Philips 286NS-05 Schematic

1

Colour Television Chassis

Comet

765320029_001

030697

ServiceManual

Content

1 Technical specifications 2
2 Connection facilities 3
3 Safety instructions, Maintenance instructions, 4

Warnings and notes
4 Mechanical instructions 6
5 Repair facilities 7
6 Overview oscillograms 9

Survey of testpoints 9

Block diagram 10

Fault finding tree 11
7 Electrical Diagrams and print lay-outs

Diagram PWB

Power supply & Line stage (Diagram A1) 12 20,21

Tuner IF (Diagram A2) 13 20,21

Sound & Chroma (Diagram A3) 14 20,21

Controls & teletext (Diagram A4) 15 20,21

CRT panel (Diagram B) 16

Deflection module 110 degrees (Diagram D) 17 17

2 x 3W amplifier (Diagram C2) 19 18

Seperate controls & Mains module(Diagram J) 22
8 Electrical adjustments 23
9 Circuit Description 24
10 Directions for use/Exploded view cabinets 28, 35
11 List of abbreviations 36

Published by LV 9865 TV Service DepartmentPrinted in The NetherlandsCopyright reserved 1998 Philips Consumer Electronics B.V. Eindhoven, The Netherlands. All rights reserved. No part of this publication may be reproduced, stored in a retrieval system or transmitted, in any form or by any means, electronic, mechanical, photocopying, or otherwisewithout the prior permision of Philips.Subject to modification5 4822 727 21614

12 Spareparts list 38

Published by LV 9865 TV Service Department Printed in The Netherlands ©Copyright reserved 1998 Philips Consumer Electronics B.V. Eindhoven, The Nether­lands. All rights reserved. No part of this publication may be reproduced, stored in a retrieval system or transmitted, in any form or by any means, electronic, mechanical,

2 Comet

I

7

1 Technical specifications

Mains Voltage: : 220-240V ± 10% AC;

1 Technical specifications

50-60Hz (± 5%)

Mains frequency : 50 Hz

: 60 Hz
Power Consumption in stand-by : < 5 Watt
Power Consumption normal mode : 28” : 75Watt +/- 10%
Aerial input impedance TV : 75 Ohm - coax
Min. aerial input VHF: : 30mV
Min. aerial input UHF: : 40mV
Max. aerial input VHF/UHF: : 180mV
Pull-in range colour sync: : ± 300Hz
Pull-in range horizontal sync: : ± 600Hz
Pull-in range vertical sync: : ± 5Hz
Picture tube range : 25” and 28”
Sound output power : 3 Watt mono

execution

: 2 X 3 Watt stereo

execution

TV Systems: : PAL I

: PAL BG

: PALBG / SECAM DK

: PAL BG / SECAM LL'
Indications: : On Screen Display

(OSD) green/red

: 1 LED ( red high

intensity, red low
intensity,

: "RC5' and error

codes blinking red)
VCR programs: : 0
Tuning and operating system: : VST
UV913E / IEC (VST): : VHFa:46 - 102 MHz

: VHFb:138 - 224 MHz
: UHF:471 - 855 MHz

UV915E / IEC (VST): : VHFa:48 - 168 MHz

: VHFb:175 - 448 MHz
: UHF300 - 860 MHz

UV917E / IEC (VST): : VHFa:48 - 118 MHz

: VHFb:118 - 300 MHz

: UHF:470 - 861 MHz
U943 / IEC (VST): : UHF:470 - 861 MHz
Local operating functions: : MENU / - / +

MENU

-+

IR LED,

76532029_002.A

03069

2 Connection facilities

2 Connection facilities

1- Audio R (0.5Vrms ≤1kΩ) k
2- Audio R (0.5Vrms ≥10kΩ) j
3- Audio L (0.5Vrms ≤1kΩ) k
4- Audio v
5- Blue v
6- Audio L (0.5Vrms ≥10kΩ) j
7- Blue (0.7Vpp/75Ω)
8- CVBS-

status 0-1.3V:INT, 4.5-7V:EXT 16:9, 9.5-

12V:EXT 4:3 j
9- Green v
10­11- Green (0.7Vpp/75Ω)
12­13- Red v
14- v
15- Red (0.7Vpp/75Ω)
16- RGB-

status (0-0.4V:INT, 1-3V:EXT/75Ω)
17- CVBS v
18- CVBS v
19- CVBS (1Vpp/75Ω) k
20- CVBS (1Vpp/75Ω) j
21- Earth

socket

3Comet

4 Comet

3 Safety instructions, Maintenance instruction,

3 Safety instructions, Maintenance instruction, Warnings and Notes

3.1 Safety instructions for repairs

Figure 3-1

1. Safety regulations require that during a repair:
– the set should be connected to the mains via an

isolating transformer;

– safety components, indicated by the symbol (see fig.

