Sanyo VB8B Crt

8/10/2019 Sanyo VB8B Crt

1/14

-1-

Training Manual VB8B Chassis

COLOR TELEVISIONTRAINING MANUAL

CIRCUIT DESCRIPTION

> Scan Velocity Modulation Circuit (SVM)

> Right and Left Unbalanced Pincushion Correction Circuit

> Inner Linearity Compensation by Dynamic S-Correction

> Dynamic Focus Circuit

Chassis Series VB8B

FILE NO.

REFERENCE NO. TI780011

Note:The VB8B chassis series is used for flat screen televisions which includesmodels DS27910 and DS32920 and is similar to the VB7C chassis.This manual contains only the different circuit descriptions from the VB7Cchassis. For the complete circuit descriptions refer to the VB7C trainingmanual reference number TI780010.


2/14

-2-


Table of Contents

1. Chassis Summary

2. Scan Velocity Modulation Circuit (SVM)

3. Deflection Circuit

3-1. Outline

3-2. Pincushion Correction

3-3. Right and Left Unbalanced Pincushion Correction Circuit

3-4. Inner Linearity Compensation by Dynamic S-Correction

4. Dynamic Focus Circuit


3/14

-3-

Training Manual VB8B ChassisPart 1 Chassis Description

1. Chassis SummaryFig. 1-1 shows a fundamental block diagram of the VB8B chassis.

FORCONTINUEDPROTECTIONAGAINST

A

RISKOF

FIRE,REPLACE

ONLY

WITH

THESAMETYPE

4A,125VFUSE.

ATTENTION:POURMAINTENIRLAPROTECTION

CONTRELESRISQUESD'INCENDIEUTILISERUN

FUSIBLEDERECHANGEDEMEMETYPE4A,125V.

CAU

TION

4A125V

BLOCKDIAGRAM

17

42

41

40

39

25

293

27

OSD(R)

OSD(G)

OSD(B)

OSD(BLK)

RESET

AFTS-CURVE

POWERFAIL

POWER

ON/OFF

STANDBYMODE

ALWAYS+5V

CONTROL

Q693,

Q695

Q635

OVERLOAD

PROTECT.

D612

PHOTO

COUPLER

IC601

POWER

REG.

Q681

12VSW

DRIVE

IC681

5VREG.

RESET

D601

FULLWAVE

RECTIFIER

Q605

ERR.

AMP.

Q604

OSC

Q601

SW. T

601

CONVERTER

TRANS.

LF601

LINE

FILTER

PS601

POSISTOR

RL601

RELAY

L901

DEGAUSING

COIL

AC120V

60Hz

F601

AC

FUSE

C601

+12V

D627

+16V

+135V

D621

D694

D624

D625

4A125V

+12V

Q627

12VSW.

Y

12

13

14

15

8

OSD

SW.

RGB

OUT

CONT

BRITE

COLOR

DEMOD

SHARP

BPA

BLACK

STRETCH

DELAY

LINE S

YNC

SEP.

HORIZ.

DRIVER

VIDEO

DET.

FM

DET.

BUS

INTERFACE

IC101

SIGNALPROCESS

SDA

9

SCL10

78/79

11

Y

52

C

54

44

49

Q135

BUFF

ER

H V

BEAM

VOLTAGE

DETECT.

ABL-IN

MUTE

INSELECT

OUTSELECT

S1-SW

S2-SW

PIPTV/AV

PIPAV1/2

MAINAV1/2

FIX/VAR

RFAGC

77

ACL

X141

SAW

FILTER

INT.VIDEO

SIF

A101

UHF/VHFTUNER

WITHPLL

&

BANDSW

IFAGC

IC802

EEPROM

65

A1901

RCPRE-AMP

SW1901-1906

CONTROLKEYS

RFAGC

BUSSCL

BUSSDA

IICSCL

IICSDA

RCIN

KEYIN

XIN

XOUT

28

34

32

33

31

10

919

13

20

11

7

SC

3

2

12

EXT. VIDEO

INT. VIDEO

SCLOCK3.58

1614

15

9

1

30

38

2

1IC801

CPU

WITH

CAPTION

DECODER

X801

ANT

C809

C808

12

357654

IC501

VERT.

