Akai GX-747 dbx User manual

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:,, -~-
STEREO TAPE DECK
MoDEtGX-7 47 dbx

l t ■ t
STEREO TAPE DECK
-'100£1.GX-7 47 dbx
SECTION 1 SERVICE MANUAL . . . . . . . . . . . . . . . . . . . 3
SECTION 2 PARTS LI ST . . . . . . . . . . . . . . . . . . . . . . . . . 55
SECTION 3 SCHEMATIC DIAGRAM .... . ... . . ..... . 70

SAFETY INSTRUCTIONS
SAFETY CHECK AFTER SERVICING
Con firm the specified insulation resistance betw een powe r cord plug prongs and externally exposed parts of the set is
greater than 10 Mohms , but for equipment with external antenna terminals (tuner , receiver, etc.) and is intended for
~ or [Al , specified insulation resistance should be more than 2 .2 Moruns (ground terminals, microphone jack s,
headphone jacks. line-in-out jacks etc.)
PRECAUTIONS DURING SERVICING
1. Parts identified by the Lt, symbol parts are critical for safety.
Replace on ly with parts number specified.
2.
In addition to safety, other parts and assemblies are specified for conformance with such regulations as those
apply in g to spurious radiation. These must al so be replaced on ly with specified replacements.
Examples: RF .converters, tuner units , antenna selector switches, RF cables, noise blocking capacitors, noise
blocking filters, etc .
3. Use specified
internal wiring. Note especially:
1) Wires
covered with PVC tubing
2) Double insulated wires
3) High voltage leads
4. Use specified insulating materials for hazardous live parts . Note especially:
1) Insulation Tape
2) PVC
tubing
3) Spacers (Insulating Barriers)
4) Insulation sheets for transistors
5) Plastic screws for fixing microswitch (especially
in turntable)
5. When replacing AC primary side components (transformers , power cords, noise blocking ca pacitor s, etc.) , wrap
ends of wires securely about the terminals before soldering.
6. Observe
that wires do not contact heat producing parts (heatsinks, oxide metal film resistors, fusible resis:o rs, etc.).
7. Check that replaced wires do n ot con t act sharp edged or pointed parts .
8. Also
check areas surrounding repaired loc:itions.
9. Use care that foreign objects (screws, solder droplets, etc.) do not remain inside the set.
2

SECTION 1
SERVICE MANUAL
TABLE OF CONTENTS
I. SPECIFICATIONS ......................................................... 4
II. DISMANTLING OF UNIT .................................................... 5
III. CONTROLS . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6
N. PRJNCIPALPARTSLOCATION ............................................... 7
V. VOLTAGE AND CYCLE CONVERSION . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8
1. VOLTAGE CONVERSION . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8
2. CYCLE CONVERSION . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8
VI. DESCRIPTION OF CIRCUIT OPERATION . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9
1. OUTLINE ............................................................• 9
2. DETAILS AND FEATURES OF EE TAPE ....................................... 9
3. dbx NOISE REDUCTION SYSTEM ................................................... 11
4. EXPLANATION OF EACH OPERATING PRINCIPLES .............................. 12
VII. MECHANISM ADJUSTMENT . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 28
1. GUIDE ROLLER LOOSE PLAY ADJUSTMENT ................................... 28
2. MICRO SWITCH POSITION ADJUSTMENT ...................................... 28
3. REEL TABLE HEIGHT ADJUSTMENT ......................................... 29
4. CAPSTAN SHAFT LOOSE PLAY ADJUSTMENT .................................. 29
5. BRAKE BAND POSITION ADJUSTMENT AND BRAKE TENSION ADJUSTMENT ........... 29
6. PINCH ROLLER POSITION ADJUSTMENT . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 30
7. PAUSE PLUNGER POSITION ADJUSTMENT . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . • 30
8. PINCH ROLLER PRESSURE ADJUSTMENT ....................................• 30
9. CUE MICRO SWITCH POSITION ADJUSTMENT .......................................• 30
10. TAPE SPEED ADJUSTMENT ................................................ 31
VIII. HEAD ADJUSTMENT . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 32
IX. ELECTRICAL ADJUSTMENT . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 34
1. CLOCK FREQUENCY ADJUSTMENT . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . • 34
2. PHOTO TRANSISTOR SENSITIVITY .........................................• 34
3. PRE-AMPLIFIER P.C BOARD ADJUSTMENT. ......................................... 35
4, dbx NR P.C BOARD ADJUSTMENT ................................... , . . . . . . . . . . .. 39
X. DC RESISTANCE OF VARIOUS COILS .......................................... 41
XI. CLASSIFICATION OF VARIO USP .C BOARDS . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 42
1. P.C BOARD TITLES AND IDENTIFICATION NUMBERS . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 42
2. COMPOSITION OF V ARlOUS P.C BOARDS . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 43
For basic adjustments, measuring methods, and operating principles, refer to GENERAL TECHNICAL MANU.AL.
-------------SERVICE MANUAL GX-747dbx-------------
3