3.1), should be replaced by components identical to
the original ones;

– when replacing the CRT, safety goggles must be worn.

2. Safety regulations require that after a repair the set must
be returned in its original condition. In particular attention
should be paid to the following points.
– As a strict precaution, we advise you to resolder the

solder joints through which the horizontal deflection
current is flowing, in particular:

• all pins of the line output transformer (LOT);

• fly-back capacitor(s);

• S-correction capacitor(s);

• line output transistor;

• pins of the connector with wires to the deflection
coil;

• other components through which the deflection
current flows.

Note: This resoldering is advised to prevent bad
connections due to metal fatigue in solder joints and is
therefore only necessary for television sets older than
2 years. The wire trees and EHT cable should be
routed correctly and fixed with the mounted cable
clamps.

– The insulation of the mains lead should be checked for

external damage.

– The mains lead strain relief should be checked for its

function in order to avoid touching the CRT, hot
components or heat sinks.

– The electrical DC resistance between the mains plug

and the secondary side should be checked (only for
sets which have a mains isolated power supply). This
check can be done as follows:

• unplug themainscordand connect a wire between
the two pins of the mains plug;

• set the mains switch to the on position (keep the
mains cord unplugged!);

• measure the resistance value between the pins of
the mains plug and the metal shielding of the tuner
or the aerial connection on the set. The reading
should be between 4.5 MW and 12 MW;

• switch off the TV and remove the wire between the
two pins of the mains plug.

– The cabinet should be checked for defects to avoid

touching of any inner parts by the customer.

– When the set is used in circumstances with higher dust,

grease or moisture levels, for example in a kitchen, the
recommended interval is 1 year.

– The maintenance inspection containsthefollowingactions:

• Execute the above mentioned 'general repair
instruction'.

• Clean the power supply and deflection circuitry on the
chassis.

• Clean thepicturetube panel andtheneck of the picture
tube.

3.3 Warnings

1. ESD
All ICs and many other semiconductors are susceptible to
electrostatic discharges (ESD). Careless handling during
repair can reduce life drastically. When repairing, make
sure that you are connected with the same potential as the
mass of the set by a wristband with resistance. Keep
components and tools also at this same potential.
– Available ESD protection equipment:
– anti-static table mat (large 1200x650x1.25mm) 4822

466 10953

– anti-static table mat (small 600x650x1.25mm) 4822

466 10958
– anti-static wristband 4822 395 10223
– connection box (3 press stud connections, 1 M ohm)

4822 320 11307
– extension cable (2 m, 2 M ohm; to connect wristband

to connection box) 4822 320 11305
– connecting cable (3 m, 2 M ohm; to connect table mat

to connection box) 4822 320 11306
– earth cable (1 M ohm; to connect any product to mat or

connection box) 4822 320 11308
– complete kit ESD3 (combining all 6 prior products -

small table mat) 4822 310 10671
– wristband tester 4822 344 13999

2. In order to prevent damage to ICs and transistors, all high­voltage flashovers must be avoided. In order to prevent
damage to the picture tube, the method shown in Fig. 3.2
should be used to discharge the picture tube. Use a high­voltageprobeandamultimeter(position DC-V). Discharge
until the meter reading is 0V (after approx. 30s).

3. Together withthedeflectionunit and any multipole unit, the
flat square picture tubes used from an integrated unit. The
deflection and the multipole units are set optimally at the
factory.Adjustmentof this unit duringrepairis therefore not
recommended.

4. Be careful during measurements in the high-voltage
section and on the picture tube.

5. Never replace modules or other components while the unit
is switched on.

6. When making settings, use plastic rather than metal tools.
This will prevent any short circuits and the danger of a
circuit becoming unstable.

7. Wear safetygogglesduringreplacementofthe picture tube

3.2 Maintenance instruction

It is recommended to have a maintenance inspection carried
out by a qualified service employee. The interval depends on
the usage conditions:

– When the set is used under normal circumstances, for

example in a living room, the recommended interval is 3 to
5 years.

3.4 Notes

1. The direct voltages and oscillograms should be measured
with regard to the tuner earth , or hot earth as this is called
(see fig. 3.3)

2. The direct voltages and oscillograms shown in the
diagrams are indicative and should be measured in the
Service Default Mode (see chapter 8) with a colour bar
signal and stereo sound (L:3 kHz, R:1 kHz unless stated
otherwise) and picture carrier at 475.25 MHz.

