Toshiba ColorStream TP43H60 Manual

NTDMOD02
TOSHIBA
Service Training
PROJECTION
TELEVISIONS
Main Power Supply
& Complete
Shutdown Guide
TP43H60
TP43H95
TP50H15
TP50H50
TP50H60
TP50H64
TP50H95
TP55H60
TP55H64
TP55H95
TP61H60
TZ43V61
TZ50V51
TZ50V61
TZ55V61
TZ61V61
Copyright 1999
TOSHIBA AMERICA CONSUMER PRODUCTS, INC.
NATIONAL SERVICE DIVISION
1420-B TOSHIBA DRIVE, LEBANON, TN 37087

Contents
Main Power Supply...............3
Overall Block Diagram .................................................................................... 4
Operation........................................................................................................... 5
Surge Protection Relay ................................................................................... 6
Start-up and Over Voltage Protect................................................................. 7
Logic and Drivers............................................................................................. 8
Oscillator ........................................................................................................... 8
Oscillator Control............................................................................................. 9
Latch .................................................................................................................. 9
Thermal Shock Detection Block .................................................................... 9
Over Current Protection................................................................................. 10
Soft Start........................................................................................................... 10
Resonance Correction ................................................................................... 10
Additional Information on Q801 .................................................................... 11
Block Diagram of Q801 (STR-Z4117) ............................................................ 11
Pin Descriptions.............................................................................................. 11
Waveforms ....................................................................................................... 12
Troubleshooting Flowchart........................................................................... 13
Shutdown Guide.................15
Summary of Shutdown Circuits.................................................................... 16
Peak-Response Meter .................................................................................... 17
No Peak-Response Meter .............................................................................. 18
Monitoring Circuits ......................................................................................... 18
X-Ray Protect ................................................................................................... 19
+125V Over Current Protect .......................................................................... 20
+36V Over Current Protect ............................................................................ 21
+18V and – 18V Over Current Protect.......................................................... 22
+35V Over Current Protect ............................................................................ 23
+9V Under Voltage Protect............................................................................. 24
+35V Under Voltage Protect........................................................................... 24
+200V Under Voltage Protect ........................................................................ 24
Troubleshooting Flowchart........................................................................... 25

3
Main Power Supply

4
D882
D884
16
17
D891
D894
14
15
D886
D889
12
13
D851
D854
9
10
D856
D855
11 +36
+18
-18
+38
+12
+125
Q760 30V
Reg. To Q752
Conv. Output
To Q752 & Q751
Conv. Outputs
Q754 5V
Reg. To Digital Conv. PC,
Q764 & Q767
Q755 9V
Reg. To Q752
Conv. Output
To Q752 & Q751
Conv. Outputs
Q756 -9V
Reg. To Digital Conv. PC,
To Q601 Audio Out
Q430 9V
Reg. To Q501
Horizontal Start-up
To Shutdown & Surge protect:
Q846,SR82,Q757, & Q758
Q832 9V
Reg. +9-2 To Numerous Circuits
Q830 5V&9V
Reg. +5-2 To Numerous Circuits
+9-1 To Numerous Circuits
Q831 5V
Reg. +5-3 To PIP Module & QBB3
To Horizontal Output
D101 32V
Reg. To Tuner(s)
C870
T862
Main Switching IC
Q801
Z801
Error
Amp.
Q862
Photo
Coupler
D801
Rectifiers
Rectifier
165V
Feedback
SR81
Relay
Stand-by
Regulator
Rectifier &
Isolation Trans.
Q840
Microprocessor
QA01
QB30
Q843 Relay Drivers
D840
T840
4
7
9
Reg.= Regulator
Conv.= Convergence
+5-1
5V=Relay On
0V=Relay Off
5V
5V
AC Line
Input
From Shutdown
Stand-By Supply
Main Power Supply
VDD
Reset
Overall Block Diagram
Figure 1 is the overall block diagram of the main
switching power supply and the standby power
supply. The standby supply is always active when
the television is plugged into an AC line source.
This supply delivers a 5VVDD and a Reset 5V to
themicroprocessortokeepthemicroprocessorfunc-
tioningat all times, even when the television is not
operating. Transformer T840 isolates the standby
supply from the live ground, and D840 is a full-
wave bridge rectifier that supplies 12Vdc to volt-
age regulator Q840 and relay SR81 (connection
notshown). When the microprocessor receives an
ON command from the remote control or power
key, on the front of the television, it sends 5V to
relay drivers QB30 and Q843 to close relay SR81.
When the relay closes, theAC line input is applied
directlyto themain powersupply. Thesupplystarts
to operate and turns ON the television. D801 is
the full-wave bridge rectifier for the main power
supply. It rectifies the 120V AC line input to
165Vdc and applies it to the main switching IC,
Q801. The primary side of the power supply is
not isolated and, therefore, is at live ground. De-
tailed explanations of the main power supply are
covered in the remainder of this section.
TroubleshootingTip:
IfSR81 never closes,check the standbypowersup-
ply. Both the5V VDDand theReset 5Vare manda-
toryfor the microprocessor to operate.
Figure1.
PowerSupplyBlock Diagram

