ZENITH IQC50H94W
Manual de Servicio

CIRCUIT DESCRIPTION
connected to the negative SW-8V line and RP25 (3.3K ohm) is connected to the positive SW+8V line. The Cathode of DP28 monitors the neutral point where these two resistors are connected. If the negative voltage disappears, the zener DP28 fires. This high is routed through DP29 to the gate of the Shut Down SCR QP01 and Shut Down occurs. EXCESSIVE CURRENT DETECTION: 9. 120V Deflection Power Supply (RP17, QP02, DP15, DP16 and DP18) If an excessive current condition of the Deflection B+ is detected by RP17 a 0.47 ohm resistor, the base of QP02 would drop. This would turn on QP02 and the high produced at the collector would fire zener DP15. This High would be routed through DP16 through DP18 to the gate of the Shut Down SCR QP01 and Shut Down occurs. 10. 28V Vertical IC I601 Power Supply (R645, Q609, D615, and DP34) If an excessive current condition of the Vertical B+ is detected by R645 a 0.68 ohm resistor, the base of Q609 would drop. This would turn on Q609 and the high produced at the collector would be routed through D615 through DP34 to the gate of the Shut Down SCR QP01 and Shut Down occurs. VOLTAGE TOO HIGH DETECTION: 11. Excessive High Voltage Detection (DH31, RH54, RH55 and DH24). Sensed from the Heater Voltage generated from pin (5) of the Flyback Transformer TH01. Also, (DH42) sends a high command to the Horizontal Driver IC IH02, to defeat Horizontal Drive Output The Flyback Transformer TH01 generates a pulse called Heater. (Note: This does not go to the CRTs as heater voltage, its used for Excessive High Voltage Detection). If this voltage goes too high indicating an excessive High Voltage condition, the voltage divider comprised of RH54 and RH55 would impress a high on the cathode of DH31. This high is routed through DH34 to the gate of the Shut Down SCR QP01 and a Shut Down occurs. 12. Side Pincushion failure generating a High. (D754, and D753) The Side Pin Cushion circuit is comprised of I651, Q652 through Q657 If a problem occurred in this circuit that creates a High on the cathode of D754, the High will be routed through D753 to the gate of the Shut Down SCR QP01. 13. Deflection B+ Too High. (DP17, RP21 and RP22 RP21 and RP22 form a voltage divider. The top side of RP22 is monitored by DP17. If this voltage goes too high, zener DP17 will fire. This high is routed through DP18 to the gate of the Shut Down SCR QP01 and Shut Down occurs. 14. Heater Voltage from the Deflection Power Supply Too High Detection. (DP27 and DP28) The Heater Voltage for the CRTs filament is generated in the Deflection Power
PV152

Supply. This voltage is monitored by DP27. If this voltage goes too high, zener DP27 will fire. This high is routed through DP28 to the gate of the Shut Down SCR QP01 and Shut Down occurs.

ABL CIRCIUT
The ABL voltage is generated from the ABL pin of the Flyback transformer, TH01. The ABL pull-up resistors are RH58 and RH59. They receive their pull up voltage from the B+ 120V(V2 ) for Deflection line generated from the Power Supply via TP91 pin 13, rectified by DP11, filtered by CP33 and then routed through the excessive current sensing resistor RP17. ABL VOLTAGE OPERATION: The ABL voltage is determined by the current draw through the Flyback transformer. As the picture brightness becomes brighter or increases, the demand for replacement of the High Voltage being consumed is greater. In this case, the flyback will work harder and the current through the Flyback increases. This in turn will decrease the ABL voltage. The ABL voltage is inversely proportionate to screen brightness. Also connected to the ABL voltage line is DH33. This zener diode acts as a clamp for the ABL voltage. If the ABL voltage tries to increase above 12V due to a dark scene which decreases the current demand on the flyback, the ABL voltage will rise to the point that DH33 dumps the excess voltage into the 12 line. ACCL TRANSISTOR OPERATION: The ABL voltage is routed through the PSD3 connector, through the PSZ2 connector, to the base of QX13. Under normal conditions, this transistor is nearly saturated. QX13 determines the voltage being supplied to the cathode of DX05, which is connected to pin 45 of the Jungle IC, IX01. During an ABL voltage decrease, due to an excessive bright circumstance, the base of QX13 will go down, this will drop the emitter voltage which in turn drops the cathode voltage of DX05. This in turn will pull voltage away from pin 45 of the jungle IC, IX01. Internally, this reduces the contrast and brightness voltage which is being controlled by the I C bus data communication from the Microprocessor arriving at pin 27 and 28 of the jungle IC and reduces the overall brightness, preventing blooming. SPOT CIRCUIT: When QN04 is turned on, the 11V standby will be applied to the anode of DN11, forward biasing it. This voltage will then pass through DN11, get zenered by DN09, and go to pin 2 of PSD3, where it will activate the Video Mute circuitry Q022 - Q024 on the Signal PWB. This is done to prevent CRT burn. Another input to this circuit is the I701 DAC3 line. This will activate when accessing certain adjustment parameters in the service mode; i.e. turning off vertical drive for making CRT drive or cut-off adjustments.

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PROJO