3 Safety instructions, Maintenance instruction,

3. Where necessary,theoscillogramsanddirectvoltages are
measured with and without aerial signal. Voltages in the
power supply section are measured both for normal
operation and in standby . These values are indicated by
means of the appropriate symbols (see fig. 3.3).

4. The picture tube PWB has printed spark gaps. Each spark
gap is connected between an electrode of the picture tube
and the Aquadag coating.

5. The semiconductors indicated in the circuit diagram and in
the parts lists are completely interchangeable per position
with the semiconductors in the unit, irrespective of the type
indication on these semiconductors.

V

CL 26532098/042

140792

Figure 3-2

5Comet

tuner earth

tuner aarde
la masse du tuner

Tuner-Erde
massa del tuner
tierra del sintonizador

with aerial signal
met antenne signaal

avec signal d'antenne

mit Antennensignal
con segnale d'antenna
con la señal de antena

normal condition

normaal bedrijf
fonctionnement normal

normaler Betrieb
funzionamento normale
funcionamiento normal

massa calda

zonder antenne signaal

Figure 3-3

hot earth

hete aarde
la terre directe
heißen Erde

tierra caliente

without aerial signal

sans signal d'antenne

.ohne Antennensignal
senza segnale d'antenna
sin la señal de antena

stand by

stand by

position de veille

in Bereitschaft
modo di attesa
posición de espera

6 Comet

4 Mechanical instructions

For the main carrier two service positions are possible:

4 Mechanical instructions

a. For faultfinding on the component side of the main carrier
b. For (de)soldering activities on the copper side of the main

carrier.

Position A can be reached by first removing the mains cord
fromit'sfixation, then loosenthecarrier lips and thenpullingthe
carrier panel for approximately 10 cm.
Position B can be reached from position A after disconnecting
the degaussing cable. A stable service position can be created
with the left hand side clip on the carrier panel and the cabinet
(see fig.4.1).

B

76532029_003

170697

5 Repair facilities

7Comet

5 Repair facilities

5.1 Functional blocks

On both the service printing on the copper and the component
side, functional blocks are indicated by lines and text.

5.1.1 Test points

The L6 chassis is equipped with test points in the service
printing on both sides of mono-board. These test points are
referring to the functional blocks as mentioned above:

• P1-P2-P3, etc:Test points for the power supply

• L1-L2-L3, etc:Test points for the line drive and line output
circuitry

• F1-F2-F3, etc:Test points for the frame drive and frame
output circuitry

• S1-S2-S3, etc:Test points for the synchronisation circuitry

• V1-V2-V3, etc:Test points for the video processing circuitry

• A1-A2-A3, etc:Test points for the audio processing circuitry

• C1-C2-C3, etc:Test points for the control circuitry

• T1-T2-T3, etc:Test points for the teletext processing
circuitry

The numbering is done in a for diagnostics logical sequence;
always start diagnosing within a functional block, in the
sequence of the relevant test points, for that functional block.

5.1.2 Service default-alignment mode (SDAM)

Service default-alignment menu: new option settings are
activated immediately.

1. Software version of the microprocessor used in that typical
set is displayed in the right top corner

2. A counter in the middle of the screen indicate the normal
operation hours of the set in a hexadecimal code (every
time the set is switched "on" the counter is incremented by
1 hour, so +1 at the counter).

3. The "S" in the middle of the screen next to the counter
indicate that the set is in the service default-alignment
mode

4. Option codeThis code indicates the Options setting of the
set.

5. Error code history;

The 5 last different error codes occurred are stored in the
EEPROM memory; last error code detected will be displayed
on the left side (see for an overview of all possible error codes
Fig. 8.x), so e.g.:

0 0 0 0 0 means no error codes present in the buffer

30000meansoneerrorcode present in the buffer; error code
3

3 2 0 0 0 means two error codes present in the buffer; last
detected error code is error code 3, previous detected error
code is error code 2

The service default-alignment mode is a pre-defined mode
which can be used for faultfinding (especially when the TV
gives no picture at all). All oscillograms and DC voltages in this
service manual are measured in the service default-alignment
mode.

