Philips L9.2A, Service Manual

Circuit description new circuits

GB 56

L9.2A

9.

 (*) Remark: When option RC = OFF, the P+ and the  P- key on 
the remote control have the same functions as the MENU UP/
DOWN keys while the VOL+ and the VOL- key have the same 
function as the MENU LEFT/RIGHT keys. When RC=OFF, it is 
not possible to change the channel preset or to adjust the 
volume in SAM/SDM with the remote control.
RC = OFF for use with L7-based remote control (only cursor 
keys). RC = ON for use with A8-based remote control (cursor 
keys, P+/P- and /Volume-).

8.5

Option bits/bytes

Option bytes
OB1 bits 8, 7, ..., 1: DP, FQ, AM, HS, HT, DM, GM, VI
OB2 bits 8, 7, ..., 1: CK, CL, AT, CI, (res), (res), SS, SP

OB3 bits 8, 7, ..., 1: RC, WE, (res), (res), TW, AC, C8, VM
OB4 bits 8, 7, ..., 1: TN, FV,XT,2X, XS, CD, BM, NI
OB5 bits 8, 7, ..., 1: EX, CX, NR, CP, CT, EW, BS, AS
OB6 bits 8, 7, ..., 1: BT, IS, VL, DV, UB, LV, DU, AO
OB7 bits 8, 7, ..., 1: ST, ST, SB, SB, SB, SY, SY, SY 
An option byte value is calculated in the following way:
value "option bit 1" x 1 =
value "option bit 2" x 2 =
value "option bit 3" x 4 =
value "option bit 4" x 8 =
value "option bit 5" x 16 =
value "option bit 6" x 32 =
value "option bit 7" x 64 =
value "option bit 8" x 128 =
Total : value "option byte" =

9.

Circuit description new circuits

Power supply (diagram A1)

9.1

Introduction

9.1.1

General

The switch mode power supply (SMPS) is mains isolated. The 
control IC7520 (MC44603A) produces  pulses for driving FET 
7518. Power supply regulation is achieved by using duty cycle 
control at a fixed frequency of nominal 40 kHz in normal 
operation. In stand-by, slow-start and overload situations the 
SMPS runs at frequencies other than 40 kHz. 
Basic characteristics of this SMPS :
•

Mains Isolated flyback Converter type

•

Input range : 90 - 276 Volts AC 

•

Secondary Sensing by Opto-coupler

•

IC7520 is Featured with  Slow-Start circuitry 

•

Protection Circuits

•

Degaussing circuit

9.1.2

Output voltages

•

Audio Supply ( +16.5V ) for the AUDIO AMPLIFIER ( 
Diagram A12 )

•

Mains Supply ( +140V ) for the  HORIZONTAL 
DEFLECTION stage (A2) and the CRT discharge circuit 
(A3)

•

Vaux ( +11.3V ) for the Video IF (A5), Video processing 
(A6) and Control circuit (A7) 

9.1.3

The switching periods of TS7518

The power supply duty cycle is dependent on the T-on of FET 
7518. The FET is driven by pin 3 of IC7520. This IC controls the 
secondary voltage (VBATT via opto-coupler 7581 and 
regulator 7570. The switching period of TS7518 can be divided 
into three main phases:  Duty cycle T-on, T-off and T-dead.
•

During T-on,  FET 7518 conducts. 

•

Energy is stored in the primary winding (2-5) of transformer 
T5545 by using a linear increasing primary current. The 
slope depends on the rectified mains-voltage present 
across C2508. The T-on period is varied to provide 
regulation of the drive waveform at pin 3 of IC7520. By 

controlling the duty cycle of the SMPS in this way the 
(VBATT is controlled.

•

During T-off,  FET 7518 is switched off and therefore does 
not conduct. The energy is now transferred to the 
secondary side of the transformer and then supplied to the 
load via the secondary diodes (D6550, D6560 and 
D6570,D6590). The current through the secondary side of 
the transformer decreases until it reaches zero.

•

During T-dead FET 7518 does not conduct .The voltage at 
the drain of the FET decays and eventually reaches the 
input voltage of approximately 300V.

9.2

Primary side 

9.2.1

Mains input and degaussing

•

Mains voltage: this voltage is filtered by L5500 and L5502, 
rectified by a diode bridge rectifier 6505 and then 
smoothed by C2508 which provides a DC input voltage of  
300V DC for an ac input voltage of 230V.

•

Degaussing : R3503 is a PTC. When switching "on" the set, 
the PTC is cold and has a low-ohmic value. Relay 1580 is 
activated while the Reset signal, coming from the (P is 
present. This allows a very high degaussing current at 
initial power on. The PTC will then heat up due to the high 
current involved and becomes high-ohmic which reduces 
the degaussing-current. During normal operation, the 
degaussing current is zero, because relay 1580 is open 
due to the absence of the (P - Reset signal.

9.2.2

Start up and take over

•

Start-up :  The start-up circuitry consisting of 3510, 3530 
and 3529 use the voltage coming from the 230V AC mains 
to start-up IC7520 via the supply pin 1. The output drive 
waveform (pin 3) is blocked by using the ICs internal logic 
until the voltage on  pin 1 reaches 14.5 Volts however with 
less than 14.5 volts on Pin 1 the IC only consumes 0.3mA. 
Once pin 1 reaches the 14.5 Volts threshold, IC7520 will 
start up (FET 7518 will conduct) and pin 1 sinks a typical 
supply current of about 17 mA. This supply current cannot 
be delivered by the start-up circuitry, so a take-over circuit 
must be present. If take-over does not occur then the 
voltage on pin 1 will decrease below 9V and IC7520 will 
switch off. The supply begins a new Start-up cycle, see top 

XS

External Source Colour Select

External source colour selection available when enabled

XT

External 1

External 1 source input available when enabled