TOSHIBA
Service Training
PROJECTION
TELEVISIONS
Main Power Supply
NTDMOD02
& Complete
Shutdown Guide
TP43H60
TP43H95
TP50H15
TP50H50
TP50H60
TP50H64
TP50H95
TP55H60
Copyright 1999
TOSHIBA AMERICA CONSUMER PRODUCTS, INC.
NATIONAL SERVICE DIVISION
1420-B TOSHIBA DRIVE, LEBANON, TN 37087
TP55H64
TP55H95
TP61H60
TZ43V61
TZ50V51
TZ50V61
TZ55V61
TZ61V61
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................................................................................. 1 0
Soft Start ........................................................................................................... 1 0
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 .................................................................... 1 6
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 ............................................................................ 2 1
+18V and – 18V Over Current Protect .......................................................... 22
+35V Over Current Protect ............................................................................ 2 3
+9V Under Voltage Protect ............................................................................. 24
+35V Under Voltage Protect ...........................................................................24
+200V Under Voltage Protect ........................................................................ 24
Troubleshooting Flowchart ........................................................................... 25
Main Power Supply
3
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 acti ve when
the television is plugged into an AC line source.
This supply delivers a 5V VDD and a Reset 5V to
the microprocessor to keep the microprocessor functioning at all times, even when the television is not
operating. Transformer T840 isolates the standby
supply from the live ground, and D840 is a fullwave bridge rectifier that supplies 12Vdc to voltage regulator Q840 and relay SR81 (connection
not shown). 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.
Stand-By Supply
Rectifier &
Isolation Trans.
D840
T840
AC Line
Input
QB30
Q843
SR81
Relay
Stand-by
Regulator
Q840
5V=Relay On
0V=Relay Off
Relay Drivers
Rectifier
D801
+5-1
From Shutdown
Main Switching IC
165V
Microprocessor
5V
VDD
5V
Reset
QA01
Q801
Feedback
4
7
C870
T862
Photo
Coupler
Q862
When the relay closes, the A C line input is applied
directly to the main power supply . The supply starts
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. Detailed explanations of the main power supply are
covered in the remainder of this section.
Troubleshooting Tip:
If SR81 never closes, check the standby power sup-
ply . Both the 5V VDD and the Reset 5V are mandatory for the microprocessor to operate.
Main Power Supply
To Q752
Conv. Output
To Q752 & Q751
Conv. Outputs
To Digital Conv. PC,
Q764 & Q767
To Q752
Conv. Output
To Q752 & Q751
Conv. Outputs
To Digital Conv. PC,
To Q601 Audio Out
To Q501
Horizontal Start-up
To Shutdown & Surge protect:
Q846,SR82,Q757, & Q758
+9-2 To Numerous Circuits
+5-2 To Numerous Circuits
+9-1 To Numerous Circuits
+5-3 To PIP Module & QBB3
To Horizontal Output
To Tuner(s)
Reg.= Regulator
Conv.= Convergence
11
9
9
10
12
13
14
15
16
17
Rectifiers
D856
D855
D851
D854
D886
D889
D891
D894
D882
D884
Error
Amp.
+36
+18
-18
+38
+12
+125
Z801
Q760
Q754
Q755
Q756
Q430
Q832
Q830
Q831
D101
30V
Reg.
5V
Reg.
9V
Reg.
-9V
Reg.
9V
Reg.
9V
Reg.
5V&9V
Reg.
5V
Reg.
32V
Reg.
Figure 1.
Power Supply Block Diagram
4
Operation
The main power supply is a current resonant switching power supply. Figure 2 is a basic block diagram for this supply . The primary winding of T862
and capacitor C870 create an LC series resonant
circuit. An oscillator (OSC), dri ve circuit, and two
MOSFETs are located internally to Q801 (STRZ4117). The OSC determines the power supply’ s
switching frequency . The dri ve 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
continually builds and collapses an electromagnetic
field around T862’s primary windings. The collapsing of the electromagnetic field induces current into the secondary windings of T862. A fullwave bridge rectif ier con verts the induced current
into 125Vdc.