Activatingtheservicedefault-alignmentmodecanbe done in 2
ways:

1. By short-circuiting the service pins S1 and S2 of the
microcomputer (pin 14 of IC7600).

2. From normal operation mode by pressing the button
"DEFAULT" or "ALIGN" on the DST (Dealer Service Tool)
RC7150.

Leaving the service default-alignment mode to normal
operation can only be done by the stand-by on the remote
control or by pressing diagnose 99 followed by the OK-button
on the DST (so not via mains switch "off"; after mains switch
"off" and then "on" again the set will start up in the service
default-alignment mode again to enable easy faultfinding).

Functions of the service default-alignment mode:

1. All analogue settings (volume, contrast, brightness and
saturation) are in the mid position.

2. Set is tuned to program number 1

3. Delta volume settings are not used (delta volume setting =
a delta on the volume setting)

4. OSD error message (present available error code) is
displayed continuously

5. The OSD-key will act as search and auto store on the
maximum program number.

6. Automatic switch off function (set switches "off" after 15
minutes no IDENT) will be switched off

7. Hotel mode will be disabled

8. All other functions remain normal controllable

The errorcodehistorybufferisclearedwhenthe Service Menu
is left by the stand-by command or by diagnose 99 command.
In case the Service Menu is left by the mains switch "off" the
error code history buffer will not be cleared.

Optioncode +Counter +"S" for
service menu active +
software version->

Error code history --> 23000
Option setting row --> - SYSTE

Option setting:

In the bottom line the options are given.

Control of the options is with the following keys on the remote
control:

• PROGRAM +/-: Select the option to be changed; Via the
"PROGRAM +/-" button the option to be changed can be
selected.Theselectedoptionis implemented immediately.

• CONTROL up/down: Changes the setting of the option.

• MENU +/-: Changes toasubmenu;via"MENU +/-" buttons
a submenu is selected in which in a stereo version the
sound/sync alignment can be done.

The options are stored immediately in the EEPROM.

The following table indicates the possible hardware and
software options and their technical consequences:

001 0023S 1.0

M BG + I

+

Text displayed in the option row in the service menu The technical consequence for the selected option
SINGLE --> For a PAL BG only or PAL BG/SECAM BGDK set

8 Comet

SYSTEM I:UK --> For a PAL I only set
SYSTEM BG+LL' -->For a PAL BG/SECAM LL' set
SYSTEM BG+DK --> For a PAL BGI/SECAM LL' set
NATIONAL BRAND MAxxxxx --> Selects MENU-Layout National Brand styling

5 Repair facilities

5.1.3 Error messages

The microcomputer also detects errors in circuits connected to
the I2C (Inter IC) bus. These error messages are
communicated via OSD (On Screen Display) and a flashing
LED in the service default-alignment mode. (error code history
buffer):

"OSD error number"
(servicemenu)

0 No blinking LED No error -­1 LED blinks ones mC error IC7600
2 LED blinks twice General I2C error I2C bus is blocked
3 LED blinks three times EEPROM error IC7605

"LED behaviour" Error description Possible defective component

1. In normal operation; in normal operation no errors are
indicated.

2. In the service default-alignment mode; in the service
default-alignmentmode both the"OSDerror message"and
the "LED error" indication will display the present detected
error continuously.

8 Electrical adjustments

8 Electrical adjustments

8.1 Adjustments on the 110 module panel

1. Horizontal amplitude: Is adjusted with potentiometer
R3924

2. Vertical centring: Is adjusted with potentiometer R3921

3. Picture height: Is adjusted with potentiometer R3903

4. East-west correction: Is adjusted by potentiometer R3913

5. Horizontal centring (main pcb):Is adjusted with
potentiometer R3129 on the main PCB

6. Focusing: Is adjusted with the focusing potentiometer in
the line output transformer

7. AFC
– Adjustment of the AFC and picture demodulator (all

versions).

– Select a non secam L/L' system in the SDAM mode

(negativemodulation).Switch the tuner toHIGHBAND
(pin 11 of tuner 1100 grounded). Connect a pattern
generator to pin17ofthetunerviaacapacitorof4.7nF
andputa82Wresistor from the output of the generator
toground.Connecta DC voltmeter to pin 44 ofIC7100.

Adjust coil 5100 to get 3V5 on pin 44 of IC7100.
– The signal of the generator has to be 38.9 MHz.
– Adjustment of the AFC and picture demodulator.

(BAND 1 L. France versions only).
– Same story as a) only the frequency of the generator

has to be 33.9Mhz with positive modulation.

23Comet

8. RF AGC

9. If the picture of a strong local transmitter is reproduced
distorted, adjust potentiometer R3130 until the picture is
undistorted.
or: Connectapatterngenerator(e.g.PM5518)to the aerial
input with RF signal amplitude = 1mV. Connect a
multimeter (DC) at pin 5 of the tuner. Adjust R3130 so that
voltage at pin 5 of the tuner is 8V5 +/- 0V5 DC.