OUTPUT

SIDEPCCAMP.

Q461

Q462

INNERPIN

Q471

Q462

Q451

RLBALANCE

Q401

HORIZ.

DRIVE

T401

H-DRIVE

TRANS.

Q402

HORIZ.

OUTPUT

HOLDDOWN

DETECT.

30

40

17

18

19

HOLDDOWNV

ELOCITYMOD

T402

HORIZ.

OUTPUT

TRANS.

H.V.

AFC

DEFLECTION

YOKE

ABL

FOCUS

SCREEN

HEATER

V+B

LOW+B

56/7

11

938

1

VH

2

H-PULSE

ABL

Q721

Q711

Q701

-ROUT

-GOUT

-BOUT

RGB

Q900

PIXTUBE

D483

+26V

(7V)

D429

D428

(16V)

R428

g7yam

ANODELEAK

DETECT.

L164

X161

75

56

36

V-S

37

H-S

33

26

4.5MHz

SOUND

TRAP

IC001

AUDIO

AMP.

1085

5

34

15

2

1

13

(L)

(R)

F/VOUT

(R)

F/VOUT

(L)

SP902

SPEAKER

(R)

SP901

SPEAKER

(L)

MUTE

Q001

4339

38

AV 2 R

AV 1 L

AV 1 R

AV 2 L

33

37

36

34

13

IC3401

MTSDECODER

A/V1IN

VIDEOIN

AUDIOIN(L)

AUDIOIN(R)

D482

VIDEO

5

LR

Y

Y1

IN

C1

IN

S1

SW

C

S1

INAV1

A/V2IN

VIDEOIN

AUDIOIN(L)

AUDIOIN(R)

EXTVIDEO

MAINAV1/2

S1/SW

11

14

9

4

10

15

IC1002

SELECT

AV1/2

Y

13

12

5

3

2

1

13

V2

IN

12

Y2

IN

5

C1

IN

3

C1

IN

2

Y1

IN

1

V1

IN

HHH LLL

H

H

H

L

L

L

S2-SW11

V2/Y2

14

MAINAV1/2

9

C1/C2

4

S1-SW10

V1/Y1

15

V2/Y2

V1/Y1

IC1001

SELECT

V/Y

+C1/C2

F/V

SELECT

56

SDL

SCL

SIDE

PCC

Q202

BUFFER

Y

Y2

IN

C2

IN

S2

SW

C

S2

INAV2

910

Q1201IC

1201

Q1202

Q1212

K1001

K1051

Q1211

F/V

R

F/V

L

Q486

9VREG.

Q498

5VREG.

5V

ALWAYS

B9

12V

B4

9V

9

-B

32

5V

B9

H-S

V-S

Q658

RELAY

DRIVER

5V

B6

D801

Q208

Q1700

Q1703

VIDEO

OUT

Q1701

Q1702

Q1705

COMB

FILTER

VM

VIDEO

SW.

C1/C2

C

Y

C1/C2

F BLK

HPULSE

Q1706,Q1707,Q1708

Q1709,Q1711,Q1712

Q722

Q712

Q702

Q831

RESET

-B

T8801

D445

Fig. 1-1


4/14

-4-


2. Scan Velocity Modulation Circuit (SVM)

2-1 Outline

The SVM (Scan Velocity Modulation) circuit improves the picture resolution by changing the electron

beam scan velocity to emphasize the brightness change of the contour of the picture. Increasing the scan

velocity of the electron beam makes the activation period of the fluorescent face shorter, lowering bright-ness. Decreasing scan velocity, increases the period of activation and raises brightness. Because this sys-

tem does not change the volume of the beam current itself, it does not degrade the focus quality.

Fig. 2-1 shows the SVM circuitry of the VB8B chassis, Fig. 2-2 shows the functional waveforms respec-

tively, and Fig. 2-3 shows the SVM block diagram.