5
Load
Current
Resonant Freq.
Operating Freq.
Switching
Freq.
Increased
Load
Decreased
Load
Normal Operating Frequency is 70-80kHz
rent, the oscillator frequency decreases and oper-
atescloser tothe LCresonantfrequency. Thecloser
theswitchingfrequencyistoresonance,thehigher
the current flow through the primary windings of
T862andthelargertheelectromagneticfield. The
largertheelectromagneticfield iswhen itcollapses,
the higher the induced current is in the secondary
winding. When the load decreases and requires
lesscurrent, the switching frequency increases and
moves away from resonance. As a result, less cur-
rent is induced in the secondary windings.
Photo
Coupler
+B
Error
Amp
D
r
i
v
e
O
S
C
T862
C870
Q801
Q862 Z801
125V
Operation
Themainpower supplyis acurrentresonantswitch-
ing power supply. Figure 2 is a basic block dia-
gramfor thissupply. TheprimarywindingofT862
and capacitor C870 create an LC series resonant
circuit. An oscillator (OSC), drive circuit, and two
MOSFETs are located internally to Q801 (STR-
Z4117). The OSC determines the power supply’s
switching frequency. The drive circuit alternately
switches the MOSFETs ON and OFF. The two
power MOSFETs, in a push-pull configuration,
alternate the current flow through the LC circuit
during normal operation. The alternating current
continuallybuilds andcollapses anelectromagnetic
field around T862’s primary windings. The col-
lapsing of the electromagnetic field induces cur-
rent into the secondary windings of T862. A full-
wave bridge rectifier converts the induced current
into 125Vdc.
To regulate the 125Vdc, an error amplifier moni-
tors the voltage and supplies a negative feedback
to the oscillator through photo-coupler Q862.
Q862 isolates the primary side of the power sup-
ply from the secondary side.
Refer to Figure 3. The power supply’s switching
frequency operates above the LC resonant fre-
quency. When the load on the secondary side of
the power supply increases and requires more cur-
Figure2
OperationalBlockDiagram
Figure3
LoadCurrentCharacteristics

6
Surge Protection Relay
Figure4 showsthe surgeprotectionoperation. To
prolong the life of the power supply, a surge cir-
cuit reduces current through the main power sup-
ply at startup. When the television is OFF, relays
SR81 and SR82 are open. At turn-on, SR81
closes and the switching power supply begins to
operate. Duringthis time,the power supplydraws
a large amount of current. To reduce the current,
the ground path for bridge rectifier D801 is
through resistor R810. Once the power supply
becomes fully operational and produces output
voltages, one of these voltages is applied to the
base of transistor Q846. This voltage turns Q846
ON and allows current to flow through the coil of
SR82. SR82’s switch closes and creates a direct
ground path for D801 by bypassing R810. SR82
remains closed during normal operation.
SR82
Q801
and
T862
D801
Q846
R846
12V
125V
Relay
SR81
R810
Main Power Supply
R809
Figure4
SurgeProtect
TroubleshootingTip:
R810 is a fusible resistor. If SR82 does not close
afterthepowersupplyisfullyoperational,R810even-
tuallyopensandprevents thepowersupplyfromop-
erating. Whenevertroubleshooting the power sup-
ply, check R810 first. If R810 is open, replace it,
usingToshiba part number 24007061, and look for
coldsolderjointsaroundQ846andR846.