I . SPECIFICATIONS
TRACK SYSTEM
REEL CAP AOTY
HEADS
MOTORS
TAPE SPEED
WOW & FLUTTER
TAPE WINDING TIME
FREQUENCY RESPONSE
SIGNAL
TO NOISE RATIO
DYNAMIC RANGE
HARMONIC DISTORTION
INPUT
OUTPUT
DIN
POWER REQUIREMENTS
POWER CONSUMPTION
DIMENSIONS
WEIGHT
4 Track 2 Channel Stereo System
Up
to 10" reel
GX head for Recording x 2
GX head for Playback x 2
Erase head x 2
AC Servo motor for capstan drive x 1
AC Eddy current motor for reel drive x 2
19 cm/s ±0.8% (7-1/2 ips.)
9.5 cm/s ± 1.0% (3-3/4 ips.)
Less
than 0.03% WRMS, 0.07% DIN 45500 at 19 cm/s
Less
than 0.04% WRMS, 0.09% DIN 45500 at 9.5 cm/s
75 sec. using
360 m (1,200 ft.) Tape
25
to 33,000 Hz± 3 dB at 19 emfs
25 to 26,000 Hz± 3 dB (0 VU)
25
to 25,000 Hz ±3 dB at 9 .5 cm/s
25 to 15,000 Hz± 3 dB (0
VU)
Better than 65 dB at 19 cm/s DIN 45500
12 8 dB (dbx Switch "ON", 1,000 Hz Peak Recording level)
Less than 0.4% at 19 cm/s
MIC: 0.25 mV
Required microphone impedance: 600 ohms
Line: 70 mV
Line: 410 mV
at OVU
Required load impedance: more than 20 kohms
Phone: 1.3
mW
Input: 2 mV (input impedance 10 kohms)
Output: 0.3 V
Required load impedance: more than 20 khoms
100 V,
50/60 Hz for Japan
110 V
/120 V /220 V /240 V, 50/60 Hz switchable for the
other countries.
JPN 80W
U/T 140W
440
(W) x 493 (H) x 256 (D) mm
(17.3 x 19.4 x 10.l inches)
23.2
kg (51.0 lbs)
* For improvement purposes, specifications and design are subject to change without notice.
* "dbx" is Trademark of dbx Incorporated.
---------------SERVICE MANUAL GX-747dbx---------------
4

II. DISMANTLING OF UNIT
In case of troub l e, etc. necessitating dismantlin g, please dismantle in the order shown in the photographs. Reassemble
in reverse
order.
SIDE BOARD
6
I .
I (
PINCH ROLLER
o CAP b
jO I) .,._ -:_ _~ ]· ')
.......... -r- .6·- <~ _
j, r -
i II
L
ABCD
SCREWS S CONTROL KNOBS
.,, __ _J ======= = -- -.- =----:
~,~ 11~·
SCR E WS
SCRE ' I/S S. KNOBS
3 M
ECHA PANE L BL OCK
+
a- .
0-
1111
b -- -
I
I
b_i ·•· (· - - d
e-.'.. -;--Q
. ~ . ~- '
'1 l ,• - -:--
■ -- -
t
UP PER COVER
ii,-....,.,11,,,=,-
SCREWS
• - •~~·~ -h
AMP PA NE L B LOC K
4
5
'
SCREWS
__ l~---
_ _J
------- ---- ----- -SERV[CE MANUAL GX-74 7db.x -- -------- --------
5

Ill. CONTROLS
F ig . 3-1 Front View
Fig. 3-2
Rear View
1.
BU ILT-IN REEL RETAINE R 19.
TAPE SELE CTOR SWITCH
2.
SUPPLY REEL TABLE
20. TRACK SEL ECTOR SWITCHES
3.
MEMORY REVERSE INDICATOR
21 . BIAS A DJU STMENT VOLUME
4.
LEFT TENSION ARM (SENSI NG PIN) 22.
MICRO PHONE JA CKS
5.
PITCH CONTROL
23 .
LINE INPUT CON TR OLS
6.
REVERSE SELECTOR SWITCH 24.
MICRO PH ONE I NPUT CONTROLS
7.
ELE C TRONIC DI GITAL REAL TIME COUNTER
25.
AUTO MUTE BUT TON
8.
AUTO T IME COUNT ER S YSTEM
26. OPERATING BU TTONS
9 .
PINCH RO LL ER 27.
RIGHT TENSION ARM (SENSING PIN )
10. POWER SWITCH 28.
TAKE-UP REEL TABLE
11 . RE
EL SI ZE SEL ECTOR 29.
BUILT-IN REEL TABLE
12. CUE/REV I
EW L EVER
30. REMOTE CONTR OL JAC K
13. VU MET ERS 31. AC ADAPTER JACK
14. HE
ADPH ONE JA CK 32.
LINE IN JACK S
15. OU
TPUT LEVEL CO NTR OL
16 .
TIM ER S TART SWIT CH 34.
LINE OUT JAC KS
17.
MONITOR SELECTOR
35. POWER
CORD
18.
TAPE SPEED SE LECTOR
36.
dbx NOISE RE DU CT ION SWITCH
SERVICE MANUAL GX -747db~
6

