SONY DKC-ST5
Manual de Servicio

(3) Gamma correction circuit The video signal outputted from pin 7 of IC102 is adjusted the level by RV401, and is passed through Q401 buffer and the amplifier Q402 through Q404. That output is inputted to gamma amplifier input, the base of Q409. It is attached to load resistor R452 and diode D401 to the collector of Q411. D401 has four diodes. Their cathodes are connected to the voltages which are divided by R456, R461, R458, R455, R454 and RV404 through R453, R457, R460, R463. When signal is nothing, the diode is OFF since the cathode voltage of D401 is low. Therefore, the signal is not passed. When the signal is gradually bigger, the diode is conducted from lower voltage one gradually and the signal is outputted. This signal is negative fed-back to Q412 via RV405. When the signal becomes bigger, the quantity of feedback through the diode become bigger. Therefore, the output gain becomes smaller than signal becomes bigger. The collector signal of Q411 is entered to base of Q413 , is attenuated at R467,R465 and R466, is passed through RV405 and is negative fed-back to Q412. The Q413 gamma amplifier output is sample-held by Q414 in the CLP2 period and is fedback by the operation amplifier IC402 to the emitter of Q410. The voltage of RV406 (R PED) is passed through the voltage follower of IC402 (1/2) to become low impedance. And is entered to minus input of pin 2 of IC402. R PED control signal of pin 8 of IC105 and the master PED control signal of pin 8 of IC205 are mixed at R472, R477 and R478 and entered to the minus input of pin 2 of IC402. The feedback works to coincide this mixed voltage and the voltage of black level that is sample-held by pin 3 of IC402 . As a result, the black level is fixed by gamma circuit. Stabilization of gamma process is planned. (4) Knee Gamma correction circuit output of Q413 is passed through the buffer Q415 (1/2). This voltage is compared with the voltage that is set by RV407 at Q415 (1/2 and 2/2). When the signal is bigger than setting voltage, Q415 (2/2) becomes ON, the signal is attenuated at R496 and R497. By this mean, the signal of over setting voltage can be attenuated. After that, this signal is passed through buffer Q416, sent to AT-115 board via MB-724 board. And the other side, the signal is sent to 75 Z driver circuit. (5) 75 Z driver circuit The signal from Q416 emitter is passed through C422, and the buffer Q419 (1/2 and 2/2), entered to base of Q420 (1/2). The output from that collector is sent to the driver circuit that is composed by Q421 through Q424. The output of this driver circuit is fed-back to the base of Q420 (2/2). The output of driver circuit is sent to the MB-724 board and passed through 75 Z resistor , is sent to the processor.

DKC-ST5 (UC,CE)

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