7
Vcc
Start
OVP
R871
T862
C868
D864
R861
Q801
From
D801
16-20 Vdc 8
D876
Start-up and Over Voltage Protect
The positive cycle of the AC line input supplies a
16V start-up pulse to pin 8 of Q801 via resistor
R861. Figure 5 is the block diagram for this cir-
cuit. After start-up, a drive circuit consisting of a
secondary winding of T862, diode D864, and ca-
pacitor C868 supply 16-20Vdc to pin 8 of Q801 to
maintain its operation. The voltage developed by
the drive circuit fluctuates with the switching fre-
quency of the power supply. Therefore, the volt-
age on pin 8 is also applied to an over voltage pro-
tect (OVP) block internal to Q801. If the voltage
on pin 8 increases to 25V, the OVP triggers the
latch and switching stops. Refer to the Latch sec-
tion for further information. D876 is a 27V zener
diodethat protects Q801by preventing excessvolt-
age increases on pin 8.
Figure5
Start-upandOverCurrentProtect

8
Logic and Drivers
As shown in Figure 6, the logic block controls the
MOSFETs’ switching frequency. The outputs of
the logic block feed two drivers that are powered
by the start block. After the start-up voltage is
applied to pin 8, the start block supplies a drive
Vcc (DRI Vcc) of approximately 8V to pin 9.
Delaying the driver supplies at start-up prevents
damaging the MOSFETs. The 8V on pin 9 pow-
ers driver B internally. To power driverA, resistor
R862 and diode D862 add the voltage from pin 9
to the voltage on pin 15. D875, C863, D873 and
C873 are voltage regulators and filters for these
supplies.
Oscillator
Refer to Figure 7. Q801’s internal Oscillator de-
velops the power supply’s switching frequency by
generating a ramp waveform internally. During
normal operation, both MOSFETs are OFF for a
shorttimewhen theyare alternatelyswitching. This
OFF time is called dead time and determined by
the value of resistor R867 on the dead time (DT)
terminal pin 7.
VB
Logic
COM.
Out
V
IN
Vcc DRI Vcc
A
B
Start
R862 D862
C863
D875
C873
D873
To
T862
Q801
8915
12
1
14
B+
130V
Figure6
LogicCircuit
Figure7
Q801’sInternalOscillator
DT
OSC
R867
Q801
7

9
Oscillator Control
If the load current drawn from the 125Vdc line in-
creases, the 125Vdc voltage begins to drop, de-
creasingthe currentthrough Q862’sLED side. The
current drop causes the LED to couple less light to
thephoto transistorside andreduce thecurrentflow
into pin 4 of Q801. This reduction in current flow
varies the OSC frequency,moving it closer to reso-
nance to increase the supply of current to maintain
the 125Vdc level. Conversely, if the load current
decreases,the 125Vdc rises and increases the light
through Q862 and the current into pin 4 of Q801.
The increased current causes the OSC operating
frequency to move away from resonance to de-
creasethe current supplied to the load and level the
125Vdc.
Figure8
OscillatorControl
Latch
The latch block stops the operation of Q801 until
the voltage on pin 1 of Q801 is removed by turn-
ing OFF the television. As outlined in Figure 9,
any one of the following detection blocks can trig-
ger the latch.
• Over voltage protection (OVP) Block (Refer
toStart-up section)
• Thermal shock detection (TSD) Block
• Over current protection (OCP) Block
The charging time of capacitor C869, connected
to the capacitor delay (CD) terminal pin 6, delays
the operation of the latch circuit during start-up.
Thermal Shock Detection Block
Thethermal shockdetection blocktriggers thelatch
if Q801’s internal temperature exceeds 150°C.
OSC
4Cont.
OSC
Control
Q862
R864
Z801
125 Vdc Load
1
3
Out
B+ Q862
C891
R884
R883 Q801
6
CD
TSD OVP
LatchDelay
OC/RC
C869
Q801 1 V
IN
Figure9Latch