8.2 Adjustments on the CRT panel

VG2 cut-off points of picture tube

Apply a black CVBS signal at the input pin 20 of scart. Adjust
thebrightnessin order to have 1.6V duringtheline at the R,G,B
outputs of the BIMOS pin 18,19,20 of IC7100. Put
potentiometers R3326, R3316 and R3306 to the minimum
value (maximum voltage on the CRT cathodes). Adjust now
VG2 till the colour that luminates first is not visible anymore.
Adjust now the other two potentiometers in such a way that
they just don't luminate.

Potentiometer R3308 should always be in the mid-position.

24 Comet

9 CIRCUIT DESCRIPTION

9 CIRCUIT DESCRIPTION

9.1 Description of the power supply:

9.1.1 General

Note: The voltages +96S and +96s are not mains isolated.

Thepower-supplyused in this chassis is aself-oscillatingdown
converter with an auxiliary winding to help the FET to switch.

9.1.2 Principle of the down-converter (Fig 8.1):

WhenswitchTS7504 is closed,thevoltage on L2-3 isVin-Vout.
During this time, energy is stored in the coil and energy is
delivered to the load. When switch TS7504 opens, the energy
stored in the coil will be stored in the output capacitor (C2515).
This is due to the fact that the current through the coil has to
decrease linear. When the switch is open the current is floating
through D6504, L2-3 and C2515. By controlling the duty-cycle
of the switch, the output voltage can be regulated.

9.1.3 Start-up (see diagram A1):

When the switch TS7504 is closed, the input voltage is placed
over winding 2-3 of transformer 5500, which acts as coil L2-3
in Fig 8.1. Via resistors R3513,R3518 and R3512 the switch is
turned on for the first time. Zener diode D6502 prevents that
the Ugs of the FET becomes higher than 15V. When the input
voltage is on winding 2-3, there is also a voltage on winding 1-

2. Via winding 1-2 the correct switching voltage is obtained.
The DC-part of this voltage is blocked by capacitor C2503.
DiodeD6510actsasa protection in start-up and in short-circuit
situations.Duringstart-up the outputcapacitorC2515 is empty.
It takes a relative long time to charge the gate to a voltage high
enough to switch on the FET. This is due to the fact the diode
D6510 is conducting. When this diode is conducting, the
currentthatwould normally flow into the gateofthefet to switch
on the FET, is now flowing into C2515. In this way a smooth
start-up is guaranteed.

9.1.4 General way of working (Fig 8.2):

The state of the power-supply can be divided into three areas:

– T-on;In this state the FET is conducting and energy is

stored in the coil and in the output capacitor.

– T-off;In this state the fet is non conducting and the energy

stored in the coil is fed to the output capacitor.

– T-dead;Fet is out of conduction and there is no energy in

the coil.

T-on; In the T-on state, switch TS7540 is switched on. When
the switch is on the voltage over resistors R3514-R3515 is a
direct measure for the current through winding 2-3. This is a
negative voltage. When this voltage becomes below a certain
level, TS7501 starts conducting and will switch off the fet. In
this way it is prevented that the coil can go into saturation. This
could be the case when the output voltage is very low. (long on
time of the FET). When the output-voltage becomes too high
during T-on the FET will be switched off. (see Output-voltage
regulation)
T-off; Due to the stored energy a current will start to flow
throughD6504,C2515 and winding 2-3. Duetothefact that the
current is flowing through this circuit, a voltage with reverse
polarity is on winding 1-2. In this way the fet remains off until
thecurrentthroughwinding2-3 reaches zero. Now a new cycle
will start. The fet will be switched on and all starts over again.
T-dead; If the output voltage is too high (for example in a low
load situation) the FET remains off till the output-voltage is not
to high anymore.

9.1.5 Output voltage regulation:

This is done by the circuit D6501, R3509, TS7502, R3505,
R3507, R3510. Transistor TS7502 can only conduct when the
voltage on the base is 0V7 lower than the voltage the voltage
on the emitter. This means that the voltage drop over resistors
R3505 and R3507 should be 5V6(zenerdiode) + 0V7(base­emitter). This is reached when the output voltage exceeds the
100V. Now transistor TS7502 starts conducting, which brings
transistor TS7501 in conduction. As a consequence the gate
voltage of the fet becomes very low and the fet stops
conducting. As long as the output voltage is too high the fet
stays out of conduction.

