SHARP 20CT250
Service Manual

20CT-250

SERVICE ADJUSTMENT
115V ADJUSTMENT
No. Adjusting point 1 115V adjustment: R711 Adjusting procedure/conditions 1. Receive the Monoscope Pattern signal. 2. Set to the P-NORM mode. 3. Connect a DC milliammeter between TP602(�) and TP603(+). 4. Take the beam ammeter reading to make sure that the beam current is between 900 µA and 1100 µA. * If not, readjust the FBT screen control to obtain the beam current of 900-1100 µA. 5. Connect a digital voltmeter to TP701. 6. Adjust R711 so that the digital voltmeter should read 115 ±0.5 V. Waveform and others No. Adjusting point 1 VCO adjustment: T203

PIF/AFT/AGC ADJUSTMENT
Adjusting procedure/conditions 1. Disconnect the antenna from the tuner antenna terminal. 2. Apply DC voltage to pin (4) of IC201 (IF AGC). » DC voltage: 6.0 V (allowable +0.1 V) 3. Using a digital voltmeter, measure the DC voltage at pin (20)(PLL FIL) of IC201. * The digital voltmeter must be able to take readings down to the third decimal place. 4. Relieve pin (4) of IC201 of the DC voltage. 5. Reconnect the antenna in position and receive the E-12CH signal. (With the AFT off, adjust the receiving frequency to 224.25 MHz.) 6. Connect the DC digital voltmeter to pin (20) of IC201. Adjust T203 so that the voltmeter should read the same voltage as in Step 3. Allowable error : 0.015 V (20 kHz) 1. Receive the PAL Colour Bar signal. If this signal is not available, any signal above the E-5CH band is acceptable. » Field strength : 55-80 dBµV Make sure the frequency is almost the same as that of the received channel (±30 kHz). 2. Using the channel setting control, make a frequency of 224.25 MHz appear on the screen (the AFT turns off). If any other channel than Colour Bar is received, make its frequency on the screen. (The AFT turns off when the on-screen display turns yellow.) 3. Turn T204 clockwise to have a 6V point, and counterclockwise to have a 0.2V point. Position the coil at the center of these two points. 4. Adjust T204 so that the DC voltage at pin (1) (AFT OUT terminal) of IC201 be 3.3 ±0.1 V. (See Fig. 3) 1. Receive the PAL Colour Bar signal. » Field strength : 57 ±1 dBµV (75 ohms open) 2. Connect the oscilloscope to TP210, as shown in Fig. 4. Oscilloscope TV Set + TP210 0.1V � Bias box
+ �

Waveform and others * Warm up the unit for longer than 10 minutes in advance.

* 10 kHz at about 0.007 V.

* Position the T203 core in the range in which the Colour Bar signal can be received.

2

AFT adjustment: T204

DC voltage at TP201 6V 3.3V

DOWN A

B' B

B" C UP

Fig. 3

6

PIF CHECKING
No. Adjusting point 1 Tuner IFT (preset): TU201 Adjusting procedure/conditions 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 tuner�s IF output terminal. (This terminal must have the probe alone connected.) 5. Set the RF AGC voltage to 0-6 V with no contact with the waveform. 6. Adjust the tuner IF coil to obtain the waveform as Fig. 2. shown in Note: Be sure to keep the tuner cover in position during this adjustment. Waveform and others

IF-OUT 1000p 75� 1000p 10k� 100k� Oscilloscope
3 RF AGC cut-in adjustment: R216

Fig. 1
E-9CH

P

C � 1.5dB ± 0.8dB

Fig. 2

Fig. 4 » Bias box: About 4.5 V 3. Turn R216 to have the highest voltage. 4. Turn R216 slowly in the opposite direction until the voltage goes down 0.1 V below the highest level. 5. Adjust the signal level to 63-67 dBµV and make sure there is no noise. 6. Now adjust the signal level to 90-95 dBµV and make sure there is no chrominance modulation beat.

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