TOSHIBA 29N23G
Service Manual

LINE CIRCUIT Line and frame drive are generated by IC403. The sync pulses are separated from the incoming video signal at pins 18/20/22 and used to control the internal circuitry of the IC. Line drive is produced by counting down the external 4.43 MHz crystal at pin 40 to15.625 kHz locked to the incoming sync. This drive is output on pin 48 and feeds directly to the line drive transistor Q600. Note. that the output of IC403 Pin 48 is an open-collector and requires a pull up resistor, if the pin is open circuited for test no waveform will be seen. Q600 collector feeds the line output transistor Q601. The line output stage is conventional with a transformer containing a split diode winding for EHT generation; fifth harmonic tuning is achieved by capacitor C618/619. FIELD OUTPUT VERTICAL SHIFT A fly-back pulse is taken from pin 1 of the FBT transformer. This is required by IC403 (Pin 49) for burst / sync gating, and RGB line blanking. The ver_sync signal is output from the pin47 and fed to pin41 of IC501. The H_sync pulse is taken from pin 1 of the FBT and fed to the micro at pin 40.These two signals are required by the micro for graphics timing and also for text. IC403 generates a vertical pulse signal V_OUT that is fed to IC601 (the vertical stage IC). IC601 is supplied by a 26V DC via diode D616 and a 13V DC via diode D613 .It generates its own ramp signal and based on the V_OUT signal and produces the vertical deflection signals that are fed to connector PL602. Horizontal adjustments (Horizontal amplitude, pincushion, corner correction) are controlled by EW_OUT at pin 11.This output drives the transistor Q602. B.C.L CIRCUIT (BEAM CURRENT LIMITER) Beam current limiting is employed to protect the circuitry in the receiver, the CRT and to prevent excessive X-Ray radiation in fault conditions. The current drawn by the CRT is monitored by the current drawn through the winding of the fly-back transformer that produces the EHT for the CRT anode. The end of the winding (Pin 10) is returned to IC403 pin 46, the beam current drawn by the CRT passes through Q601 and develops a voltage on the collector proportional to the current (V=IxR). The voltage on the collector will vary depending on the beam current being drawn reducing the brightness and contrast of the picture. If the voltage is sufficiently negative (indicating very high excess beam current) the output will be reduced, reducing the picture brightness and contrast.

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