9.2 Protections:

9.2.1 Overvoltage protection:

A disadvantage of a down converter is that if the switch
becomes a short-circuit, the output voltage will increase to the
input voltage. This could damage circuits. In this power-supply
there is a protection to prevent this. If the output voltage
becomes higher than 130V, zenerdiode D6514 starts to
conduct.TheVin will be shortcircuited.This will blown themain
fuse 1501 and protect in this way all the other circuits.

9.2.2 Short-circuit and start-up protection:

The short-circuit protection works the same as the start-up
protection. If the output-voltage is very low in case of a start-up
orashort-circuit condition, The gate willbecharged very slowly
due to the fact that zenerdiode D6510 is conducting. So the
current is not only charging the gate but is also flowing into the
output capacitor. In this way it takes a few milliseconds to
switch on the fet. Diode D6510 takes also care that the fet
never remains in his power consuming (linear) area.

9.2.3 Undervoltage protection

Iftheoutputvoltage is very low, it also takes a largetimebefore
the current through winding 2-3 reaches zero. The power
supplied to the circuit is in this way very low and protects in this
way the circuit.

9.2.4 Other output voltages:

The output voltages +8S, +9S and +5S are made by winding 5-

6.Duringthetime that the fet TS7504 is notconducting,energy
is transformed to this winding (flyback principle) and the
voltages mentioned above are created. From the +9S, the +5S
voltage is derived. This voltage is stabilized by transistors
TS7505, TS7500 and zenerdiode D6500. D6500 is the
reference voltage and TS7505 is delivering the current. When
zenerdiode D6500 starts conducting, the voltage over resistor
R3502 becomes high and a POR signal is created.

9.3 Degaussing:

R3516 is a dual PTC (2 PTC's in one housing). After switching
"on" the set, the PTC is cold, so low ohmic. This makes the
degaussing current high. After degaussing the PTC is heated,
so high ohmic. This makes the degaussing current low. After
degaussing the PTC remains heated by the mains.

9.4 Line-circuit (Diagram A1):

The primary side of the line-circuit and the deflection coil are
connected to the hot earth. The driver-circuit contains an opto­couplertocreate isolation between the low-signal partsandthe

9 CIRCUIT DESCRIPTION

25Comet

mains. The optocoupler is driven by pin 37 of IC7100-6E via
transistorTS7103.When TS7103 isnotconducting,(the LED of
the optocoupler is also out of conduction) TS7421 is also not
conducting. In this way TS7422 will conduct and the 96V is
placed over winding 2-1 of the LOT. A voltage over winding 2­1 of the LOT will cause a voltage over the windings 8-10, 6-10
and 9-10. Now energy will be transformed from the primary to
the secondary-side and charge capacitors C2424 and C2425.

C2430 will be charged to the difference of the +40D and +14D
(=26V) when TS7422 is conducting. When TS7422 stops
conducting, the voltage of pin 8 of the LOT will become very
negative. This forces C2430 to be charged to 26V plus the
absolute value of pin 8. When TS7422 starts conducting again
the voltage of pin 8 of the LOT will increase and so the voltage
on the anode of D6422. In this way the 160V is created. This
means that during the off-time of TS7422, C2430 is charged
andduringthe on-time of TS7422, theenergyinC2430 is given
to C2426. When transistor TS7103 conducts, the LED of the
opto-coupler will be activated. This causes the transistor of the
opto-coupler to conduct, which drives TS7421 in conduction.
ThisbringsTS7422outofconduction.Due to this construction,
this circuit is protected against missing line-drive pulses. When
a line-drive pulse is missed, the line-transistor stays out of
conduction, due to the fact that the diode of the opto-coupler is
forced into conduction by TS7103. In this way nothing can be
damaged when there is no line-drive.

Winding 4-3 is an extra winding to help TS7422 to switch.

On the secondary-side of the LOT there is a circuit consisting
of TS7423, R3422, R3433, R3434, C2431 and C2432. This
circuit creates a pulse when TS7422 switches off. This pulse
indicatesthathorizontal flyback takes place. Thisinformationis
fed to IC7100-6E to blank the picture.

9.5 Stand-by:

Thestandbysignal from the mC is lowincaseof stand-by. Now
TS7103 is brought into conduction by R3100. As mentioned
before this will switch off the line-output stage completely.

