Zenith R50V26 Service Manual

Also see for R50V26: Operating guide

Contents

Zenith R50V26 Service Manual Manual pdf 29 page image

CIRCUIT DESCRIPTIONSPV154 - 923-03506 3-5 ZP26/28 - SERVICINGNOTE: If the 5.5V or the 3.3V regulators OPEN, the setwill not go into shut down, they must have a short toproduce a shut down input on PROTECT 1.The set will shut down after the Screen go excessivelybright, but this is from the 115V over current sensor,not PROTECT 1. When the power supply is in Stand By,the Short Detection circuit could activate. By turningoff Q919, no accidental shut down operation can oc-cur.SUB POWER SUPPLY VISUAL LEDThe Chassis has 1 Green LED on the Sub Power SupplyPWB. This chassis utilizes 1 Green LED in the power sup-ply cold side. The power supply operates in two differ-ent modes, Standby and Projection On mode. The LED islit in the Standby or Power On mode with the AC ap-plied and the TV ON or OFF;· SRS Audio Front 29V Regulator +29V indicated byD912 Color GREENLED USAGEThe Visual LED is very useful in Trouble Shooting. With-out removing the back cover, some diagnostics can bemade. By observing the operation of the Green LED, thetechnician can determine if the Sub Power Supply isrunning or not. The following will examine how the LEDis illuminated.POWER ON ENERGIZE:When the Microprocessor outputs a High from pin 53when power is turned ON, the high is inverted by Q003to a LOW. This low is inverted by Q004 to a HIGH androuted through the PQS1 connector pin 2. This high isrouted through R954, R951, D926 to the base of Q914.This transistor turns ON and it’s collector goes LOW.This low is the Ground return for the Relay S901. The B+for the primary of the relay is the SBY +5V generated bythe switching transformer pin 15, rectified by D920,filtered by C935, L915, and C936.When the relay S901 turns on, the contacts close andAC is routed to the Deflection circuit power supply andthe Deflection Power Supply is Energized.DEFLECTION HORIZONTAL DRIVE ACTIVATION(IC01 Rainforest IC)When the set has AC applied, one of the switch pulsesgenerated on the Sub Power PWB is the +11V pulse frompin 10 of the Switching Transformer I901. This +11Vpulse is rectified by D919 and filtered by C929. This+11V DC voltage is then routed to the +9V regulatorI906 pin 1. When the set is turned ON, the HorizontalDrive Power supply I906 is activated. This happens whenpin 2 of I906 goes high with the Power On/Off controlline from PQS1 pin 2. The +9V is output via pin 3 to thePQD2 connector pin 5. From here the +9V is routedthrough the Deflection PWB to the Signal PWB via PSD3connector pin 11, through the coil LC03 to pin 19 ofIC01 the Rainforest IC. When this voltage is applied topin 19, the horizontal circuit inside the Rainforest ICis activated and a horizontal drive signal is output frompin 26. This H. Drive signal is routed through the PSD2connector pin 5 to the base of the horizontal drivetransistor Q709 on the Deflection PWB. The collector ofQ709 produces a drive signal routed through the drivetransformer T702 and output from pin 4 to the base ofthe Deflection Horizontal Output Transistor Q777 tobegin driving the deflection circuit which in turn, ac-tivates the High Voltage circuit.The sub power supply in the chassis works very similarto the previous models, with some very significantexceptions. The use of the power supply creating theSBY+11V supply eliminates the need for a Stand-Bytransformer. The following explanation will describe theSub Power Supply Shut Down Circuit.VOLTAGE LOSS OR EXCESSIVE LOAD DETECTIONThe second most common circuit used is the VoltageLoss Detection circuit. This is a very simple circuit thatdetects a loss of a particular power supply and suppliesa Pull-Down path for the base of a PNP transistor.This circuit consists of a diode connected by itscathode to a positive B+ power supply. Under normalconditions, the diode is reverse biased, which keepsthe base of Q1 pulled up, forcing it OFF. However, ifthere is a short or excessive load on the B+ line, thediode in effect will have a LOW on its cathode, turningit ON. This will allow a current path for the base bias ofQ1, which will turn it ON and generate a Shutdown Signal.B+ VOLTAGE TOO HIGHIn this circuit, a Zener diode is connected to a voltagedivider or, in some cases, directly to a B+ power supply.If the B+ voltage increases, the voltage at the voltagedivider or the cathode of the zener diode will rise. If itgets to a predetermined level, the zener will fire. Thisaction creates a Shutdown Signal.SHUT DOWN CIRCUITShut down occurs when the shutdown Photo CouplerI903 is activated by pulling pin 2 low.When I903 is activated by pulling pin 2 low it gives aground path for the emitter of the LED inside I903. Thelight produced by turning on this LED turns on theinternal photo receiver and generates a high out of pin(3). This high is routed to the base of Q902, turning iton. This grounds pin (4) of I901, removing Vin, andthe power supply stops working.The reason for the photo sensor I903 is to isolate hotand cold ground.