RCA CTC 185 Service Manual

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THOMSON TECHNICAL TRAINING

This publication is intended to aid the electronic technician in servicing the CTC185 television
chassis. It explains the circuit theory of operation of the prominent or otherwise new and
different circuits associated with the digitally controlled chassis. This manual covers power
supply, system control, horizontal deflection, vertical deflection, tuner, IF, video signal
processing and audio. It includes practical, proven troubleshooting methods that are designed
to help the technician become more familiar with the chassis layout, increase confidence and
improve overall efficiency in the servicing the product.

Note: This publication is intended to be used only as a training aid. It is not intended to replace
service data. Thomson Consumer Electronics Service Data for these instruments contains
specific information about parts, safety and alignment procedures and must be consulted
before performing any service. The information in this publication is as accurate as possible
at the time of publication. Circuit designs and drawings are subject to change without notice.

FOREWORD

SAFETY INFORMATION CA UTION

Safety information is contained in the appropriate Thomson Consumer Electronics Service
Data. All product safety requirements must be compiled with prior to returning the instrument
to the consumer. Servicers who defeat safety features or fail to perform safety checks may be
liable for any resulting damages and may expose themselves and others to possible injury.

All integrated circuits, all surface mounted devices, and many other
semiconductors are electrostatically sensitive and therefore require special
handling techniques.

Prepared by
Thomson Consumer Electronics, Inc.
Technical Training Department, Mail Stop INH-905
P.O. Box 1976
Indianapolis, IN 46206-1976
U.S.A.

First Edition 9624 - First Printing
Copyright 1996 Thomson Consumer Electronics, Inc.
Trademark(s)® Registered Marca(s) Registrada(s)
Printed in U.S.A.

®

Contents

Power Supply .................................................................................................................... 5

System Control ................................................................................................................ 13

Horizontal Deflection ..................................................................................................... 22

Vertical Deflection .......................................................................................................... 29

Tuner ................................................................................................................................ 32

Video / Audio IF.............................................................................................................. 45

Video Processing ............................................................................................................. 47

Audio Processing ............................................................................................................ 54

Introduction

The power supply in the CTC185 is a non-isolated switching power supply that uses a
MOSFET (Metal Oxide Semiconductor Field Effect Transistor) as the switching
device. The supply uses a winding on the IHVT to provide a voltage boost and a series
PWM controlled MOSFET to regulate the output voltage to 130 volt DC. The supply
has no alignments because it uses a precision 1% voltage reference IC (U4103). The
control circuit is synchronized to the horizontal oscillator by using a flyback pulse as
a timing reference for a sawtooth ramp generator. When the chassis is in standby mode,
Q4114 is kept on by a drain to gate resistor which forces the output voltage to be equal
to the RAW B+ voltage.

Power Supply 5

CTC185 Main
Regulator

RAW B+

T4401

IHV T

3

C4007

CR4105

1

V2

CR4103

L4102

I1

Q4114

C4153

V1

GATE

DRIVE

V3

FLYBACK PULSE
PIN 8 OF IHVT

HORIZ. SYNC
CIRCUIT AND

SAWT OOTH GE N.

V4

V5

COMPARATOR

ERROR AMP

AND

REFERENCE

Figure 1, Main Regulator Block Diagram

The winding on the IHVT at pins 1 and 3 sums an inverted retrace pulse on the RAW
B+ (V1 in figure 1). This provides the pre-boost so the supply will regulate even when
RAW B+ falls below 130 volts (figure 2). The number of turns on the winding
determines the lowest voltage at which the supply will regulate. The 25 and 27 inch
chassis will regulate the +130V with as little as 95 volts AC RMS and the 19 and 20 inch
chassis with as little as 90 volts AC RMS.