I V.
PRINCIPAL PARTS LOCATION
F USEIBI PCB ~ J _..... '.,'.-;
TI 009C2450 - ·;
J r. \a r~
,
AC ADAPTER J ACK --b tl ~ ::.
dbx NOISE REDUCTION PCB_,_ -
TI01 585010 -
MIC AMP PCB
TIOl5C5020
TAPE SPEED SWITCH
R L
L....J
IN
I
R L
L....J
OUT
BRAKE M
ICRO SW I TC H
SW902
Fi g. 4-1 Front Vie \V
F US E ! AIPCS
TI009D2440
_ PRE AMP PCB
TI015A5050
Fig. 4-2 R ear View
----- ------------SERVICE M ANU AL GX-747dbx ------ -----------
7

V . VOLTAGE AND CYCLE CONVERSION
D 1
0 g D
VO L TAGE SE LE CTOR
VS90I
0~
POWER CORD - ---;,,,
-,~ r D
i
'--
__, 1 I I
!
FUSE(A) PCB
TI00902440
SYSCON PCB
t
CYCLE CHANGE SWITCH --~ =1
SW I '-- ~-= -'
60 Hz ---- 50Hz
Fig. 5 -1 Voltage Conversion (U/T Model) and Cycle Conversion (U/ T, JPN Model)
1. VOLTAGE CONVERSION (Refer to Fig. 5-1)
U/T Model (1 10V / 120V/220V / 240V , 50 /6 0 Hz)
Remove
the back boa rd and l ook for th e voltage
se l ector on th e u pper right portion of the ma -
chine. Chan
ge th e pos i tion of voltage se l ecto r
plu
gs so th at the vo lt age to be used co incides
w
ith the voltage shown throug h the open ing of
voltage
selector plug . Fuse change is not nece s-
sary.
2. CYCLE CONVERSION (Refer to Fi g. 5-1)
Remove th e back board and select the position of
switch l ocated in the recess at right port i on of Syscon
P .C B oard to correspond to the frequency of power
to be used. Move the switch to the ri ght for 50 Hz
and to the l
eft for the 60 Hz.
---------------S ERVICE MANUAL GX - 747 <.l bx ------ - --------
8

VI. DESCRIPTION OF CIRCUIT OPERATION
GX-747 dbx is made by adding the dbx noise reduction
system to GX-747. It
is functionally identical to the
GX-747 except for the dbx noise reduction system.
The VU Meter
is used for GX-747 dbx.
1. OUTLINE
The open deck has made a progress by increasing the
performance
of deck itself from 4 track 19 cm/sec.
to 2 track 38 cm/sec. On the other hand, the cassette
has tried to
catch up with the performance of the
open deck by rapidly improving the software, i. e.,
the recording
tape.
But now for the open deck, too, a software, EE tape
(extra efficiency tape) has been developed adding
an advantage
to the special features of the open deck
such
as long time recording and wide dynamic range.
By using the
EE tape, GX-747dbx 9.5 cm/sec. will
have the same performance
as that of the convention-
al 19 cm/sec., and the largest shortcoming of the open
deck, i. e., troublesome tape loading has been
eliminated by using the power lock system (tension
arm
is automatically locked by motor).
Improvement in Record/Playback S/N ratio
2. DETAILS AND FEATURES OF EE TAPE
1) "High range MOL" can be improved and "high
linearity" can be obtained by using the cobalt
addition type magnetic substance having high
coercive force.
2) Playback equalizer
of EE tape position
Tape Speed Conventional Type EE Position
38 cm/sec. NAB 3,180 µ.S + 50 µ.S NAB 3,180 µ.S + 35 µS
19 cm/sec. NAB 3,180 µS + 50 µ.S NAB 3,180 µS + 35 µS
9.5 cm/sec. NAB 3,180 µS + 90 µS NAB 3,180 µS + 50 µS
4.75 cm/sec. NAB 3,180 µS + 90 µS NAB 3,180 µS +70 µS
3) The REC bias is set 30% or more larger than the
conventional position.
4) Since the REC equalizer has high peaking fre-
quency and less equalizing amount, the transient
characteristic
is improved.
Tape Speed Tape Type 1 kHz Noise Level (dB) 10 kHz Noise Level (dB)
(cm/s)
EE Tape
-74
-66
19 Conventional
Hi Fi Tape
-71
-63
EE Tape -71 -66
9.5 Conventional
Hi Fi Tape
-71
-62
Fig. 6-1
Improvement in MOL (3% distortion level)
Tape Speed I
(cm/s) Tape Type 1 kHz (VU) 10 kHz (VU)
15 kHz (VU) 20 k!Iz
(VU)
EE Tape +15 +10
+8 +5
19 Conventional
Hi Fi Tape +15
+7
+3
+0.5
EE Tape +14 +4 0
9.5 Conventional
Hi Fi Tape +13
+1
-6
Fig. 6-2
1OkHz D Range Comparison
Tape Speed Tape Type 10 kHz MOL Noise Level (dB) 10 kHz D Range
(dB)
(emfs)
EE Tape +10 ~
-66
76
19
Conventional
Hi Fi Tape + 6 ~
-63
70
EE Tape +
4~-66
70
9.5 Conventional
Hi Fi Tape + 1~
-62 63
Fjg. 6-3
---------------SERVICE MANUAL GX-747dbx---------------
9

