Circuit Descriptions - Kenwood KR-6600 Service Manual

CIRCUIT DESCRIPTIONS
FM RF
Qgi: MOS-FET, RF amp = Dg1: Limiter
Qg2: Mixer Dg2: Poping-noise elimina-
tion for AM-FM switch-
over
Qg3: Local oscillator Dg3: Reverse current pre-
vention
Qg4: IF amp -
IF CIRCUIT
The IF circuit employs IC HA1137W in which quadra-
ture detector circuit, muting circuit, and meter circuit
are incorporated. (See block diagram.)
Quadrature detection is a sort of phase detection. A
signal from the third IF AMP is directly applied to one
side of input circuits of the' multiplier of quadrature
detector, and another signal is applied to the other side
through the phase shifter of Tg6. The variable in phase
difference obtained from these two signals is utilized
for detection.
50
iF 7
Jin
IF IN 'uedrotu ie
o> [] Md
pet deta:
MPX IC
1Cq2
'Sub-muting
{X13-2350-10)
S moter
Fig. 1 HA1137W BLOCK DIAGRAM
FM MUTING CIRCUIT
The FM muting circuit employs Qd2 (Qd3) of the pre-
amplifier unit as a switching element. The operating
voltage for muting is taken out of the muting circuit
within the IC.
—DETECTION PROCEDURE-—
Intensity of input level is detected from a position where
a signal equivalent to the quadrature detection input is
obtainable (NOISE DETECTOR). The obtained signal
is supplied to the MUTE DRIVER CIRCUIT. On the
other hand, DC output (discrimination curve) of the
quadrature detector is fed to the OV SWITCH and out-
put is generated as shown in Fig. 2-(a). This output and
above-mentioned input level detecting signal are put in
the OR CIRCUIT, thus generating an OR output as
shown in Fig. 2-(c).
(a) O V Switch Output (b) Mute Driver Circuit Output
\F sali characteristic
1 Discri
curve
bc
aig Hpi i
TALE 7 MHz
10.7 MHz ey
<< circuit
{ (c) OR Circuit Output
|
|
——+++ —— Muting level
10.7 MHz
Fig. 2. HA1137W MUTING DETECTION
Namely, when the input signal is weak or deviates from
the discrimination curve of the FM detector, operating
voltage for muting is generated exceeding the muting
level. (The muting start !evel corresponds to the antenna
input below 8uV.) This operating voltage is processed
for wave-form trimming at the Schmidt circuit of the
SUB-MUTING unit (X13-2350-10) and then used as a
trigger signal for ON-OFF switching of Od4.
Muting start voltage = Qp2: ON > Qp1: OFF > Qd4:
OFF
When S3 is ON (OQd4: OFF), FM +B is directly applied
to the gate circuit of FET Qd2 (Od3) and FET is switch-
ed on (FET turned on). When Qd4 is ON, gate voltage
of Qd2 (Qd3) is lowered and FET is switched off (FET
turned off). When S3 is OFF, +B is always maintained
at the gate circuit of FET which is therefore turned on.
This operating voltage for muting is also apptied to
terminal No. 12 of MPX IC (the forced MONO circuit)
for the prevention of misoperation of ST lamp.
Tx13- 2350-10
Tx05-1320-111
Qp2 Qd2 (ad3)
18 4 = 1
fe ee Le ee
$3: Muting ON
Fig. 3 DETECTION PROCEDURE
FM-MPX
The FM-MPX circuit employs newly designed MPX IC,
HA1196. (See block diagram.) Compared with con-
ventional HA1156, the distortion factor is very low.
VCO is adjusted to 76 kHz. For FM dolby broadcast,
the de-emphasis circuit is switched over to 25usec (S4).