REG B+
130VDC

After the pre-boost, the supply acts as a buck converter in that the output voltage is equal
to the input voltage times the duty cycle. When Q4114 is turned on, current flows
through CR4105, L4102 and Q4114 charge up C4153 . When retrace begins, the
voltage on the anode of CR4105 begins to drop but the current I 1 (figure 3) continues
to flow through it because of inductor L4102 . When the voltage V2 (figure 3) decreases
to -0.7 volts, CR4105 turns off and CR4103 turns on and conducts I 1 until it drops to
zero. During this time Q4114 remains on to conduct the current I1 until it drops to zero.
The timing of the turn off of Q4114 can be seen in figure 3. The falling edge of voltage
V3 is the signal that turns off Q4114. The turn off time of Q4114 is fixed and the turn
on time varies in response to the PWM (Pulse Width Modulator) control circuit.

6 Power Supply

Figure 2

Figure 3

Power Supply 7

V2

CR4103

L4102

I1

Q4114

C4153

RAW B+

T4401

IHV T

3

C4007

CR4105

1

V1

GATE

DRIVE

V3

FLYBACK PULSE
PIN 8 OF IHVT

HORIZ. SYNC
CIRCUIT AND

SAWT OOTH GE N.

V4

COMPARATOR

V5

ERROR AMP

AND

REFERENCE

Figure 4, Main Regulator Block Diagram(repeated)

The PWM is made up of the horizontal sync circuit and sawtooth generator block, the
comparator block, and the error amp and reference block (figure 4). The horizontal sync
and sawtooth block generates a sawtooth voltage V4 (figure 5). This signal and the error
voltage V5 from the error amp block are fed into the comparator block which generates
the gate drive voltage V3. There is a small time delay after voltages V4 and V5 cross
and a state change in V3 occurs. This is due to the slow response of the comparator
block.

REG B+
130VDC

Figure 5

8 Power Supply

The gate drive block generates a 9 volt supply which floats above Reg B+. This voltage
is needed to switch Q4114 on. The floating supply is generated using a charge pump
coupled to the cathode of CR4105. When V2 is negative, charge is stored on a C4138
and when V2 rises after the retrace interval, this charge is dumped to the floating 9 volt
supply.

Circuit Description

Raw B+ comes from a full wave bridge rectifier (not shown) and filter capacitor C4007.
R4172 is a bleeder to discharge C4007 when the ac power is disconnected. Capacitors
C4104, C4122, C4124, C4134, and C4135 are for RFI suppression. R4146 and C4137
form a snubber for MOSFET Q4114. R4103 is a current limit resistor for Q4114. It
limits the gate drive by reducing the 9 volt gate supply with respect to the source
terminal of Q4114. R4108 provides gate drive when the chassis is in standby. CR4106
is a protection diode for the gate of Q4114.

RAW

+12V

RUN

V7

B+

C4007

C4007

Q4104

R4148

C4136

R4172

IHVT

PIN 8

R4116

C4108

R4145

IHVT

PIN 3 PIN 1

C4124

R4117

Q4108

R4118

C4109

V6

V1 V2

CR4105

C4122

V4

R4127

R4120

R4124

R4123

CR4103

Q4103

R4146

I1

L4102

R4108

R4128

C4138

R4125

3

C4104
Q4114

R4147

U4103

2

R4110

V5

C4123

R4142
R4126

C4103

C4137

R4103

CR4106

+

­+2.5V

REF

R4149

R4114

Q4113

1

CR4111

R4138
C4135

C4106

CR4113

V3

R4137 R4111 R4112

C4134

CR4112

R4136

130V
REG B+

C4153

Figure 6, CTC185 Main Regulator Circuit

The 9 volt gate supply consists of C4106 and CR4111. The charge pump is C4138,
R4147, CR4113, and CR4112. C4138 is charged through CR4112 during retrace, and
discharged into C4106 during trace through CR4113. MOSFET Q4114 is switched on
though R4138 and is turned off by Q4113 turning on and bleeding the gate charge off
through R4114.

The comparator block consists of Q4102 and Q4103. The inputs to the comparator are
the bases of the two transistors. Q4113 serves as a high voltage buffer to switch Q4114.
Once input is fed from the error amp and reference block which is formed by U4103 and
the voltage sense resistors R4136, R4137, R4111, and R4112, the parallel combination
allows the output voltage of the supply to be trimmed to exactly 130 volts. U4103
contains a 1 % voltage reference and the error amplifier in a three lead TO-92 package.
C4103 and R4149 provide the gain compensation for the error amp to ensure the power
supply is stable into any expected load.