F characteristics comparison of newly developed EE tape and conventional Hi Fi Tape
19cm/s tape speed
+ 10 /
I
0 /
I
-10
/
I
-20
Iv
20
50 100 200
9.5 cm/s tape speed
v-
I
0
~
,
-10
~
I
-20
/
I
20
50 100 200
S/N Ratio for Record/Playback
EE tape/Conventional HiFi tape
20
50 100 200
-::::..:
~'
i. ldB
i\ T
\
--
'\ \
--
"\
-
--
i
500 1000 2000 5000 10000 20000 40000
~'\
.i.. I dB
T
..... \,\
\ \'
.....
t
500 1000 2000 5000 10000 20000 40000
----- Newly developed EE tape
Conventional
HiFi tape
Fig. 6-4
500 1000 2000 5000 10000 20000 40000
------ Newly developed EE tape
Conventional HIFI tape
Fig. 6-5
-----------------SERVICE MANUAL GX-747dbx -----------------
1Q

3. dbx NOISE REDUCTION SYSTEM
The dynamic range of actual sounds of orchestra,
rock
band, etc. far exceeds 100 dB. On the other
hand, the dynamic range of recording systems such as
disk and tape has so far been limited to 50-70 dB
due to the saturation level and noise characteristics.
The
dbx noise reduction system is one of the attempts
to improve
on the above drawback.
TAPE
LINE IN PUT o----<XJ--
.___ ____.
MIC INPUT,~~ --
11
In this system, the dynamic range in decibels of the
input signal is halved during recording, and doubled
during playback after recording
to return it to
original range. The system makes
it possible to
record and playback signals whose dynamic range
exceeds 100
dB through a tape deck whose dynamic
range is the normal 50-60 dB. (Refer
to Fig. 6-6)
DECODER -----o LINE OUTPUT
(dB)
Saturation level (dB)
+20~----
W/«fl////2
________ (dB)
0
-20
-40
-60
-80'-----.T
Source
Signal
+10
ENCODER
The dbx system largely consists of the encoder and
decoder. The basic operation
of the encoder/decoder
is as follows:
Encoder operation
As shown in Fig. 6-7, the input signal range in
decibels is halved. For example,
-60 dB input signal
is changed to -30 dB, and +20 dB signal is reduced
to+ 10 dB. After the encoder, the signal is sent to the
recording amplifier, and recorded on the tape. That
is, the 80
dB dynamic range of the original signal
is reduced to 40 dB. In this case, when the user tries
to record the original signal with the tape deck
having 60
dB dynamic range, the 20 dB difference
from the 80 dB signal range should have been
sacrificed
by the tape noise or saturation distortion,
but thanks to the operation of the dbx encoder,
the dynamic range
of 40 dB thus obtained covers the
80
dB input range sufficiently.
Decoder operation
The encoded playback output signal of the tape deck
which has passed through the playback amplifier
from the
PB head is sent to the decoder. The
dynamic range
in decibels is doubled according to
the decode characteristic shown in Fig. 6- 7. For
Tope
-----~ +20
--
---
--
0
--
------
-20
-40
-60
...._ ___ ..J-80
DECODER Output
Signal
Fig. 6-6 Encode and Decode
example, a -30 dB signal is changed to -60 dB, and
the noise level
is also lowered by 30 dB, which gives
the so-called noise reduction effect. Similarly, a +10
dB signal is increased to +20 dB, so that the saturation
characteristic on the tape has apparently been im-
proved
by 10 dB. The saturation ceiling level is
increased, hence the dynamic range
of the tape deck
is effectively increased, and the original sound
can be
reproduced without its wide dynamic range being
affected.
(dB)
a
.J
~-10
~-20
!:i -30
~ -40
::,
0 -so
-60-+Ec----+-------I
-10
-80
-80 -70 -60-50·40 -30 -zo -10 0 +10 +20 (dBJ)
INPUT LEVEL
Fig. 6-7 Input/Outpl,lt
----------------- SERVICE MANUAL GX-747dbx ----------------
11

