Zenith R50V26 Service Manual

Also see for R50V26: Operating guide

Contents

Zenith R50V26 Service Manual Manual pdf 36 page image

CIRCUIT DESCRIPTIONSPV154 - 923-03506 3-12 ZP26/28 - SERVICINGThe ABL voltage is determined by the current drawthrough the Flyback transformer. As the picturebrightness becomes brighter or increases, the demandfor replacement of the High Voltage being consumed isgreater. In this case, the flyback will work harder andthe current through the Flyback increases. This in turnwill decrease the ABL voltage. The ABL voltage is inverselyproportionate to screen brightness.Also connected to the ABL voltage line is DH16. Thiszener diode acts as a clamp for the ABL voltage. If theABL voltage tries to increase above 11V due to a darkscene which decreases the current demand on theflyback, the ABL voltage will rise to the point that DH16dumps the excess voltage into the 11 line.ACCL TRANSISTOR OPERATIONThe ABL voltage is routed through the PSD2 connectorpin 2, through the acceleration circuit RC62 and DC02to the base of QX18. Under normal conditions, thistransistor is nearly saturated. QX18 determines thevoltage being supplied to the cathode of DC01, whichis connected to pin 53 of the Rainforest IC, IC01.During an ABL voltage decrease due to an excessivebright circumstance, the base of QX18 will go down,this will drop the emitter voltage which in turn dropsthe cathode voltage of DC01. This in turn will pullvoltage away from pin 53 of the Rainforest IC, IC01.Internally, this reduces the contrast and brightnessvoltage which is being controlled by the I 2C bus datacommunication from the Microprocessor arriving at pins30 and 31 of the Rainforest IC and reduces the overallbrightness, preventing blooming as well as reducing theColor saturation level to prevent color smear.MAIN MICROPROCESSOROVERVIEWThe Microprocessor must keep communication with theChassis to maintain control over the individualcircuits. Some of the circuits must return informationas well so the Microprocessor will know how to respondto different requests.The Microprocessor uses a combination of I 2 C Buscommunication and the Serial Data, Clock and Loadlines for control. The I2 C communication scheme onlyrequires 2 lines for control. These lines are called SDAand SCL. Serial Data and Serial Clock respectively.The Microprocessor also requires the use of what arecalled Fan Out IC or DACs, (Digital to AnalogConverters). This allows the Microprocessor to use onlytwo lines to control many different circuits.Also, due to the fact that this Microprocessor operatesat the new 3.3Vdc voltage, it requires a Level Shift ICto bring up the DC level of the control lines to make itcompatible with the connected ICs.The Microprocessor communicates with the followingICs:ON THE SIGNAL PWBMain Tuner U501PinP Tuner U502EEPROM I002Flex Converter I301DAC1 I006DAC3 I007Level Shift I0043D Y/C I301Main Video Chroma I501Rainforest IC01ON THE TERMINAL PWBA/V Selector IX01Sub Video Chroma IX03Main Y Pr/Pb Selector IX04Sub Y Pr/Pb Selector IX05ON THE SURROUND PWBFront Audio Control IA01DATA COMMUNICATIONThe following explanations deal with thecommunication paths used between the Microprocessorand the respective ICs.ON THE SIGNAL PWBMain Tuner U501 (with MTS outputs). TheMicroprocessor controls the Main Tuner by SDA (Data)and SCL (Clock) I2 C communication lines. SCL1 andSDA1 lines for the Main Tuner are output from theMicroprocessor at pins 2 (SDA1) and 3 (SCL1)respectively. These lines go directly to the Main Tuner,SDA1 at pin (5) and SCL1 at pin (4). These lines controlband switching, programmable divider set-upinformation, pulse swallow tuning selection, etc.PINP TUNER U502The Microprocessor controls the Main Tuner by SDA(Data) and SCL (Clock) I 2C communication lines. SCL1and SDA1 lines for the Main Tuner are output fromthe Microprocessor at pins (2 SDA1 and 3 SCL1)respectively. These lines go directly to the Main Tuner,SDA1 at pin (5) and SCL1 at pin (4). These linescontrol band switching, programmable divider set-up information, pulse swallow tuning selection.EEPROM I003The EEPROM is ROM for many different functions ofthe Microprocessor. Channel Scan or Memory List,Customer set ups for Video, Audio, Surround arememorized as well. Also, some of the Microprocessorsinternal sub routines have variables that are stored