The horizontal sync circuit and sawtooth generator is comprised of two transistors
(Q4104 and Q4108). Q4104 provides the time delay which is necessary to keep the
MOSFET on until the current through it has dropped to zero. The collector voltage V7
is shown in figure 7 along with the flyback pulse V6 which drives the emitter to illustrate
the time delay. The collector is capacitively coupled into the base of Q4108 to provide
a short duration pulse which charges C4109. C4109, Q4108, R4118 and R4120 form
the ramp generator. The short duration pulse from Q4108 charges C4109 quickly and
R4120 discharges C4109 at a slower rate.

Power Supply 9

Figure 7

10 Power Supply

Troubleshooting

TECH

TIP

Troubleshooting of the power supply can be somewhat difficult because the supply will
not operate unless horizontal deflection is working and horizontal deflection will not
work unless the power supply is operational. The solution is to bypass the regulator
circuit to power deflection and then check the waveforms and DC voltages in the power
supply.

1. Apply a short across the drain and source terminals of Q4114 and use a variac on
the AC line to control the regulated B+ voltage. If the regulated B+ is allowed to
get too high, the XRP circuit will shut off the horizontal oscillator. Therefore, in
order to get the chassis to turn on, the variac must be set so as to provide between
90 and 95 volts AC RMS to the chassis.

2. After the chassis is operational use the variac to set the regulated B+ voltage as close
as possible to 130 VDC.

3. Use the supplied voltages and waveforms to check the various stages of the power
supply for proper operation.

Two parts that are more likely to fail than others are Q4114 and U4103. U4103 fails
most often from excessive voltage on one of its pins. Q4114 failure is most often caused
by applying an excessive load to the output. If the horizontal output transistor (Q4401)
were to short, Q4114 will also short and blow the fuse.

TECH

TIP

Do not de-energize any power supply or yoke lead by directly shorting it to ground. If
it is necessary to de-energize any point on the chassis or deflection yoke, do so by
grounding that point through a 1K ohm resistor. The reason is that discharging power
supply or yoke leads directly to ground can cause a failure of the regulator output
transistor Q4114.

This does not apply when discharging the CRT anode. In that case, discharge by
directly shorting to the CRT ground braid.

Power Supply 11

Overview

The standby power supply in the CTC185 is derived from a dropping resistor connected
to a three terminal 12 volt regulator IC (U4102). The power comes directly from the half
wave rectified AC line. The microprocessor turns off the T4-Chip (U1001) in the
standby mode to remove the load from the standby supply. This is to conserve power
because the standby supply cannot continually supply the power needed to operate the
T-Chip. The standby supply timing for the T4-Chip is important because the current
needed to run the micro and the T4-Chip together is derived from C4154 until the
+26V run supply supplements it. When the charge stored on C4154 is depleted, the
+12V standby supply will drop and the T4-Chip will be turned off automatically.

F4001

R4001

CR41 09

R4002

C4154

CR41 10

R4104

CR47 03

+26V RUN

C4102

U4102

C4127

CR41 04

5.6V

Q3901

C4114

CTC185
Standby
Power Supply

12V ST BY

R4107

5.9V RE F
5V ST BY

R4109

5.6V RE F

C4113

Figure 8, CTC185 Standby power Supply

Circuit Description

The AC power line is rectified by CR4109 and CR4004 and fed through R4002 to filter
capacitor C4154. Zener diode CR4110 limits the voltage to 27 volts so that regulator
U4102 is not damaged by excessive voltage. Diode CR4703 supplements the standby
power supply when the chassis is running from the +26V run supply. The power
necessary to run the chassis during turn-on comes from C4154. U4102 provides a
regulated 12 volt output to run the standby loads in the chassis. Zener diode CR4104
is used to provide a 5.6V and 5.9V reference for the system control circuitry.