4. EXPLANATION OF EACH OPERATING PRINCIPLES
Amplifier Block Diagrams
MIC IN ov-{>-!•---"-"------'
TAPE MONITOR
SWITCH SOURCE
fLINE
OUT
LINE
OUT(Qt---------.-
HEAD PHO!£ AMP
METER
AMP
VU METER
TAPE
.
L __
4-1 dbx NOISE REDUCTION SYSTEM CIRCUIT
=v
ERASE HEAD
~~
REC HEAD
dBx NR SWITCH
RB AMP
PB HEAD
Fig. 6-8 Amplifier Block Diagram
+B (+I l,6V)
RI Cl
The main circuits composing this system provide the
pre-emphasis, de-emphasis, VCA (Voltage Controlled
Attenuator or Amplifier),
RMS (Root Mean Square)
action, level sensor, etc. (Refer to Fig. 6-8). Each
circuit will be described below:
N ~
TRI
4-1-1 INPUT FILTER (HPF)
This filter, which
is a high-pass filter (HPF) of about
20
Hz cutoff (fcut) frequency and -12 dB/oct slope,
is formed by a positive feedback active filter. Here,
the filter
is made into an active filter using a
transistor to improve the shoulder characteristic of
the filter and to act as a buffer to eliminate the
effect of following circuits due to their impedance
characteristics. (Refer to Figs.
6-9 and 6-10)
..,.
er
.--->---o OUT
-B H l,6Vl
Fig. 6-9 INPUT Filter Circuit
[ 12d8/oct
g Yf
~ I
I
fcut::; 20Hz
Fig. 6-10 Input Filter Charactelis tic
---------------- SERVICE MANUAL GX-747dhx ----------------
12

4-1-2 PRE-EMPHASIS CIRCUITS
(Refer to Figs. 6-11 and 6-12)
r--------- -7
I RI I I0K C6 I00P I
I Rl2 33K I
,------, I ______ __. I
I C3 R5 I I C7 I00P I
I 0,01 IOK I I -------- I
I I IC I }JPCl252H2 I I
FROM R6 V CA
H.P.F I C4 I 33K I I
I I 1------~+-- ➔ TO REC AMP
I I I
L _J
'---+------, ► TO 8, P.F
- - - - LPre Emphosis Circuit ___ _.,,,..I
I
(dB)
10
5
0
., -5
>
~
-10
I I
L _________ J
Fig. 6-11 Pre-emphasis Circuit
I
Pre -emphasis
\De-emphasis
-15 '----------~--~
10 100 IK I0K I00K (Hz)
Freq
Fig. 6-12 Pre-emphasis and De-emphasis
Characteristics
The primary role of this circuit is to play the same
role
as the equalizer used with the record or other
recordingmedia
by pre-emphasis of the high frequency
band to reduce high band noise, and to reduce the
breathing phenomenon (Note 1).
During playback, the high frequency band
is de-
emphasized
to return the signal to the original state.
In Fig. 6-11, the time constant
of the Low band is
determined in the first stage, and the time constant of
the high band and the emphasis value is determined
in the last stage. C3 and C4 provide AC coupling
to prevent the DC offset of the previous stage from
being modulated
by the change in the VCA gain.
The first stage also serves to convert the
input voltage
to a current source, while the last stage converts the
current
to a voltage output. C7 is an oscillation
preventing capacitor. (Refer to Fig. 6-11)
Note
I. It is generally well-known that noise (hiss noise)
added
by the tape recorder or tape deck has a
high
content of high frequency band com-
ponents.
In the dbx system, compander gain changes
according
to the level of the intermediate and
low frequency band components which ·are the
main components in the input,
but hiss noise
also varies according to the gain change. As a
result, hiss noise changes
as the level of the
main signal component changes, and
"bre8thing"
noise is heard. This is called the breathing
phenomenon.
Of course intermediate and low frequency band
noise changes similarly, but since in term .ediate/
low band signal components are stron.g, this
noise is masked
by these strong signals, a.11d it is
not sensed by the ears. Hiss noise consisting of
high frequency band components is not oiasked
by the signal components and is therefore easily
detected by ear.
----------------SERVICE MANUAL GX-747dbx ----------------
13