10
5
10
OC
Css
OC
Q801
T862
C870
R870
C867
To pin 14,
Q801
C866
R866
R872
165V from D801
OSC
OSC
Control
C874
RC
C872
11 RC
Over Current Protection
Figure 10 is the schematic diagram for the over
current protection and soft start circuits. The over
current detect (OC) senses excess current in the
LC series resonant circuit. As current in the LC
series resonant circuit increases, a voltage devel-
ops at the over current protect (OC) terminal pin
10. Resistor R870 and C874 detect the current
flowthroughthe LCcircuit. Resistor R866samples
thevoltage andappliesit topin10. Capacitor C867
is a filter to reduce ripple. Depending on the input
voltageon pin 10,theover current protect responds
in one of two ways:
(1) OC Low Threshold Voltage: +2V
When the input voltage at the OC terminal is
higher than +2V, the voltage at the soft start (Css)
terminal pin 5 lowers and the soft start engages.
Byreengaging the softstart, the mainoscillatorfre-
quencyincreases,reducing thecurrent flowthrough
the LC circuit. The soft start continues to operate
until the voltage on pin 10 drops below 2V.
(2) OC High Threshold Voltage: +2.5V
If the input voltage at the OC terminal exceeds
+2.5V,theoscillatorfrequencyincreases toits maxi-
mum frequency and C866 discharges rapidly.
When the Css terminal voltage decreases to 0.7V,
thecircuitresetsand C866charges again. Themain
oscillator frequency decreases gradually. If this
conditioncontinues, the latch engages and oscilla-
tionstops.
Soft Start
Still referring to Figure 10. At start-up, the soft
start is engaged by capacitor C866 on pin 5, soft
start(Css) terminal. Whilecapacitor C866 charges,
the switching frequency increases to reduce surge
currentthrough the MOSFETs. Once C866isfully
charged, the switching frequency goes to its nor-
mal operating frequency (approximately 70-80
kHz).
Figure10
OverCurrent Protect and Soft Start
Resonance Correction
Bymonitoring thecurrentflow throughpin11, The
resonance correction (RC) block prevents the
oscillator’s switching frequency from dropping
below the LC resonate frequency. As the switch-
ingfrequency decreases, current increases through
pin 11 of Q801. When the switching frequency
comes close to resonance, the RC block engages
the over current protect to increase the switching
frequency and move it away from resonance.

11
Additional Information on Q801
Caution: Different input signals may cause a variance in voltage readings. The Voltages and waveforms
below were recorded while displaying a color bar signal.
Block Diagram of Q801 (STR-Z4117)
15 VB98
Logic
1
14
12
354 7
6
10
CD
OC
Css Cont. DT GND
COM.
Out
VIN
Vcc DRI Vcc
TSD OVP Start
Ref.
Latch
Delay
OSC
OSC
Control
OC
A
B
To
R861 Out to VB From DRI Vcc
From
C866
From
R864 From
R867
From
R866
From
C869
Q801
RC
11
RC
From
C872
Pin Name Vdc Description
1V IN 165 Half bridge power input
3 GND 0 Control unit ground
4 CONT 7.2 Oscillator control terminal
5 Css 3.9 Soft start capacitor terminal
6 CD .4 Delay latch capacitor terminal
7 DT 5.7 Dead time resistor terminal
8 VCC 19.6 Control unit power terminal
9 DRI 8 Gate drive power output
10 OC .7 Over current detection
11 RC .06 Resonance correction terminal
12 COM 0 Half bridge GND
14 OUT 82 Half bridge output
15 VB 90 High side gate drive power input
Pin Descriptions

12
Waveforms
On Off On
Off On Off
Across C870 140Vpp
Q801 Internal
Oscillator
Pin 14, Q801 160Vpp
Internal Q801
MOSFET (B)
Internal Q801
MOSFET (A)
Dead Time
Internal Q801
1.5Vpp