2-2 Operation

At the SVM circuitry, the input video signal with positive polarity (a) is amplified and inverted by

Q1701. After passing through Q1702, the signal is differentiated by R1711 and L1701 and becomes (b).

This 1st differential signal passes through Q1705 and is further shaped by C1708, C1709, L1702, and

R1717, and amplified by Q1706. The result is the current waveform (c) which flows in the SVM coil.Q1707 and Q1708 compose a complementary-amplification circuit connected directly at the base. The

base clipping provided by Q1707 and Q1708 VBE eliminates the fundamental noise generated as a result

of primary differentiation and improves the signal-to-noise ratio.

Q1709 and Q1711 compose a class B push-pull current-amplification circuit. The voltage at the con-

nection points of the two output transistor collectors is balanced by half of the 130V source voltage.

Direct feedback is applied to the two output transistors by R1736. In operation, when the negative direc-

tion modulation voltage is applied to the output circuitry, Q1709 conducts and current flows into the

SVM coil charging C1722. At this time the deflection speed is increased and the brilliance decreases.

Next, when the positive direction modulation voltage is applied to the output circuitry, Q1711 conducts

and current flows through the SVM coil discharging C1722. At this time, the deflection speed isdecreased and the brilliance increases.

The current is 2nd differentiated by the SVM coil and C1722. The SVM coil consists of two coils assem-

bled in the DY/CPM (Convergence Purity Magnet), which is located on the CRT neck and positioned

over the electron beam gun. One coil is positioned on the upper side of the gun and the other one is at

the lower side of the gun. The current through the SVM coil generates a magnetic field at right angles

with the electron beam and is added to the electromagnetic field produced by the horizontal deflection

yoke. Thus the deflection velocity of the electron beam is modulated by the SVM magnetic field result-

ing in a change of brightness. The total horizontal deflection magnetic field applied to the electron beam

results in waveform (d). The horizontal electron beam velocity changes as shown in (e) and the result-

ant brightness change is shown in (f).When OSD (On Screen Display) characters are displayed, including captions, the blanking pulse is added

to transister Q1701 by Q1703 stopping the SVM so the characters will not be distorted.

Note:

This SVM circuitry is designed to be most effective at a frequency range of 3MHz to 4 MHz of the video

signal where the resolution of the picture is most visible.

Part 2 Circuit Description


5/14

-5-


2

1

3

1

V/M COIL

L1703/1704/1705

C1740

R1740

C1714

C1716

C1713

L1706 R1704

R1716

C1749

D1707

D1705

Q1711

Q1709

C1726

C1724

D1708

Q1712

R1720

R1744

R1723

C1711

R1722

C1748

R1734

C1715

C1717

L1703

L1705

L1704

Q1707

Q1706

Q1708

R1728

R1724

R1736R1742

R1729

R1731

C1705

C1721 C1719

R1730

R1727

R1733

R1732

R1737

R1741

R1735

R1738

C1722

C1712

R1725

K17H

K17C

EM1

1/2DJ2.7B

KK820

KK820

KK680

L2B95R6KN:L2E15R6KN

DJ100

J

35EM100

SB07-03N

SB07-03N

2SC4793LB

2SA1837LB

500CK270GAA

KK270

R

AE

1.8K

1.2K

33K

FK0.01(D:BF)

120K

KZ0.01F

100K

200FK4700(H:B)

FK4700(D:BF)

ZZ0122:B4Z21B0080N

AE

AE

AC

DJ47B

39K

12K2SJ150

J

J

16EM100

16EM47

160EM10BE

DJ1K

DJ10B

100K

1/2DJ1.2K

1/2DJ39B

1/2DJ2.2B

1/2DJ1.2K

1/2DJ39B

160EM22BE

FJ0.012(D:BF)

