Kenwood KR-7400 User manual

KENWOOD
Hl/FI STEREO COMPONENTS
Service Manual
KR-7400

CONTENTS
EXTERNAL VIEW . 3
TOP & BOTTOM VIEW. 4
DISASSEMBLY/CORD STRINGING. 5
BLOCK DIAGRAM. 6
CIRCUIT DESCRIPTION. 7~9
ADJUSTMENTS. 10,11
PACKING .12
PARTS LIST
MODIFICATIONS....13
TOTAL. 1 4
AUDIO SECTION ASS'Y . 1 4
TUNER SECTION ASS'Y. 1 5
POWER SUPPLY (X00-1430-10). 16
RF (X01-1160-10). 17
IF (X02-1050-11) . 18 ~ 20
MPX (X04-1040-10). 21
MAIN AMP (X07-1270-10). 22,23
PREAMP (X08-1270-00) 24
TONE AMP (X11-1210-00). 25
PUSHBUTTON SWITCH (X13-1800-10). 26
CONNECTION (X13-1810-10). 26
PC BOARD
POWER SUPPLY . . . .. 27
RF . 27
IF . 28
MPX. 28
MAIN AMP. 29
PREAMP . 30
TONE AMP .. 30
PUSHBUTTON SWITCH . 31
CONNECTION. 31
TROUBLESHOOTING. 32
SEMICONDUCTOR SUBSTITUTIONS AND LEADS. 33
MODIFICATIONS. 34
SCHEMATIC DIAGRAM. 35
SPECIFICATIONS. 36
Note:
The products are subject to moi
regions. This is because each p
provides information of modific
ordering a soda: 3 d components
We employ the following abbr i
U.S.A.
Cat i 3da
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Australia .
Europe .
on in components and circuits in differ*
t must be used under the best conditic
jased on the standard in the U.S., for th
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of respective countries:
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England . . .
Scandinavia .
South Africa
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EXTERNAL VIEW
The KR-7400 is one of the NEW KR series and the highest
grade products. Its tuner section consists of frequency
linear 4 gangs variable capacitor, linear scaled front dial
calibrations, LED (light emitted diode) dial pointer, 3 MOS
FET's of front end, local oscillator with buffer amplifier,
PLL 1C of MPX stage, 2 meters for accurate tuning, and FM
DETOUT jack.
PLL assures excellent channel separation and improved
stability. FM DET OUT jack is available, permitting this
receiver to be ready for FM 4-channel broadcasts whenever
they became available. Its tuner section is also super
sensitivity and higher spurious response rejection ratio.
Meanwhile, main amplifier section consists of direct-
coupled with differential amplifier, and SOA limiter and
DC drift of center voltage protection.
They assures to protect the damage of power transistor and
spea kers.
Driver stage is pure complementary circuit. Equarizer and
tone amplifier have dual power supply and low noise
dual-can type 1C. One of the special feature of KR-7400 is
the availability of two independent tape monitor circuits
which permit tape dubbing from one tape recorder to
another while listing to a completely different source
such as an FM broadcasts. 4-CH OUT-IN for those who
wish to enjoy 4-channel reproduction can do so through
this receiver by connecting a SQ, RM, or CD^l- type
adaptor to these jacks. Tuner and audio section in
KR-7400 can stand comparison with separated TUNER
and AMPLIFIER.
Pin jack Terminal srtips
(El 1-0104-05) (E20-0418-13)
GND terminal Slide switch Ferrite ant. AC outlet * Fuse holder
(N08-0128-05) (S31-2007-05) (T90-0031-05) (E08-0221-05)
Pin jack * Power cord
Push terminal
(E21-1201-05)
Pin jack
(El 3-0604-03)
(El 3-0410-03)
* Refer to parts list
the unit is K type.
3

TOP & BOTTOM VIEW
Power transformer
Power supply unit
Electrolytic
(C90-0213-05)
Electrolytic
(C90-0213-05)
* RF unit
Power transistor
MAIN AMP unit
(X07-1270-10)
* MPX unit
* IF unit
PREAMP unit
(X08-1270-00)
Connection unit
(XI3-1810-10)
Push switch unit
(X13-1800-10)
TONE AMP unit
(X11-1210-00)
Refer to parts list

DISASSEMBLY / CORD STRINGING

CIRCUIT DESCRIPTION
FM-RF (X01-1160-10)
Two dual-gate MOS FETs are used for RF stage. FET in
the mixing stage, too, are of this type and produces ideal
mixing effects. A buffer follows the local oscillator
(OSC) to inject a stable oscillator output into the mixer,
so that the front end is high in both stability and
sensitivity.
FMMPX (X04-1040-10)
The MPX has PLL MPX integrated circuit (Refer to
page 8). Unlike the conventional circuitry by which a
38-kHz switching signal is derived from the 19-kHz pilot
signal contained in the incoming signal, this MPX produces
a very accurate switching signal by error voltage of phase
difference between the incoming signal and VCO (Voltage
◄ TA7060P
CIRCUIT
(ICbl)
PREAMP (X08-1270-00)
Metal can sealed monolithic 1C is used here. This 1C
consists of the differential amplifier of the first stage
and emitter followers of next stage, operating to provide
Class A drive and pure complementary output. The circuit
is a wide dynamic range circuit, operating with high input
impedance and low output impedance and drawing two
power supplies, positive and negative, and thus ensures
stabilized equalizer characteristics.
7