12 Power Supply

The T4-Chip power control circuit consists of Q4115, R4143, R4144, and the
microprocessor. The signal labeled as STBY_SW is a pin on the microprocessor which
is pulled low to turn on Q4115 and apply power to the chassis.

Troubleshooting

U3101

STBY
SW

Q4115

20

R4144

29

R4143

+12V STBY

U1001

STBY
VCC

Figure 9, T4-Chip Power Control

Dead Set

The turn on sequence for the T4-Chip requires precise timing. Recall that C4154 only
stores enough charge to get the set going. The scan derived +26 volts is necessary to
re-supply the voltage to keep the T4-Chip powered.

1. Measure the output of U4102 for +12 volts. If it is not present, verify that the T4­Chip (U1001) is not turned on by measuring pin 20 for any voltage greater than zero.
If there is, the switch transistor (Q4115) is turned on causing the standby supplies
to pulled down.

2. The standby supply can be supplemented with an external +26 volts applied to the
cathode of CR4703. This will provide a constant supply so the T4-chip can run.

System Control 13

The system control circuit in the CTC185 chassis is responsible for controlling all
functions in the TV. Like the CTC177 and CTC179, the CTC185 control system
includes a microprocessor (U3101), a signal processing IC, referred to as the T4-Chip
(U1001), the Electrically Erasable Programmable Read-Only Memory (U3201), and
the tuner PLL IC (U7401).

The microprocessor receives input from the infrared remote receiver (IR3401) and the
front keyboard and executes instructions to control the television based on those inputs.
The EEPROM is a storage device that stores data such as alignment values, T4-Chip
register values and the channel scan list. It is non-volatile, meaning that it stores its
contents even with power removed. The tuner PLL controls the frequency synthesizer
of the tuner in order to tune the various channels. The T4-Chip is the heart of the
television and controls all aspects of the deflection and signal processing. The T4-Chip
is discussed in more detail in the signal processing section of this manual.

POWER

VOL. UP

Q3201

5V

14

EEPROM

3

KS1

POWER

4

KS2

DATA

CLOCK

1,2,3

4

19

20

8

VCC

U3201

EEPROM

GND

DATA CLOCK

56

44

DATA

CLOCK

43

System Control

VOL. DN.

CH. UP

CH. DN.

MENU

5V

2

IR3401

3

5

KS3

39

1

U3101

KD1

µP

RUN

SENSE

RESET

VDD

OSC

OSC

1

OUT

36

Y3101

GND

IN

37

12V

13

5V

2

21

22

5V

19 18

DATA CLOCK

7.6 V

STBY

20

46

BUS
GND

U1001

T4-CHIP

HORZ.

OUT

22

U7401

5V

TUNER PLL

VCC

9

GND

12

Figure 10, CTC185 System Control block Diagram

14 System Control

Data Communications

Data communications are carried via the two wire I

2

C (“I Squared C”) bus. All four
devices, U3101, U3201, U1001 and U7401, communicate using the data and clock lines
related to this bus.

After reset, the microprocessor loads some initial configuration data from the EEPROM
(U3201). If it can't read fom the EEPROM, the micro continues to try to get a response
from the EEPROM. This can be seen as constant data activity on the data line. The
microprocessor also turns power off to the EEPROM and then back on repeatedly, via
Q3201, in an attempt to reset the EEPROM. Once EEPROM has been read succesfully,
the data bus is quit while the TV is in the standby mode.

When a power button is pressed or an ON command from the remote control is received,
data activity begins. The CTC185, like the CTC177/187, reads status information back
from U1001. The status register reports back the following information:

• Power-On-Reset (POR)

• X-ray Protection Fault (XRP)

• Horizontal Lock Detector

• Automatic Fine Tuning (AFT)

POWER

VOL. UP

VOL. DN.

CH. UP

CH. DN.

MENU

5V

2

IR3401

3

Q3201

5V

14

EEPROM

KS1

3

POWER

KS2

4

KS3

5

U3101

39

KD1

OSC

1

OUT

36

1

Y310 1

µP

OSC

IN

37

DATA

CLOCK

RUN

SENSE

RESET

VDD

GND

1,2,3

19

20

13

21

22

8

4

2

VCC

U3201

EEPROM

GND

DATA CLOCK

56

12V

5V

5V

19 18

DATA CLOCK

7.6 V

STBY

44

DATA

CLOCK

43

U100 1

T4-CHIP

HORZ.