100

J10EA030N:J10KR030N

130V

9V

VCC

IC101

LA76834NMP

R1753

C1707

R1709

R1700

Q1700

C1704

R1718

R1714

Q1703

R1708

C1709

R1705

R1717

L1702

C1708R1719

C1706R1710 R1712

L1701

R1711

R1707

R1702

Q1702

R1706

Q1701

R1703

Q1705

D1701

C1701

100

KK4700LZB

J

1/10GJ4.7KC

AC

CJ270CL

1/10GJ1.2KC

1/10GJ820C

2SC1740SS:2SC945AKA:2SC1815GR:2SC536G-NP

1/10GJ3.3KC

CJ56CL

1/10GJ560C

1/10GJ

470C

L2B9330KN:L2E1330KN

CJ120CL 1/10GJ

560C

25EM100220 1/10GJ1.8KC

L2B9100KN:L2E1100KN

1/10GJ1KC

1/10GJ3.3KC

1/10GJ22KC

AE

1/10GJ1.8KC

AC

1/10GJ68KC

AE

TVR1B

FK0.015(D:BF)

R208

Q208 1/10GJ1KC

AC

49

L256 C257C258

J KK0.01

LZB16EM47

5315

5VB6

VideoSignalAmplifier

1st Differentiate& Amplifier

Amplifier

Q1701 Q1702,Q1705,R1711,L1701

C1708,C1709,L1702,R1717

Q1706 Q1707,

Q1708

NegativeFeedback

Q1712

Shaping 2nd Differentiate& Output

Q1709,Q1711

Buffer

Q1700

Electron Gun

SVM Coil

Current

Neck of CRT

SVMMute

Q1703

VideoSignal

FastBlankingPulse

R G B

Video input signal

Differentiated waveform

Current waveformin SVM coil

Horizontal deflectionmagnetic field

Velocity ofhorizontal deflection

The brightness changeof fluorescent substanceon the CRT

+ +

- -

(a)

(b)

(c)

(d)

(e)

(f)

time

amplitude

amplitude

current

magneticfield

velocity

luminance

+

-

+

Fig. 2-1

Fig. 2-3

Fig. 2-2


6/14

-6-

3-1 OutlineFig. 3-1 shows the block diagram of the deflection circuit of VB8B Chassis.

The horizontal drive pulse is output from pin 30 of IC101 (signal processor) and input to the horizontal

output stage. The vertical sawtooth signal is output from pin 26 of IC101 and input to pin 6 of IC501.

The vertical output signal is output from pin 3 of IC501.

The signal processor (IC101) is controlled and compensated for V-size, S-correction, V-linearity, Pin

distortion correction, H-size and H-shift by the CPU via the I2Cbus.

Pincushion correction (PCC) is accomplished by adding the east-west parabola pulse output from pin 25

to the horizontal output circuit.

Inner pincushion correction (Inner PCC) and right & left pincushion unbalance correction are accom-

plished by using FETs in parallel with C417, the S-correction capacitor.Dynamic focus is also accomplished using T8801 in parallel with the S-correction capacitor C417.

3-2 Pincushion Correction

In the PCC correction circuit, consisting of Q461 and Q462, the east-west parabola signal supplied from

pin 25 of IC101 is added at the horizontal output stage and pincushion correction is performed by vary-

ing the DC bias.

Further, the ABL and ACL voltage is fed back to the base of Q461 to compensate for the width change

by the beam current.

H-Drive

Q401T401

Drive Trans

Q411H-Output

IC501V-Output

6 3

V-Drive

25

30

26

EW-Out

Q461 Q462

C417

C477

Q471Q451

Vert.Coil

Horiz.Coil

DeflectionYoke

T402FlyBack Trans.

L414

L416

PincushionCorrection

IC101IVCD

L413

S-correctionCapacitor

LinearityCoil

Dummy

Coil

Right & LeftUnbalanced Pincushion

Correction

Inner PincushionCorrection

11

C8804

T8801

DynamicFocus

Fv Fh

Screen

ABL/ACL

Fo(dynamic)

Fo(static)


3. Deflection Circuit

Fig. 3-1


7/14

-7-

V H

DEFLECTION-YOKE

CRT

COMPONENT

F.B.T

H-OUT

KX1P/...