13
Open fuse F470.
Does a
constant voltage
appear at F470’s
supply side?
Yes
Connect a 100W
light bulb to F470’s
supply side.
Does relay
SR81 close when the
Power button is
pushed?
Using the isolated
ground, check the
voltage on the power
supply side of fuse
F470 with the fuse
still removed.
Yes No
Check the relay drive,
microprocessor and standby
power supply circuits.
Does the
voltage regulate
at 125Vdc? Yes
The power supply is
operating normally.
No
Check Q862, C870, R865,
R870, C866, C869, and Z801.
Does the
power supply make
a soft "tick-tick"
sound?
No Yes
Check D864,R871 and C868.
Start
No Turn to next page.
Check R810
Caution:
Before removing or adding fuses,
remove all power from the
television and always use an Isolation
transformer when troubleshooting.
Troubleshooting Flowchart

14
With fuse F470 still open,
check the voltage on the power
supply side of F470 when the
television is first turned on.
Does the
voltage at F470 rise
to >100V then drop, or does
the voltage never
appear?
Voltage never appears.
Voltage appears then drops.
Using the
live ground, check
for 165V on pin 1
of Q801. No voltage.
Is pin 14
or 15 of Q801 shorted
to pin 12?
Voltage is present.
Yes No
Check F860,C810,
D801 and R810.
Change IC801 Check D876, R861, C869,
D873, D862 and D875.
Check Q862, C870, R865,
R870, C866, and C869.
Continued from the
previous page.
Caution:
Before removing or adding fuses,
remove all power from the
television and always use an Isolation
transformer when troubleshooting.

15
Shutdown Guide

16
Summary of Shutdown Circuits
Toshiba incorporates an elaborate shutdown cir-
cuit in every projection television for customer
safety and to prevent damaging the television if a
failure occurs. If the shutdown circuit engages,
neither the front panel power switch, nor the re-
mote power switch operates the TV. Refer to Fig-
ure 1. Although the standby powersupply remains
operational, power relay SR81 disengages to cut
the power to the main supply and the power LED
in the front of the TV blinks. To reset the shut-
down circuit, the AC power cord must be un-
plugged. If the power LED blinks, but the relay
stilloperates or remainsclosed,the television is not
ina shutdowncondition. Inthis scenario,theblink-
ingLEDindicatesthattheproblemisrelatedtothe
microprocessor’sserial clockand data lines. Ifthe
relay doesn’t energize or remains closed and the
power LED remains steady, the television is not in
shutdown. The main indicator of a shutdown con-
dition is that the relay immediately disengages af-
ter closing, and the power cord must be unplugged
and plugged back in for the relay to reengage.
Beware! The relay may disengage so quickly af-
terclosing, a person may only hear one “click” and
not realize the relay disengaged. To determine if
therelayis engaged, check the voltage drop across
the relay solenoid. An 11V drop across it engages
the relay.
Main Power Supply
Stand-By Power
Supply
Shutdown Curcuit
(Z801)
Microprocessor
(QA01)
Relay Drives
Q843 and QB30
SR81
12V
5V
The shutdown circuit
bypasses the microprocessor
to turn OFF the relay drivers.
AC Line Plug
+125V for main power
Because Z801 is supplied by the stand by 5V,
the TV must be unplugged to reset shutdown.
5V TV ON
0V TV OFF
15
16
11V drop
activates relay.
Figure1.
ShutdownOperation