4-1-3 DE-EMPHASIS CIRCUIT (Figs. 6-12 and 6-13)
In contrast to the pre-emphasis circuit which pre-
emphasizes the high frequency band during recording,
this circuit de-emphasizes the high frequency band
during playback to restore the
signal to its original
4-14 VCA (Voltage Controlled Amplifier (or At-
tenuator)) CIRCUIT (Refer to Figs. 6-14,6-15,6-16)
.--------- + B ( +11,6 VJ
'---+-------+--- -B (-11,SV)
From R,M.S Sencor
Fig. 6-14 Encoder
GND I set
Vee
GAIN
CELL
IDLING
CURRENT SUPPLY
GC I
I out
GC 3
Fig. 6-16 IC µPC1252H2 Internal Block Diagram
ICl and IC4 (µPC1252H2) are dbx VCA circuit ICs.
Pin (D is a virtual ground, and accepts only an input
signal current. Pin @ is a high impedance current
output.
Fig. 6-13 De-emphasis Circuit
state. The circuit configuration is the same as the
pre-emphasis circuit, but
has some differences in
component values.
INPUT
.--------,;. +9 (+11,SV)
1-----<;; OUTPUT
L......---1-------B (-11.SV)
From R.M.S Sencor
Fig. 6-1 S Decoder
Pin ~ is connected to negative power supply through
the current setting internal bias circuit. The set
current is nominally 2 mA. The range is co:rnpressed
in the encode mode when the RMS sens~r cutput is
positive. In the decode mode, since the RN.IS sensor
output
is fed to Pin a) with Pin @ beini GND, the
range is expanded when the RMS sensor output is
positive, and it is compressed when the MS sensor
output
is negative, which is just the opposi1e action
to the encode mode.
The circuit consisting of R9(R37), Rl0[R.38), and
VRl (VR3) can make symmetrical adjusiroent pos-
sible and minimize DC offset voltage -..Nhich is
multiplied by the circuit gain, and controlth.e second
order harmonic distortion. When a fast gain 4;:hange is
commanded, DC offset appears at the Oiltj)Ut. R8
(R36) are provided to reduce this. In tie encoder
(Fig. 6-14), it acts as a logarithmic amplifier, while
it acts
as an anti-logarithmic amplifier in ie decoder
(Fig. 6-15). Fig. 6-16
shows IC µPC125lff2 block
diagram.
---------------- SERVICE MANUAL GX-747dbx ----------------
14

4-1-5 LEVEL SENSOR FILTER OR
BAND PASS FILTER (Figs. 6-17 and 6-18)
H.P.F L.F.F
r-----~r----------7
I C9 CIO I I Rl7 Rl8 I
INP UT ~
10,1/63 0,1/63
I
I
I
I
Q.
0
<D
IO
I I I I
I 11 I
I 11 I
L-----J 1... ___________ J
+B (+ll,6Vl
TR2
2SC2320
---
0
0UTPUT
-B Hl.6Vl
Fig. 6-1 7 Level Sensor Filter Circuit
(dB)
0
~ -IO J----12_d_B_/_O_C_T ___ .,..... __
z
0
g; -20
w
o:: -30
I I
I I
I I
I I
I I
1
fL= 17,5Hz I fH = 20KHz
-40-1--L----~--.,........L----
IO 100 IK IOK IOOK (Hz)
FREQ
Fig. 6-18 Level Sensor Filter Characteristic
This filter circuit is a BPF (Band Pass Filter) in which
the third order LPF (Low
Pass Filter) and the second
order HPF (High
Pass Filter) are combined.
the level sensor
in the region where the frequency
response
of the tape deck is not flat. (Misoperation
prevention).
The purpose
of this filter circuit is to prevent gain
error occurrence by attenuating the characteristic
of
4-1-6 WEIGHTING NETWORK (Figs. 6-19 and 6-20)
Cl4 R20 R21
INPUT o--t I-I -c-....w,•8,--__., _ _,
3
•w.,\K•,,-------O OUTPUT
0.1/63 27K
Cl~
0,0033/100
Fig. 6-19 Weighting Network Circuit
If signals in which the high frequency band is
emphasized by the pre-emphasis circuit, are directly
recorded by the tape deck, the high frequency band
saturation level
of the tape might be exceeded.
The weighting network prevents this by lowering the
amplification
of the VCA (Voltage Controlled Ampli-
fier) by a corresponding amount
in the case of
As shown, in the GX-747 dbx, fL=17.5 Hzmd fH=20
kHz.
{dB)
+15
-10
100
IK
FREQ
IOK (Hz)
Fig. 6-20 Weighting Network Cha1a.cteristic
signals containing many high frequency baod com-
ponents. For this reason, a weighting nitVJ"Ork as
shown in Fig. 6-19 which has the charac:e:J'.'istic as
shown in Fig. 6-20 is provided before th.-e RMS
sensor circuit. The weighting network rai-ses the
sensitivity
of the level sensor for signals With a high
content of high frequency band compo1er:,.ts, and
lowers the amplification of the VCA.
----------------SERVICEMANUALGX-747dbx----------------
15