/KX5P

BEFORES

AFTERS

C417

FO

SC

A

FC

HEATER

LOW

+B

HV

V+

B

FO

INNERPIN

RLUNBALANCE

DA

F

PCCAMP

BEFORES

AFTERS

Q462A

1AA2HEA0206--

KX

J407

R408

T401

KX5P

KX4P

KX3P

KX1P

L413

R418

L414

C419

C413

C414

C412

D406

D407

L403

L402

Q402

L401

C408

R406

C407

C406

Q401

R416

DAF

R463

C463

R445

D445

D446

C445

H

PCC

KV

KP

GND

-B

L416

R407

C411

R413

C422

C434

R434

T402

R451

R452

R453Q

451

R458

R459

C474

D472

R474

R476

R472

R470

Q472

R475

C477

C476

R477

Q471

D473

R473

C8802

C8801

Q

461

R460

R465

R468

D463

D462

C472

C478

K16V

K16P

C475

T8801

C8804

C479

R454

R455

R456

R457

V6451

R450

C8803A

Q462

J

1SJ15K

AD0008

:AD0008C

J30B0250N

:J30B1380N

J30B0250N

:J30B1380N

J30B0250N

:J30B1380N

L71B0250N

2SJ2.7

K

250MJ0.1

8

(UAP:AT:AU)

LM0045

:LM0045D

:L26B0170N

:L26B0260N1

00GK2.2

P

400NJ

0.0

12

EAQ

630NJ

0.0

22

EAQ

1500MH

7100

(XK:AN)

ERB44-04

ERD07

-15L

ZZ0208

L30042

SD2645YB

L26B

0800N

160EM

4.7

BE

1/2DJ

5.6

K

500KK

2200A

500KK

470A

2SC2271

(D-C:E-C

:D:E)

DJ33

1SJ3.3

FK0.0

15

(D:BF)

1SJ560

EU1

MTZJ20A

:RD20EB1

25EM22

:EM22LBZA

L26B0770N:

L26B4090N

2SJ6.8

K

1500MH

6500

(XK:AN)

5SJ

3.9

VCA

250GJ

2.2

P

160PM

0.4

7D

27K

L40B13100

1/2DJ

100K

8.2

K

1KT

XXGA

000868N

2SJ3

.9K

2SJ3

.9K

400GK

0.1

P

MTZJ10A4

70K

2.7

K

680

1/2DJ

100K

AE

1SJ

47K

250M

J0

.15

(AT:AP:AU)

500KK

1000A

100

TXXGA

000868N

RD10EB2

:MTZJ10B

5.6

K

J

X

2SC3114

(R

:S:T)

1/6NF

560

1/6NF120K

56K

1SS133

1SS133

63GJ

0.1

V:

HJ0

.1

X

16PM10

L18B0450N

1000KK2200

(CRD:NHA)

KK4700

820

680

2SJ1

.8K

2SJ1

.8K

XX

1000KK

2200(CRD:NHA)

2SB1274

(QRA:RRA)

1 2

3

4 5

6 7 8

9

10

11

R462

C4

62

C464

1/10GJ

4.7

KC

CJ

10

0CL

63GJ

0.1

5V:

HJ0.1

5

D487

R485

R276

R492

R471

D471

S5277B

:ED0445

:MPG06D

:ERA15-02

18K

1/6NF

33K

12K

1/10GJ

180KC

R

TP51

TP50

R494

R493

C493

R491

X

X

DJ390K

DJ180K

100PM

2.2

X

DJ1K


Fig. 3-2 shows a deflection circuit diagram of VB8B chassis

Fig. 3-2


8/14

-8-


3-3 Right and left unbalanced pincushion correction circuit

3-3-1 Outline

The DY and CPM are adequate for landing the electron beam on the fluorescent screen after passing

through the shadow mask. However, as for the pathway of the electron beam, unbalance of right and left

occurs because of pincushion distortion. Particularly in the flat face CRT, the unbalanced pincushion dis-tortion can stand out as shown in Fig. 3-6-1.

This is because the migration length of the electron beam is different between the central part and the

corner parts of the CRT face.

