Akai VS-2EGN

11-3-4 AFT CIRCUIT
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3MHz —————>}
FINE VARIABLE RANGE
fir
65V
m<— 1.SMHz 4
The AFT (Automatic Fine Tuning) circuit is a circuit
for the automatic control of local oscillator frequency
by converting the frequency changes of the IF signal to
DC voltage changes through FM detection and applying
them to the variable capacitance diode of the tuner local
oscillator.
1) Fig. 7-113(D) shows the AFT circuit configuration.
The VIF carrier having passed through the limiter, en-
ters the FM detector, which subjects it to frequency
deviation detection, amplifies its DC output, and makes
11-3-5 VIDEO EQUALIZER AND OUTPUT CIRCUIT
AUDIO DET
Fig. 7-165 AFT characteristic
it an AFT control value on demand.
2) Carrier is fed to the AFT tuning circuit via C11 and
C15, and then fed to the detector (IC).
3) The AFT tuning circuit consists of LS, C14, C10,
and variable capacitance diode D4. Fine tuning in the
AFT lead-in condition can be made by varying the capa-
citance of D4. The variable range is +500 kHz with bias
voltage at 6£3V.
Fig. 7-165 shows the AFT characteristic.
1) L7 and CF2 (ceramic filter) forms a trap for the
audio. sub-carrier frequency, and the audio signal con-
tained in the video signal is separated here, TR2 is an
Emitter follower, and D5 is a diode for clamping the
leading edge of the SYNC signal.
2) TR2 and TR3 are cascade-connected, to lower the
AC load impedance, and improve the high band frequen-
cy characteristic.
3) Since the level around the CHROMA signal falls due
to the audio trap, the high band is compensated for by
the equalizer amp (TR3, R32, C23, L8). The resonant
frequency of L8 and C23 is about 5.2 MHz, and the gain
characteristic of the equalizer amplifier is as shown in
Fig. 7-167.
96
+B
Lio.
R36
VIDEO OUT
Fig. 7-166 Video equalizer and output circuit
ee
GAIN
fo 5.2MHz
Fig. 7-167 Equalizer characteristic
Service Manual VS-2EGN