CIRCUIT DESCRIPTION
PROTECTION CIRCUIT
Current limiter protection operates out of transistor's SO A
(Safe Operating Area). This protection circuit is accom¬
plished by detecting the ic of power transistor. Safeguard
against overcurrent is decreased by the bias on the
complementary stage. For DC drifts of the center voltage
level, a relay is employed to cut the speaker line out of
service when the center level drifts more than ±7 volts.
This protective action, as well as the SOA protection
mentioned above, is self-return. All these protective
schemes operate free from the influence of speaker load
impedance. Confirm the current limiter protection to
operate. The following is the method: connect the dummy
resistor to both speaker terminals, and the oscilloscope
across the dummy resistor of the left channel.
And then feed the signal (1 kHz) to AUX jack of the
receiver. Next short-circuit the right speaker terminal, and
the left output is increase. Other channel in the same.
L-ch waveform
L-ch waveform
When R-ch is shorted
MUTING CIRCUIT
When the power switch is on the protection relay does no^
operate to eliminate the shock noise from speaker. And
then the relay operates after power switch is on.
▲ MAIN AMP (X07-1270-10)
SOA protection DC drift protection
9

ADJUSTMENTS
* Tuning dial is set to the proper point corresponding to no radio stations.
* The sweep and the r.f. generator are set to the lowest response possible on oscilloscope.
* When connecting the r.f. generator to the antenna terminal use the dummy antenna . . . refer to figure 1.
* Use the insulated screwdriver adjusting the i.f.t.
* SELECTOR is FM position.
* FM MUTING is OFF position unless it is required.
* Test point shown in the schematic diagram.
* INPUT means antenna input level.
No. ALIGN
TEST EQUIPMENTS
RECEIVER
SETTING
OUTPUT
INDICATOR
ADJUSTMENT
POINTS
REMARKS
CONNECTION SETTING
FM SECTION
i
IFT
SWEEP to TP1
via. 5pF cap.
10.7 MHz
Non-station
VTVM & SCOPE
to TP2 via.
100k£2 resist.
Lbl
Maximum
deflection
(Fig. 2)
2
IFT
RF-SG to ANT
via. dummy ant
98 MHz
75 kHz (Dev.)
400 Hz (Mod.)
98 MHz
same
La8
Maximum
deflection
3
DISCRIMI¬
NATOR
SWEEP to TP1
via. 5pF cap.
10.7 MHz
Non-station
VTVM & SCOPE
to TP3 via.
lOOkfi resist.
Lb3
S-response and its
symmetry on each
side of 10.7 MHz
center frequency
(Fig. 3)
4 TRACKING
RF-SG to ANT
via. dummy ant
90 MHz
75 kHz (Dev.)
400 Hz (Mod.)
90 MHz
VTVM & SCOPE
to REC jack
La 1-4
Maximum
deflection
5
TRACKING
same
105 MHz
75 kHz (Dev.)
400 Hz (Mod.)
105 MHz
same
CTal-4
same
6
TRIGGER
SWEEP to TP1
via. 5pF
10.7 MHz
Non-station
VTVM & SCOPE
to TP4 via.
100kT5 resist.
Lb6
same
7 BEACON
RF-SG to ANT
via. dummy ant
98 MHz
75 kHz (Dev.)
400 Hz (Mod.)
60 dB (Input)
98 MHz
VTVM & SCOPE
to TP5 via.
lOOkft resist.
VRb3
TP5 is 5V (d.c.)
8 BEACON
same
98 MHz
75 kHz (Dev.)
400 Hz (Mod.)
22-23 dB (Input)
same
same
VRbl
TP5 is 3.5V (d.c.)
9 MUTING
same
98 MHz
75 kHz (Dev.)
400 Hz (Mod.)
17 dB (Input)
98 MHz
MUTING on
-
VRb3
MUTING
operates
10 BEACON
MPX-SG to
RF-SG ext jack
98 MHz
67.5 kHz (Dev.)
400 Hz (Mod.)
17 dB (Input)
98 MHz
-
-
STEREO
indicator lights
11
OUTPUT
RF-SG to ANT
via. dummy ant
98 MHz
75 kHz (Dev.)
400 Hz (Mod.)
60 dB (Input)
same
VTVM & SCOPE
to REC jack
VRb2
Output is IV
10