9.6 Deflection :

9.6.1 Horizontal deflection:

The voltage over capacitor C2422 is the same as the voltage
over C2515 (96V). When TS7422 is conducting this voltage is
placed over the horizontal deflection coil. This causes a linear
increasing current through this coil. In this way deflection is
created. When TS7422 switches of flyback takes place and it
starts all over again. L5424 is used for linearity correction.

9.6.2 Vertical deflection:

Vertical deflection is based on a balance amplifier. Or TS7401
or TS7402 is conducting. This depends on the signal V-drive. If
V-drive is high TS7401 conducts and the voltage of C2401 is
placedoverthe deflection coil. Now thepictureis written. When
V-drive is low, TS7402 conducts and the +40V supply voltage
minusthevoltage over C2401 is placed over thedeflectioncoil.
Flyback takes now place. In this way deflection is generated.

R3407 is used to adjust the vertical shift. With this resistor the
level of the signal VFB is adjusted. R3402 and C2404 are used
to damp oscillation of the deflection coil with his parasitic
capacitance. The signal NIL from the mC is used to create a
non-interlaced mode. This is done by creating a small DC
current through the deflection coil.

9.7 110 degrees deflection module

For the 25" and 28" sets a 110 degrees module is needed for
East/West correction. This panel is allocated on the right hand
side of the mainboard (seen from the rear). East/west
correction in this module is based on the diode-modulator
principle; the current through the horizontal deflection coil is
modulated. As this is done by a parabolic-shaped voltage, E/W
distortion is corrected. This parabolic-shaped voltage is
derived from a saw-tooth-shaped voltage of the frame
deflection.

9.7.1 Frame (time base frame)

Because the raster part is fed by the primary side a galvanic
isolator must be applied between IC 7100 (= so called Bimos
ic) in the secondary side and the raster amplifier on primary
side. This is realised by opto coupler (7422); this opto coupler
will be switched and it will block the saw-tooth of the Bimos ic.
So we don't use the saw-tooth of the Bimos((pin 42) or the
feedback frame input(pin 41). The only information from the
Bimosic(=IC7100) is the flyback command(pin43).The output
of this pin is a pulse of 6 to 0 Volts during 1 mS with a period of
20mS. This signal blocks transistor 7424 and this causes
conduction of the opto coupler diode (7422). The internal
transistor also conduct and pins 11 and 12 (connector 00820)
of the 110 module will be short circuited.

9.7.2 Raster part

A saw-tooth must be created because we don't use it from the
Bimos ic(see annex 5). Via 150V C2901 will be charged via
R3901, R3900 and D6900; the function of D6901 is to
determine the lower part of the potential level. After 20mS a
signalcomingfromthe Bimos ic will short-circuit pins 11and12
of connector 0082 and C2901 will be discharged. It is a must to
have an amplitude on the screen independent of the 50Hz or
60Hz frequency of the mains; see circuit diagram annex 6. The
emitter voltage of T7900 can be adjusted with potentiometer
3903; this is the top Voltage of the saw-tooth. This is the circuit
for adjusting the vertical amplitude independent of the 50/60Hz
frequency. The saw-tooth will control T7901 and this transistor
controls the amplifier (= T7902, T7903 and T7904).D6902,
D6903, T7905 and C2904 determines the flyback. This flyback
pulse is negative and is created by an inverted polarity of
C2904.During the deflection T7905 is blocked and C2904
charges; during the flyback T7905 conducts and the flyback
pulse will be made.

9.7.3 East-West modulator

The parabola is taken on C2907; R3916 and D6905
determines the shape of the parabola and they corrects the
upper and lower parts. The parabola is fed via C2908 to
potentiometer 3913; this for adjusting the pin-cushion
correction. Via T7904 this signal goes to MosFet 7908; the Vgs
command has two functions by changing the Voltage of Vgs by
potentiometer 3924: pin-cushion correction and horizontal
amplitude adjustment.

Special components:

• D6904 + R3916: temperature compensation of Vbe
(T7904)

• R3935: trapezium correction

• C2909: to avoid external radiation

• C2918: to avoid "twisted or broken" lines

26 Comet

9 CIRCUIT DESCRIPTION

9.7.4 Line timebase

The control voltage of pin 37 of the Bimos ic (=ic7100) is
derived via opto coupler pos 7420 to transistor T7421; then
send via C2428 and C2421 to pins 5 and6 of connector 0082;
this is the control of the base of T7906 (=BU1508AX). At the
flyback diode between collector and mass there are two parts
present to allow the East/west modulation. One part of this
modulator consists of D6906, T7908, C2910 and C2911. The
second part another diode is not visible in the circuit diagram
but it is present in the MosFet 7908.

