Sharp VT-G14 Service Manual page 13

Ë Ë Ë Ë Ë PIF/AFT/AGC ADJUSTMENT
1. TUNER IFT : TU201 (preset)
1. Get the tuner ready to receive the E-9CH signal,
but with no signal input. Adjust the PLL data.
2. Connect the sweep generator's output cable to
the tuner antenna. (RF sweep)
3. Adjust the sweep generator's output level to 80
dBµV.
4. Connect the response lead (use a low-impedance
probe with wave detector; see Fig.1 ) to the tun-
er's IF output terminal.
(This terminal must have the probe alone con-
nected.)
5. Set the RF AGC voltage to 0V ~ 6V with no con-
tact with the waveform.
6. Adjust the tuner IF coil to obtain the waveform as
shown in Fig. 2 .
Note : Be sure to keep the tuner cover in posi-
tion during this adjustment.
2. VCO and AFT ADJUSTMENT : T253 (VCO), T254 (AFT)
1. VCO coil (T253)
1) Disconnect the antenna from the tuner antenna
terminal.
2) Apply DC voltage to pin (4) (IF AGC, TP252)
of IC251.
» DC voltage: 6.0 ± 0.1 V
3) Using a digital voltmeter, measure the DC volt-
age at pin (20) (PLL FIL, TP253) of IC201.
» (The digital voltmeter must be able to take
readings down to the third decimal place.)
10 kHz at 0.007 V
4) Relieve TP252 of the DC voltage.
5) Reconnect the antenna in position and receive
the E-12CH signal.
» (With the AFT off, adjust the receiving fre-
quency to 224.25 MHz.)
6) Connect the DC digital voltmeter to TP253.
Adjust T253 so that the voltmeter should read
the same voltage as in Step 3).
» Allowable error: 0.015 V (20 kHz)
2. AFT coil (T254)
1) Receive the E-12CH signal in the same condi-
tion as in Step 5) above.
2) Connect the oscilloscope to pin (1) (AFT,
TP251) of IC251.
3) Adjust T254 so that the DC voltage reading be
3.3 V.
» 3.3 ± 0.1 V (from the 0-6.6 V variable range)
SERVICE ADJUSTMENT
13
IF-OUT
1000p
1000p
75Ω
Fig. 1
E-9CH
P
Fig. 2
DC voltage at TP251
6V
3.3V
DOWN A B' B
Fig. 3
VT-G14
VT-G21
10kΩ
100kΩ
Oscilloscope
C
– 1.5dB
B" C UP