Generally, conventional pincushion correction circuitry can correct the isometric distortion, but it can-

not correct pincushion distortion that is right and left unbalanced.

This unbalanced pincushion correction circuit is constructed with a FET (field effect transistor) and a

resistance circuit which are connected in parallel with the S-correction capacitor to resolve the unbal-

anced pincushion distortion.

3-3-2 Operation

Fig. 3-3 shows the diagram of the right and left unbalanced pincushion correction circuit.

When the S-correction capacitor is charged by the horizontal deflection current ia in the polarity as

shown by the arrow in Fig. 3-3, a positive voltage is impressed between G (gate) and S (source) of the

FET by the bias circuit. The FET turns ON between D (drain) and S (source) and current flows accord-

ing to the voltage across the S-correction capacitor.

When the S-correction capacitor begins to discharge and the bias voltage between gate and source of the

FET becomes lower than the threshold level the FET turns OFF.

Fig. 3-4-1 and Fig. 3-4-2 show the waveforms of the horizontal deflection current during the horizontal

scan period. In the scanning interval last half the horizontal deflection current decreases so that the dis-charge current from the S-correction capacitor flows towards the FET, and amplitude of the upper part

of horizontal deflection current becomes less.

Fig. 3-5-1 and Fig. 3-5-2 show the waveforms of horizontal deflection current in the vertical scan peri-

od. When the voltage impressed across the S-correction capacitor is high due to the horizontal deflec-

tion current being high, the FET current will become high. This results in the greatest amount of cor-

rection. Therefore, the middle of the envelope of the upper part of horizontal deflection current becomes

dented as shown in Fig.3-5-2.

Because of this, deflection of the right side of the image becomes less, and the middle of the right side

of the image becomes dented as seen in Fig. 3-6-2. In this way the distorted image, which is right and

left unbalanced as shown in Fig. 3-6-1, is corrected as shown in Fig. 3-6-2.

Note:

Since correction of the right and left unbalanced pincushion will make the right side of the image small-

er than the left side, additional consideration should be given to the linearity coil.


9/14

-9-


Scanning timeBlankingtime

Horizontal time 1H 1V

C417

Horiz.Coil

L413LinearityCoil

S-correctionCapacitor Q451

Right & LeftUnbalanced PincushionCorrection

R456 R459R451

R452---R455

Horizontal Deflection Current

a b ca = b > c

Q411H-Output

L416

t

ia

ta tb

D407ia ib

ic

ia= - ib

ib

t

ia

ib

ic

ib' = ib - ic ( at Q451=ON)

ib'

G

D

SFig. 3-3

Fig. 3-4-1

Fig. 3-4-2

Fig. 3-5-1

Fig. 3-5-2

Fig. 3-6-1 Fig. 3-6-2


10/14

-10-


3-4. Inner Linearity Compensation by Dynamic S-Correction

3-4-1 Outline

Generally, a flat picture tube has a geometric error that is called inner pin distortion. Inner pin distor-

tion is a geometric error resulting in a different cross hatch width between the sides and the center of the

image as shown in Fig. 3-8-1.Compensating for this error is almost impossible by conventional PCC circuits. This inner pin distortion

can be overcome by adapting the value of the S-correction capacitor during a certain interval of the scan

time. In this way an improved S-shaped deflection current can be realized to compensate for the above

mentioned error of any picture tube.

3-4-2 Operation

Fig. 3-7 shows the diagram of the inner pincushion correction circuit.

Perfect horizontal linearity can be realized by a continuous adaptation of the deflection current during

the horizontal scan. Creating a more linear deflection current during the start and end of the scanning

time can be realized simply by increasing the value of the S-correction capacity during these scan inter-

vals.

This is accomplished by switching another S-correction capacitor (C477) in parallel with C417 during

the proper scan interval or duty cycle.

Generally, the horizontal width is dependent on the voltage across the S-correction capacitor. Therefore,

by modulating this voltage the envelope of the horizontal deflection current becomes parabola shaped.

