Akai VS-2EGN - Page 92

10-2-4 LOCAL OSCILLATOR
AFT VOLTAGE
c7e
OUTPUT <——+lI
TUNING VOLTAGE
Tn
HIGH CH l2v <=
1) The local oscillator oscillates at a frequency is high-
er than the received signal frequency by an amount
equal to the IF frequency. This system is called the upp-
er supertheterodyne system, and the following relation
exists,
fo= fp + ffF
where fo= local oscillator frequency
fp = video frequency of received channel
f[F = video IF frequency (5.5 MHz)
2) Important requirements of the tuner local oscillat-
or are as follows:
@ Oscillation frequency is stable. It should not devi-
ate beyond the AFT locking range.
® Oscillator output is sufficient without affecting the
oscillator frequency stability.
3) Fig. 7-143 shows the oscillator.
D13 is a switching diode, which conducts when HIGH-
CH signals are selected C83, L02, and C85 are then
shorted to earth. Asa result, both inductance and capa-
citance are decreased, which raises the tuned frequency
to a higher value.
4) Fig. 7-144 shows the tuner oscillator excluding the
bias circuit and switching circuit of Fig. 7-143, during
LOW-CH reception.
Fig. 7-145 shows the circuit during HIGH-CH reception.
OUTPUT
Li
Fig. 7-144 The circuit components specific to
the LOW-CH are enclosed in a broken line
OUTPUT
il
aa ap |
Fig. 7-145 The circuit components specific to
the HIGH-CH are enclosed in a broken line
N
Reece
5) Fig. 7-144 can be redrawn in a simplified from as
Fig. 7-146.
()
Fig. 7-146 Clapp Oscillator
Fig. 7-146 shows a Clapp oscillator. Its oscillation fre-
quency is highly stable. The oscillator frequency is
adjusted by the variable capacitance diode D8 of Fig.
7-143. DIS is a variable capacitance diode for AFT.
This corresponds to varying C3 of Fig. 7-146. The oscil-
lation fre f this circuit is fo=-
quency 0 circ mVICy
The capacitance of C3 is the total capacitance of C77,
C80, C81+83, D8, and D15. At the time of manufac-
ture, the frequency is adjusted using Lol and Lo2.
Service Manual VS-2EGN
89