On pins 1 and 2 of the module the primary side of the LOT is
connected. The LOT supplies the following voltages:

• 3-5 : 26 Volts after smoothing

• 10-8: 14 Volts

• 9 : 160 volts for video amplifiers.

9.8 BIMOS IC: PAL I Version

9.8.1 Introduction

The TDA 8361 is a single chip video and audio processor and
it incorporates a built in IF-detector, Luminance and
Chrominance separator, PAL/NTSC Chroma Decoder, RGB
processing, Horizontal and Vertical Oscillators, Sync
Separators and the FM Sound demodulation circuit.

9.8.2 Bimos Start Up

The Bimos device (Line Oscillator) starts up via pin 36,
however it will only start up when the voltage on this pin has
reached 5.6 Volts. At 5.6 Volts the output frequency will be
about 25kHz. The supply voltage at this pin (Pin 36) is
produced by the Switched Mode Power Supply and not by the
Line Out Put Stage. Once the supply voltage on Pin 10 has
(Line Output Stage) reached the 8-volt threshold the output
frequency will switch to 15.625 kHz. The voltage on this pin is
prevented from exeading 8 volts by zener diode 6106 (Page

10)

9.9 Vertical Synchronisation and Frame Amplifier

9.9.1 The Vertical Sync Separator

This functional block separates the Frame Synchronisation
pulses from the CVBS signal, once these pulses have been
separated the pulses are then used to synchronise the Frame
Oscillator.

9.9.2 Amplitude

The voltage on pin 42 of the BIMOS device determines the
amplitude of the frame sawtooth.

9.9.3 Frame Pre Amplifier

This internal amplifier increases the amplitude of the Frame
sawtooth, from here the frame sawtooth is available at pin 43.

9.9.4 BCI Input Pin

This Input is fed into the BIMOS device form the Tube Base
Board, It is used to provide frame correction for changes in the
EHT voltage. For example if the EHT decreases (more white)
the picture will get larger. This results in the BCI decreasing
and the picture size being automatically corrected by the
BIMOS.

9.9.5 The IF Demodulator

The IF bandpass characteristic is determined by SAW filter

1001. The IF signal is then fed from here into BIMOS pins 45
and 46. Pin 1 of the IC is used for the selection of positive or
negative video modulation. It is high for positive modulation
and low for negative.

9.9.6 AGC

Pin 47 is used for AGC (AGC is used to adjust the gain of the
Tuner so that over modulation is avoided).

9.8.3 Horizontal Sync Separator

Thisfunctionalblockseparatesthehorizontal sync pulses from
the CVBS and then locks the pulses to the free running
horizontal sawtooh generator. However both the vertical and
horizontal oscillators are also internally locked to the 4.43 MHz
Chroma reference Crystal item 1100.

• The Horizontal Oscillator Saw Tooth Generator The
sawtoothisconverted to a squarewaveformwith a variable
duty cycle. This square waveform is then fed to the Line O/
P stage via a Galvanic Isolator. The correct line time
constant is automatically determined, internally by the
BIMOS IC.

• Pin 38 This is pin has two purposes, these are.

1. The Sandcastle Pulse O/P

2. Horizontal Flyback Pulse Input

1. The Sandcastle Pulse has an output current of a few

micro amps; the amplitudes of the sandcastle pulse
components are: Burst 5V3; Line Blanking 3V and
Frame Blanking 2V.

2. TheHORFLYBACK input has a currentof100-300 uA.

The Horizontal flyback pulse input, allows the phase of
the flyback pulse to be compared internally with the
phase of the horizontal oscillator; if the phase is not
correct, the horizontal oscillators duty cycle will be
adjusted accordingly.

9.9.7 AGC Threshold Adjustment

Variable resistor R3130 which is connected to pin 49 adjusts
the AGC Threshold

9.9.8 Automatic Frequency Control/ Pin 44

The AFC control signal is available at pin 44 of the BIMOS
device. It is obtained from the internal IF reference signal.
C2100 is used to smooth the AFC voltage.

9.9.9 IDENT Signal/ Pin 4

The IDENT signal on pin 4 is "High" when horizontal sync is
detected in the video signal and low when no sync is detected.
The IDENT line is connected to the main Microprocessor.
(Pin15)

9.9.10 What does the IDENT line do ?

It is used for tuning control (for switching from fast to the slow
tuning mode) Pin 4 (BIMOS) is also used by the
microprocessor (Pin15) for "No Signal Standby" after 15
minutes. Used to provide a stable OSD - Time constant
switched by the BIMOS.