22

OUT

20

BUS

46

GND

U7401

5V

TUNER PLL

VCC

9

GND

12

Figure 11, CTC185 System Control block Diagram

POR (Power-ON-Reset)

U1001 has a standby power monitor called POR. This circuit detects when the Standby
Vcc has dropped below approximately 6 volts and shuts the IC off by stopping both the
PWM and horizontal outputs.

The POR circuit output is latched and reset on the OFF to ON transition of the ON/OFF
bit. This means when the TV is ON and a standby transient occurs that triggers the POR
circuit, it is necessary to send an OFF command followed by an ON command to get the
set started again. If the Standby Vcc is still too low when an ON command is received,
the IC will stay in the OFF mode requiring the process to be repeated.

XRP

Although there is an XRP bit in the status register sent back from the T4-Chip, the run
sense line (U3101 pin 13) informs U3101 when an XRP or POR condition has occurred
When the XRP input is above the reference value, the comparator’s output will turn the
TV off by stopping both the PWM and Horizontal outputs.(See the XRP circuit in the
Horizontal Deflection section of this manual.)

System Control 15

The XRP bit is latched internally and gets reset at the ON to OFF transition of the ON/
OFF bit. This means to restart the TV after an XRP trip, the microprocessor must first
send an OFF command followed by an ON command.

Horizontal Lock Detector

This detector compares the position of the flyback pulse with the sync of the video signal.
While this detector can be used to detect the presence of an active channel, it is not used
for tuning. A separate sync pulse input is applied to pin 12 of U3101 for that purpose.

Periodic Updates

In addition to reading the status register, the microprocessor continually updates the
registers in U1001 approximately once every second with data stored in the EEPROM.
In other words, alignment information stored in the EEPROM is constantly being loaded
into the T4-Chip. This prevents electrical disturbances such as kine arcs from corrupting
the information in U1001. This updating does not take place, however, when the TV
is in the service mode.

Run Sense

U3101 pin 13 monitors the presence of the run 12V. This is a scan derived supply from
pin 8 of T4401, the IHVT (Integrated High Voltage Transformer). If this supply is not
present, the micro will place the TV in the off mode and try to re-start the set. If there
are three failed attempts in one minute, the micro places the television in the off mode.
The ON command from the front keypad or remote control must be initiated again to
start the process over.

16 System Control

Keyboard Interface

The keyboard interface is similar to the other schemes used on the color television
chassis. The POWER, VOL. UP and VOL. DN switches short their corresponding sense
line to the drive line, which is normally held low. The other three switches pull KS1, KS2
and KS3 to ground. When U3101 detects a low on any sense line, this indicates a key
has been pressed. The drive line is then raised to determine which particular key. If the
active sense line remains low, the key pressed was the one shorting to ground. U3101
will then initiate the appropriate function based on which key was pressed.

IR Input

Infrared remote signals are demodulated and amplified by IR3401 and appear at U3101
pin 1 as 5 Vp-p negative going data pulses. When no IR is received, the DC level at
U3101 pin 1 is 5V. IR3401 is powered by the 5V standby supply.

Reset Circuit

The reset circuit starts the microprocessor at a known place in its program. U3101 reset
is an active low to pin 2. When AC power is first applied, the reset line will remain low
a minimum of 12ms once the 5V STBY supply reaches 4.5 volts. This allows the
internal clock of the microprocessor to come up and stabilize before carrying out
instructions. The reset circuit also monitors the condition of the 12 volt standby supply.
If the 12 volt standby supply drops below 10 volts, the reset circuit activates and holds
the microprocessor in a reset state.