17
Peak-Response Meter
The main component in the shutdown circuit is
Z801. When the shutdown circuit engages, Z801
holds pin 16 low (0 volts), causing the relay driv-
ers to turn OFF and de-energize the relay. This
causes all power in the TV to drop, except for the
standby power supply. Because Z801 is powered
by the standby supply, the television must be un-
plugged to reset Z801’s internal latch. Refer to
Figure 2. Three inputs to Z801 monitor for shut-
down. The first one is between pins 1 and 2 of
Z801. It is the 125V over current protect (OCP).
This monitors the current through the main 125V
supply and triggers the shutdown if the current is
excessive.
The second input to Z801 is an x-ray protect be-
tween pins 11 and 13. Pin 11 is the reference volt-
ageand pin 13 monitors a dc voltage developed by
the flyback transformer. Shutdown takes place
when the voltage on pin 13 rises above the voltage
on pin 11.
The third shutdown input is pin 14. This input
connects to several monitoring circuits. If any
monitoring circuit triggers, 1V or more is applied
to pin 14 to engage the shutdown.
When the relay opens at shutdown, the condition
that caused the shutdown disappears. This makes
troubleshooting difficult. Therefore, a peak-re-
sponse(or min-max)meter is requiredfor pinpoint-
ingthe momentary cause oftheshutdown. Apeak-
response meter measures and holds the highest
voltage that occurs at a test point.
D7701
Z801
13
11 R889
X-RAY REF.
14
16
Protect out to
relay driver QB30
0V = Shutdown
+ 5V = Normal
R7782
Q768
R7750
Q759
R7765
Q762
Q340
Q341
D315
R370
Q370
D370
D471
R470
+ 36V OCP
+ 18V OCP
- 18V OCP
+ 200V UVP
+ 35V OCP
X-Ray Protect
+ 125V OVP
1
2
OVP = Over Voltage Protect
OCP = Over Current Protect
UVP = Under Voltage Protect
Shutdown occurs with one of the following:
(a) the voltage on Pin 14 rises above 1V.
(b) the voltage on pin 13 rises above the 25V on pin 11.
(c) the voltage drop across R470 rises above 1.5V.
15
5V from standby
power supply
25V from main
power supply
+ 9V UVP
D428
+ 35V UVP
D429
Figure2.
BlockDiagramofShutdownCircuit

18
No Peak-Response Meter
Ifapeak-responsemeterisnotavailable,tryusing
anoscilloscopeon the dc setting. The scope reacts
quicker that the digital voltmeter, and the change
in dc level can be seen on the CRT of the scope.
However, most scopes do not have a dc voltage
readout or the ability to record the value. This can
makeitdifficulttogetanaccuratedcvoltageread-
ing. Therefore,the peak-response meter is the pre-
ferredmethod for measurement.
Caution: Alwaysuse an isolationtransformer when
troubleshootingtelevisions.
Monitoring Circuits
To help find the cause of a shutdown condition, it
is necessary to know the operation of each moni-
toring circuit and the conditions that trigger shut-
down. Thefollowing circuit explanations describe
theoperation of each monitoring circuit, give a test
point for each circuit, and provide troubleshooting
tipsto help in the repair of theTV. Use the trouble-
shootingflowchart at the end of this section to help
determinewhich monitoringcircuit is causingshut-
down. Please take note: the troubleshooting tips
and flowcharts in the following sections are in-
tendedas a troubleshooting guide,NOT an answer
toall situations.
Warning!!
Toshiba does not recommend disconnecting the
shutdowncircuit for troubleshooting. Disconnect-
ingtheshutdownincreasesthepossibilityofafail-
ure damaging the television.