At the vertical scanning time of the top and bottom corners the voltage across the S-correction capacitor

is low. At the vertical scanning time of the center part the voltage across the S-correction capacitor is

high.

When the voltage across the S-correction capacitor C417 is low, D472 is OFF, Q451 is OFF, Q471 isON and C477 serves as the additional S-correction capacitor, and the horizontal width is widened.

When the voltage across the S-correction capacitor C417 is high, D472 is ON, Q451 is ON, Q471 is OFF

and C477 is removed, and the horizontal width is not changed.

In this way the inner linearity can be fully compensated over the complete picture by modulating the duty

cycle as a frame period.

Fig. 3-9-3 shows the horizontal deflection current with the inner Pincushion correction circuit and Fig.

3-8-4 shows the picture after geometrical adjustment.


11/14

-11-


1H 1V

C417

Horiz.Coil

L413LinearityCoil


AdditionalS-correction

Capacitor

R475

R470

R472---R473

Horizontal Deflection Current

C477Q471

Q451

Inner PincushionCorrection

R474

C474

D473

R476

C476

R477D472

C475

Without InnerPincushion Correction(the best adjustment)corner = straightcenter = pin

Without InnerPincushion Correction

corner = barrelcenter = straight

With InnerPincushion Correction

The corners are widened.

After Geometric Adjustment

G

D

S

Fig. 3-7

Fig. 3-8-1

Fig. 3-9-2

Fig. 3-9-1

Fig. 3-8-2

Fig. 3-8-3

Fig. 3-8-4

Fig. 3-9-3


12/14

-12-


4. Dynamic Focus Circuit

4-1 Outline

The migration length of the electron beam is different between the central part and the four corners of

the screen. This difference is further increased in a flat face CRT compared to a conventional round

shaped CRT. The most suitable focus voltage is usually different according to the migration length ofthe electron beam. Therefore, it is necessary to change the focus voltage according to the beam landing

position to achieve optimum focus. This is called DAF (Dynamic Astigmatism Focusing).

The optimum focus voltage waveform is a parabola shape in the horizontal and vertical periods

(Fig. 4-2). Dynamic focus is realized by superimposing the amplified deflection current wave upon the

focus electrode.

4-2 Operation

The parabola voltage of the horizontal period is impressed across the S-correction capacitor C417 and

coupled by C8803A and C8804 to the primary of T8801. Any DC voltage component is blocked byC8803A and C8804. The parabola voltage is boosted up to approximately 1000V at the secondary coil

of the step-up transformer T8801. The parabola voltage is supplied to FBT T402 pin 11 and superim-

posed upon the DC voltage through the coupling capacitor in the FBT.

The static focus voltage and the dynamic focus voltages are supplied to their respective focus electrodes

of the CRT.

Note:

The vertical parabola correction function is not used in the VB8B chassis.

For example, on the specification of CRT A68ERF031X013, the dynamic focus voltage is as follows:

Line parabola (screen edge-to-edge) on Vdyn = typ. 800V ---horizontal focus voltage

Frame parabola on Vdyn = typ. 350V ---vertical focus voltage

Because the specification for the CRT shows the voltage for effective image screen, it is different from

the actual voltage and is more than 1000V as horizontal parabola voltage.


13/14

C417

Horiz.Coil

T402Flyback Trans.

L413LinearityCoil

11

C8804

T8801

DynamicFocusInput

Fv Fh

Screen

C8803A


(1)Gfoc(2)Gdyn

G5G6 G4 G3 G2 G1

Dynamic Focus

Static Focus

Dynamic Focus

Static Focus

Example of Electron Gun

1V

1HEffectiveScreen(edge to edge)

V

H

Specificationof CRT

LineParabola

FrameParabola

t

t

t

t

CathodeVideo

DynamicFocus

1H

1V

1H

1H

-13-


Fig. 4-1

Fig. 4-2

Note:These waveforms are the ideal concept of dynamic focus, not actual.


14/14


J uly / 2002 / SMC Printed in U.S.A.

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Sanyo VB8B Crt