5.6V REF 12V STBY 5V STBY 12V STBY

C3121

Q3102

C3126

CR3102

R3145 R3107

R3130

R3132

R3133

Q3101

R3129

R3120

R3119

2

RESET

CR3111

5.1V

R3152 R3151

C3113

C3115

Y3101

36 37 22 21

U3101

C3127

CR3110

C3114

5V STBY

Figure 12, Reset Circuit

A stable 5.6 volt reference is applied to the emitter of Q3102. The 12 volt standby supply
is divided by R3132 and R3133 so approximately 6 volts is applied to the base of Q3102.
The collector of Q3102 is tied to the base of Q3101. The collector of Q3101 is connected
to the 5 volt standby supply and to the reset pin 1 of U3101. Under normal operating
conditions, the voltage on the base of Q3102 is at 6 volts which is high enough to keep
Q3102 off. If the 12 volt standby supply drops far enough to allow the voltage on the
base of Q3102 to drop to 5 volts, Q3102 will turn on. When Q3102 turns on, Q3101
will also turn on pulling the reset line to ground initiating a reset to U3101.

The 8MHz crystal oscillator is also disabled by grounding it through CR3110 and
R3151. This ensures that the standby supplies are up and stable before the oscillator is
allowed to run. When the 12V STBY rises sufficiently to overcome the 5.1 volt zener
voltage of CR3111, CR3111 begins to conduct, raising the voltage on R3151. This will
reverse bias CR3110, allowing the oscillator to start

On Screen Display (OSD)

The CTC185 uses the same OSD circuitry configuration as the CTC177 chassis. Red,
green and blue outputs from U3101 (pins 17, 16 and 15) drive the red, green and blue
OSD inputs on U1001 (pins 34, 35 and 36) producing a full color OSD.

System Control 17

Horizontal and vertical sync are input to pins 26 and 27 of U3101 and are used to control
the position of the OSD on the screen.

The Fast Switch (FSW) signal from U3101 pin 18 is an active high signal that switches
U1001’s RGB signal path to the OSD signal. This switching signal is only present during
the time interval that OSD is being displayed. If this signal should ever become “stuck”
high, a no video symptom would result.

+7.6

C2709

+

GRN OSD

C2710

C2711

35

IN

U1001

T-CHIP

BLU OSD

+

+

36

34

IN

RED OSD

IN

H. SYNC

V. SYNC

CC. VIDEO

Q2707

GRN OSD

26

U3101

OUT

16

+7.6

µP

27

11

BLU OSD

OUT

RED OSD

OUT

Q2706

15

+7.6

Q2705

17

18 33

FSW FSW

Figure 13, On Screen Display Circuit Block Diagram

18 System Control

Power On Sequence

The power on sequence timing diagram for the CTC185 is shown in figure 14. When
the television is first plugged in, the microprocessor “awakens” and goes into the standby
mode within 20ms. The micro will then wait for a keyboard button press or IR input from
the remote control. Also, when the micro comes on line, it turns the power to the
EEPROM off for approximately 50ms and then back on to start the EEPROM in a know
state. The EEPROM power is also switched off/on when there is an acknowedgement
problem with the EEPROM in an attempt to reset it.

When an ON command is received from the keyboard or remote control, the standby
switch line on pin 29 of U3101 goes low. This turns on Q4115, which supplies power
to the T4-Chip (U1001) turning it on. Video and Audio are muted in the T4-Chip; the
12V run supply is detected; the T4-Chip registers are initialized with data from the
EEPROM (U3201). About the time the run supply is detected, CRT degaussing takes
place for approximately 1.5 sec while vertical deflection is killed for approximately .75
sec. Once degaussing is complete, the speaker mute is turned off along with the video
and audio mute in the T4-Chip.

STAND BY TIMING

+12v_STBY

STBY_SW

EEPROM PWR

POWER UP TIMING

STBY_SW

T4-Chip Off/On ONOFF

Video & Audio

Mute

+12v_run

Initialize

Registers

(Micro awake)

20mS

Power to T4-Chip

(No T-Chip Pow er)

30mS

Run_Sense

1 mS

Spkr Mute

Degauss

Vertical Kill

Figure 14, CTC185 Power On Sequence

200 mS

1.5 Sec

.75 Sec