Magnavox 27TP83 C101 Manuel de service télécharger pdf (Page 69)

F8 CHASSIS INTRODUCTION

The F8 chassis is a leader TV chassis produced by Philips Consumer Electronics Company for the

1999 model year. The F8 chassis is used in sets with 25“ and 27“ screen sizes. The F8 chassis is a

global design and is oriented front to rear, or ”north to south“, as it has been called. The F8 chassis

tuning system features 181 channels with on-screen display (OSD). The main tuning system uses a

tuner, a microcomputer IC, and a memory IC mounted on the main chassis. The microcomputer

communicates with the memory IC, the customer keyboard, remote receiver, U/V tuner, signal

processor IC and the audio output IC via the I

C bus.

The memory IC retains the settings for favorite

stations, customer-preferred settings, and service/factory data.

The F8 chassis uses a Very Large Scale (VLSI) Integrated Circuit for signal processing. This IC

performs video IF, sound IF processing, AGC control, horizontal and vertical drive and synchronization,

also luminance/chrominance processing. The on-screen graphics and closed caption decoding are

done within the microprocessor, and then sent to the signal processor IC to be added to the main

signal.

The F8 chassis utilizes a switch mode power supply for the main voltage source. The chassis has a hot

chassis ground reference on the primary side of the power supply, and a cold ground reference on the

secondary side of the power supply and the rest of the chassis. ALWAYS USE AN ISOLATION

TRANSFORMER WHEN SERVICING THIS CHASSIS FOR YOUR SAFETY.

SIGNAL FLOW DESCRIPTION

(Display Signal Flow Block Diagram)

The incoming RF signal is applied to the U/V tuner where the 45.75 MHz IF signal is developed and

amplified. The IF signal then exits the tuner from pin11to pass through the SAW filter, (1003). The

shaped signal is then applied to IC 7250, the IF processor. The automatic gain control (AGC) is

performed in IC 7250 and applied to the U/V tuner at pin1. An automatic fine tuning signal (AFT) is also

generated in the IC and then routed to the tuning system via the I

C bus, to provide frequency

correction as needed.

The IF signal exits IC 7250 at pin 6, the audio and video signals are separated and each processed

accordingly. The sound IF signal enters IC 7250 at pin 1, while the video re-enters at pin 13. The audio

signal passes through the pre-amp, volume control and volume limiting circuits before application to the

final audio stages.

The video signal entering at pin 13, is separated into its luminance and chrominance components, and

exits at pins 21, 20, and 19 as R, G, and B respectively. The R G B signals are sent to the CRT DRIVE

board, amplified and applied to the CRT pins as drive. Drive and cutoff controls for the CRT are also

located on this board.

Brightness, Picture, Sharpness, Color and Tint control voltages are varied by the microprocessor, IC

7600 through individual control lines. IC 7600 also controls selected inputs, as selected by the

consumer.

Video and audio muting are an internal function of IC 7250. Automatic volume limiting also takes place

inside this IC.

Vertical and Horizontal drive signals are also developed inside IC 7250. Vertical drive exits the IC from

pin 47, while Horizontal drive exits at pin 40. These signals are a product of a Voltage Controlled

Oscillator Circuit (VCO) and divider network located inside the chip. Vertical drive is applied to pin 1 of

IC 7401, amplified, shaped, and output from pin 5 where it is applied to the vertical winding of the yoke.

A feedback signal is returned to pin 46 of IC 7250. The vertical sync pulse is also applied to the

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Magnavox 27TP83 C101 Manuel de service Page 69

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