned OFF.

Diode D3(1N34A) is also inverse biased, TRl base
current passes R2(lO kn), R3(4.7 kft), and D2
(lSZ473VE) and is biased, and TRl is turned ON
and maintained. Consequently, reverse relay RL-l
returns to its original condition (FWD RB.) and
FWD playback mode is effected. Refer to MUTING
CIRCUIT OPERATION for mute function during
change from REV to FWD playback.

5. RECORDING MODE (Refer to Schematic 4)

SYS.CON. operation at recording time is exactly the
same as Stop mode.

lS

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LEFT REEL MOTOR

F. FWD.
IOOV

RIGHT REEL moron
Fig. 11

6. FAST FORWARD AND REWTND MODE

(Refer to Schematic 5, 6)

In this case,SYS.CON.operation is also exactly the
same as Stop mode. However, the voltage supply to
both reel motors are as shown in Fig. 11.

. MUTING CIRCUIT OPERATION
(Refer to Schematic 2, 3, and 7)

As shown in Schematic Diagram-7, the muting
circuit consists of an electronic switch from Recording
Amp. P.C. Board Transistor TR3 and a one-shot
multi-vibrator comprised of SYS.CON. P.C. Board
TR3 and T R4.

When stabilization is attained, TR3 of the one-shot
multi-vibrator is turned OFF, and TR4 turned 0N.
Accordingly, the collector voltage of TR4 is lowered,
Zener Diode D1 is biased in reverse direction, and
TR3 of Recording Amp. RC. Board is turned OFF. ,

1f Button SW-S is now depressed for FWD to REV
Playback, or REV to FWD Playback, a charge
current flows to condenser C3 or C4. That is to
say, the positive direction trigger signal is introduced
at the base of TR3 of SYS.CON.P.C. Boardand
therefore, the one-shot multi-vibrator which is com-
prized of TR3 and TR4 is inverted and, after a
certain period of time (2 to 3 seconds), again returns
to its stabilized condition. While this one-shot
multi-vibrator is inverted (while TR4 is OFF), because
TR4 collector voltage is high, the voltage exceeding
the Zener Point of Zener Diode D1 is applied to the
cathode of D1 and D1 assumes a lead-through
condition. Thus, electronic switch Transistor TR3
is turned ON, and the signal is muted. (The internal
resistance of electronic switch TR3 at lead-through
time is extremely low.)

At recording time, a charge current flows to con-
denser C9 in the same way as described above.
That is to say, the one-shot multi-vibrator is inverted
and the mute circuit functions. Also when stop is
effected from recording mode, TRS is turned OFF,
and because a charge current flows to C10 through
R18, the mute circuit functions. Thus, at recording
time or when returned to stop mode, these functions
prevent a click noise from being recorded on the
tape.

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l. REEL TABLE HEIGHT ADJUSTMENT

As shown in Fig. 12, loosen reel table height adjust-
ment screws, and adjust reel table height by moving
table in direction of arrow and positioning so that the
tape winds in the center of the reel.

. PlNCH WHEEL PRESSURE
' MEASURING METHOD

Measure pinch wheel pressure with a tension gauge as
shown in Fig. 13 Read the value on the tension gauge
as soon as the pinch wheel separates from the tape
and tape travel stops. Ideal pinch wheel pressure is
1.8 kg. '

Fig. 15

3. BRAKE TENSION ADJUSTMENT

'1) As shown in Fig. 14, use a 60 mm diameter tape
wound on a 5" reel and measure the brake tension
with a tension gauge. Ideal brake tension is from
300 to 370 grams.

2) Brake tension adjustment can be made as follows:
(Refer to Fig. 15)

a) Change position of suspended spring (a).

b) Loosen screws (1)) and (c) and adjust the
vertical position of spring suspension metal (d).

c) Loosen screws (6) and (f) and adjust the
horizontal position of brake band suspension
metal (g).

d) Only the left side is shown in Fig. 15, but the
right side must be adjusted in the same way.

NOTE: In making brake tension adjustment, at all
modes except stop mode, confirm that the
brake band completely separates from the
cloth tape on the brake drum. (Refer to
Fig, 15)

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VI. HEAD ADJUSTMENTS

Some width

015mm"

Sumo width

4-CH.E.H. 2-CH. E.H. REG/RB. l-EAD Fig. 18

1. HEAD HEIGHT ADJUSTMENT

(See Fig. 18)

l) Erase Head (2-CH)
Loosen Erase Head Screws (b) (b') and adjust
Erase Head height so that the upper edge of the
tape is about 0.15 mm lower than the upper
Erase Head core.

2) Erase Head (It-CH)
Loosen Erase Head Screw (a) (a) and adjust
Erase Head height so that the tape is equidistant
in relation to the top and bottom edges of the
Erase Head core (width between top edge of tape
and top edge of head core is the same as the
distance between the bottom edge of the tape and
the bottom edge of the head core).

3) Recording/Playback Head

Adjust Recording/Playback Head Height by turning
Recording/Playback Head Height Adjustment
Screws (0) (d), to left and right until the width
between the upper edge of Channel 1 Head Core
and the upper edge of the tape is equal to the
width between the lower edge of Channel 4 Head
Core and the lower edge of the tape.

2. RECORDING/PLAYBACK HEAD SLANT

ADJUSTMENT (See Fig. 18)

By turning Head Height Adjustment Screws (c) (d)
to left and right, adjust so that the Recording]
Playback Head contacts the tape surface at a right
angle (head is at right angle in relation to tape
surface).

. RECORDING/PLAYBACK HEAD

AZlMUTH ALIGNMENT ADJUSTMENT
(See Fig. 18)

Connect a High Sensitivity V.T.V.M. to the Line
Output and playback a Head Alignment Test Tape
(8,000 Hz, 3-3/4 ips.). «Adjust by turning Head
Alignment Adjustment Screw (e) to left and right
until the various line outputs reach maximum.

. Repeat adjustments outlined in 1-3) as well as

2. and 3. above two or three times to
obtain optimum adjusted condition.

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FIG. 1 ILLUSTRATION 0F HEAD BLOCK

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