4-1-7 RMS (ROOT MEAN SQUARE) LEVEL SENSOR
(Figs. 6-21 and 6-22)
R22 22M
,-....,1--<1,__ __ +BC+ 11.6V l
....._ ______________ -BC-ll.6Vl
Fig. 6-21 RMS Level Sensor Circuit
Vee
SQUARELAW
AMPLIFIER
)-----16 )---------'
Vee Is
CAP.
Fig. 6-22 IC µPC1253H2 Block Diagram
The RMS level sensor circuit detects the effective
value (RMS)
of input signal current, and converts it
to a
DC voltage proportional to the logarithm of the
detected level.
IC2 and IC5 (µPC1253H2) are for the dbx RMS level
sensor circuit.
Fig. 6-21 shows the RMS level sensor circuit. For
pre-setting the control signal (OV at zero point level)
for the input level, zero point level control
is made
possible by varying the operating current
of IC by
means
of YRS. (Zero point level=410 mV=-5.5 dB
ref lmW 600 ohms (0.774 volts))
The attack time
is 15 ms when there is a IO dB level
change (increase), 5
ms when there is a 20 dB change,
and 3
ms when there is a 30 dB change. The decay
rate (Note 2) is 125 dB/sec as standard. Time
constant is determined by the current (11) flowing
through R26, and by Cl 7.
In the GX-747 dbx, since IT=7.5 µA, in the equation
for obtaining the decay rate of 125 dB/sec, C=IOµF
from IT=0.75 C.
Cl8 is a by-pass capacitor, and the charging iurrent of
Cl7 flows through Cl8. Cl6 is a capacitorprovided
to obtain stability and low distortion chanc1:eristics
in the high frequency band.
R25 serves as ale'-'el shift
for bringing the control
value to the center. VR2 is
a volume control for symmetry adjustment.
R22 prevents any ''thump" sound when tle~e
is no
signal.
Note 2. Slope
of the output voltage change a.Jter an
abrupt large drop of the input signal lere 1.
Note 3. GND is used for common, othenise mis-
operation may result due to the larl,'! current
flowing during the recovery time.
---------------SERVICE MANUAL GX-747dbx---------------
16

4-1-8 POWER SUPPLY CIRCUIT (For dbx NR Circuit)
(Refer to Figs. 6-23 an_d 6-24)
+ISV n------,-------..
(I)
..,
u
R58
330
TR5
25B764
7
..,
~
(I)
:::::
u
ci
IC?
..,
M5230L
,._
..,
u
R59
330
N
'°
i '
ci
.,.
,0:
,n
(I)
!!?
CX)
N 0
a:
..,
'
(I)
u Q
,n
0
:8
GND
o------+----+-----------------------1-----uGND
-ISV n---------
------------------------u -II. 6V
TR6
2S0863
Fig. 6-23 Power Supply Circuit for dbx NR Circuit
STARTER
STANDARD
1--+----1
VOLTAGE
GENERATOR
_>---_._---I OVER CURRENT 1-------<
PROTECTION
VOLTAGE
ADJ
'------+--------------------------~I GNO
IC7 (M5230L) is an IC for the dual tracking regulator.
The tracking regulator
is a stabilized power supply in
which a change in either the positive
or negative
output voltage due to the load (or otherwise), is
followed by a similar change in the other voltage.
When it is used for the DC amplifier, etc., any offset
voltage due to supply voltage fluctuation can be
prevented.
Figs.
6-23 and 6-24 show the power supply circuit
and
IC M5230L block diagram respectively. In the
power supply circuit of Fig. 6-23, the output voltage
is controlled by changing the reference voltage
BALANCE ADJ
H OU1 PUT
'-----I OVER CURRENT!-------<
4
(-)IN p UT
PROTECTION
Fig. 6-24 IC M5230L Block Digram
applied to Pin Ci) of IC (M5230L) though the
resistor dividing circuit consisting of R6 l, R.69, R60,
and VR7. In addition, C38 sets the rise
tim constant
of the output voltage, improves the ripple suppres-
sion ratio, and improves the output noise v~lt age.
R64 and R65 are resistors for balancing tle positive
and negative output voltages. Though
th1 positive/
negative output voltage ratio can be cliar--iged by
changing this resistance ratio, the po1[tjve and
negative output voltages are made identical 1y- making
R64 equal to R65.
---------------- SERVICE MANUAL GX-747dbx ----------------
17