19
X-Ray Protect
Refer to Figure3. The x-ray protect circuit senses
excessive high voltage. Pin 13 of Z801 compares
a voltage developed by the flyback transformer to
a reference voltage on pin 11 (approximately 25
volts). If the voltage on pin 13 increases to a value
GREATER than the voltage on pin 11, Z801 acti-
vates shutdown. To determine if x-ray radiation
protection is causing a shutdown condition, con-
nect a peak-response meter to pin 13. The normal
operating voltage is around 22V. If the voltage at
pin 13 rises above 25V, then excessive high volt-
age is most likely the cause of the shutdown. An-
other way the x-ray protect can trigger shutdown,
is by a loss of the 25V reference on pin 11. This
voltageis developed by themainpower supply and
usedfor the audio amplifier and horizontal start-up
circuit. The 25V on pin 11 should also be checked
using the peak-response meter.
TroubleshootingTips:
Problemswiththe horizontaloutput’sresonance ca-
pacitors,flyback transformer,deflection yokes, an-
ode caps, loss of the 25V-reference voltage, or a
shortedCRTmaytrigger theshutdown.
•TheCRTsare the most likely culprit with an x-
rayprotection shutdown. EachCRTcan bedis-
connectedseparatelyby disconnectingthe drive
PC board. The television can operate with one
oftheCRTs disconnectedwithout damagingthe
remainingCRTsortelevision. ACRTmayinter-
mittently arc and cause intermittent shutdown.
LIGHTLYtapping on theneck of the CRTmay
duplicatethe symptom. Takecaution whentap-
ping. Tappingtoohard can damage theCRT.
•The sealant around an anode cap on one of the
CRTsmaybecomeleaky. Ifthisoccurs,theseal-
ant has to be removed, the area on the CRT
cleaned with denatured alcohol, and the anode
capmust be replaced. Alsocheckthe other end
of the anode lead for proper contact with dis-
tributorZ450. The anode lead maybendinside
thedistributor block. Theanode leadcan beun-
plugged,straightened, and tined to reinforce its
integrity;otherwise,itshouldbereplaced. Thisis
coveredinservice bulletinTV9610.
•Ashortedsecondarywindingoftheflybacktrans-
formerordistributorblockcancauseanincrease
inthehighvoltage. Aringingcheckmayindicate
a bad flyback transformer. However, replace-
ment of the flyback transformer or distributor
blockmay bethe onlyway todeterminefor cer-
tainif theyare bad.
•If the resonant capacitors C444 and C440 be-
come leaky, the resonant frequency of the LC
circuit formed by the capacitors and flyback
transformer,wouldchangeandmayincreasethe
highvoltage. However, thisis very uncommon
withToshibatelevisions.
•Ashortedaudioamplifiermaypullthereference
25Vtoground causing the voltage on pin 11 to
dropbelowthevoltageonpin13,triggeringshut-
down. Pin 11 must be checked with a peak-
responsemeter because the main power supply
isnotoperational aftershutdown.
Figure3.
X-RayProtection
T461
D471
C471
R472
D885 R889
Z801
13
11
25V Reference
22V Normal
> 25V Shutdown

20
+125V Over Current Protect
Figure 4 shows the over current protect (OCP) for
the main 125V B+ line. Resistor R470 is the over
current sensing resistor. As the current increases
through the load, the voltage drop across R470 in-
creases. If the voltage drop rises to or above 1.5V,
Z801 engages the shutdown. The normal operat-
ing voltage across R470 is about 0.2V, and it can
have approximately a 1V drop at turn on due to
surgecurrent. To test thiscircuit,connect the peak-
response meter directly across resistor R470 and
measure the voltage drop at shutdown. Because
ofthe physical location of the resistor,it is easier to
takethe measurement from R470ratherthan Z801.
TroubleshootingTips:
Manythingscancausethe125VOCPtotriggershut-
down. A shorted horizontal output, flyback trans-
former, orthehorizontal output’sresonance capaci-
tors can pull excessive current through R470. An-
otherpossibilityisimproperpowersupplyregulation.
Themainpowersupply mayproduce toomuch cur-
rentandcausetheshutdown. Finally,R470canchange
valueandincreasethevoltagedropacrossitandcause
shutdown.
•Tocheckthemainpowersupply,refertotheMain
PowerSupply troubleshooting chart within the
mainpowersupplysection ofthismodule.
•Ashortedhorizontaloutputisthemostlikelycul-
pritof thisproblem. However,the outputmight
be shorted because of a shorted flyback trans-
former, arcing in the CRTs and anode caps, or a
shorted yoke. A ringing test may indicate a
shortedyoke or primary windingofthe flyback
transformer. Nevertheless, replacement of the
yokeand transformer mightbenecessary to de-
terminethefailed part. Fortipson troubleshoot-
ing the CRT, resonance capacitors, and anode
caps,refer to thex-rayprotect circuit in thepre-
viousparagraphs.
•Theover currentsensing resistor R470can increase
invalue and cause a false shutdown. R470’s value
mayincreaseonlyslightlyandcauseashutdowncon-
ditionintermittentlyorwhenthehighvoltagefirstcomes
updue to surgecurrent.
T862 D883
D884
R470
C472
R479 R471
Z801
1
2
Over Current
Monitor Point
To
Load
0.2V Normal
1.5V Shutdown
>
Figure4.
+125OverCurrent Protect