4-2. SYSTEM CONTROL
The system control circuit using a microcom-
puter
will carry out the following controls.
CD Operational Control of tension arm lock
Q) Indication of real time counter
G) Operation control of function SW
@ Control of reel motor brake system
G) Flashing control of indication lamps
Since the LED indicator and the electronic
counter indicator are operated by the timing
of
pulse, no tester can find the failure.
The failure can only be found by checking the
waveform on the oscilloscope.
(!) ICl (µPD553C-152) Terminal Description
CLOCK
~'""{
INTERRUPT
RESET
SEGMENT[:
TAPEPULSE
REC OUT
OSC DRIVE
QT RELAY
PB MUTE
REV OUT
PAUSE PLANGER
MOTOR NORMAL
REVOLUTION
REVERSE REVOLUTION
BACK UP
L-------
EACH FUNCTION
INPUT
PLAY/FF OUT
Fig. 6-25
@ IC2 (µPD553C-110) Terminal Description
CLOCK
[3 DIGIT H2
DIGIT INPUT 4 DIGIT
H3
5 DIGIT lo
SEGMENT INPUT g
RESET
_,,,,~{
RELAY CTL
0
STOP OUTPUT
TEST
+IOV
PDr
PD2
PEo
PE1
PE2
PE3
PFo
PF1
PF2
PF3
PH1
CLOCK
DIGIT INPUT 2 DIGIT Hr
DIGIT INPUT I DIGIT Ho
REV UP
AUTO STOP
QUE
REEL SIZE
REV H
REW H
REV
UP LAMP
AUTO STOP LAMP
REEL
MOTOR CTL
Fig. 6-26
----------------- SERVICE MANUAL GX-747dbx -----------------
18

4-2-1 FUNCTION MODE SELECTOR CIRCUIT
Selection
of the function mode is controlled
by the pulse timing using a microcomputer.
For easier understanding, one input terminal
@ will be explained as an example. The
terminal @ receives four
~als, i. e., the
strobe signals
from@ ,@
6@
and~.
Each pulse signal from @ , @ , Q§) and
@ has a different timing. Assuming that the
signals from these terminals are 1000, 0100,
0010 and 0001,
signal 1000 of Terminal
@will pass D84, and through the collector of
TR85, it will enter the emitter of TR60.
If FWD (SW3) button is pressed, the base of
TR60 is earthed through R202, TR60 is
energized, and Signal 1000 will be outputted
into Terminal @
of ICl through D60.
r-----------7
If any input is made into ·@ , it is processed
within the microcomputer depending on the
signal and the signal
is outputted from the
respective terminal.
If 1000 is inputted into
@ , it is judged as FWD and the related
signal
is inputted to the output terminal.
(See Fig. 6-27). Similarly, Signal O100 from@
passes the emitter and collector of TR86 and
enters the Counter
P.C Board. When the pro-
gram
SW (SW6) is pressed, Signal 0100 is
inputted into 1c1@. Signal 001 O from@pas-
ses the collector/emitter of TR87, and through
D66 and the reverse selector
SW L__ ,it is
inputted into ICl @. Signal 0001 from @
passes SRI and SW906 and is inputted into
@.
I IC-I ~
1, ~31
5
_6 __ F_wo __ 6_P_RO_G_RAM __
6
_C-~
0 SW906
KEY
SW REVERSE
: ~ PHo 26 SELECTOR
I ~
I
I ~
I
I ~
L _________ ....1
Fig. 6-27
Input to
® Judgement and Output by Microcomputer
1000 Output H to Terminals(}
2) ,QA),@ and (25) of ICl (FWD output order).
0100 Act
as clear/program SW only when MEMORY AUTO REV SW is ON.
0010 Act once
as a reverse equipment.
(SW 906 is ON and tension arm is locked)
0001
When FWD, REV, FF or RWD key is pressed, process the other signals after making
Terminal
(is) H.
Fig. 6-29 shows the truth table when the function switches are pressed.
Function
SW
Term~
FF FWD STOP
12
H H
13
L L
14
H H
15
H L
16
L L
17
L H
24 H L
25
H H
Note: H of Terminal@: _jl__
H of other Terminals: _r-
H
L
L
H
L
L
L
L
REV REW
PAUSE
H H H
L L L
H H L
L H L
H L L
H L H
L H L
H L L
Fig. 6-28
REC REC MUTE
REC PAUSE
L H a
H H a
H L L
L L L
L L L
H H
ll
L L L
H H L
Fi~- 6-29
---------------SERVICE MANUAL GX-747dbx---------------
19