To regulate the 125Vdc, an error amplifier moni-
rent, the oscillator frequency decreases and operates closer to the LC resonant frequency . The closer
the switching frequency is to resonance, the higher
the current flow through the primary windings of
T862 and the larger the electromagnetic field. The
larger the electromagnetic field is when it collapses,
the higher the induced current is in the secondary
winding. When the load decreases and requires
less current, the switching frequency increases and
moves aw ay from resonance. As a result, less current is induced in the secondary windings.
Load
Current
Increased
Load
O
S
C
D
r
i
v
e
Q801
T862
C870
Photo
Coupler
Q862
125V
Error
Amp
Z801
+B
Figure 2
Operational Block Diagram
tors the voltage and supplies a negative feedback
to the oscillator through photo-coupler Q862.
Q862 isolates the primary side of the power supply from the secondary side.
Refer to Figure 3. The power supply’s switching
frequency operates above the LC resonant frequency. When the load on the secondary side of
the power supply increases and requires more cur-
Decreased
Load
Resonant Freq.
Normal Operating Frequency is 70-80kHz
Operating Freq.
Switching
Freq.
Figure 3
Load Current Characteristics
5
Surge Protection Relay
Figure 4 shows the surge protection oper ation. T o
prolong the life of the power supply , a sur ge cir cuit reduces current through the main power supply 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. During this time, the power supply draws
a large amount of current. T o 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.
Troubleshooting Tip:
R810 is a fusible resistor . If SR82 does not close
after the power supply is fully operational, R810 eventually opens and prevents the power supply from operating. Whenever troubleshooting the power supply , check R810 first. If R810 is open, replace it,
using T oshiba part number 24007061, and look for
cold solder joints around Q846 and R846.
Relay
SR81
R809
R810
Main Power Supply
Q801
D801
T862
SR82
Figure 4
Surge Protect
and
125V
12V
R846
Q846
6
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 circuit. After start-up, a dri v e circuit consisting of a
secondary winding of T862, diode D864, and capacitor C868 supply 16-20Vdc to pin 8 of Q801 to
maintain its operation. The voltage developed by
the drive circuit fluctuates with the switching frequency of the power supply. Therefore, the voltage on pin 8 is also applied to an over voltage protect (OVP) block internal to Q801. If the v oltage
on pin 8 increases to 25V, the OVP triggers the
latch and switching stops. Refer to the Latch section for further information. D876 is a 27V zener
diode that protects Q801 by preventing excess voltage increases on pin 8.
From
D801
R861
16-20 Vdc
OVP
8
Start
D876
Vcc
Q801
D864
Figure 5
Start-up and Over Current Protect
R871
T862
C868
7
Logic and Drivers
DT
OSC
R867
Q801
7
Oscillator
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 powers driver B internally . T o power dri ver A, 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.
D873
C873
R862 D862
D875
C863
IN
V
1
B+
130V
Out
14
COM.
12
To
T862
8
Start
Vcc
DRI Vcc
915
Logic
VB
Q801
A
B
Refer to Figure 7. Q801’s internal Oscillator develops the power supply’ s switching frequenc y by
generating a ramp waveform internally. During
normal operation, both MOSFETs are OFF for a
short time when they are alternately switching. This
OFF time is called dead time and determined by
the value of resistor R867 on the dead time (DT)
terminal pin 7.
Figure 7
Q801’ s Internal Oscillator
Figure 6
Logic Circuit
8
Oscillator Control
6
CD
TSD OVP
LatchDelay
OC/RC
C869
Q801
1 V
IN
Latch
If the load current drawn from the 125Vdc line increases, the 125Vdc voltage begins to drop, decreasing the current through Q862’s LED side. The
current drop causes the LED to couple less light to
the photo transistor side and reduce the current flow
into pin 4 of Q801. This reduction in current flow
varies the OSC frequency , mo ving it closer to resonance 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 decrease the current supplied to the load and level the
125Vdc.
125 Vdc
Load
OSC
Control
OSC
The latch block stops the operation of Q801 until
the voltage on pin 1 of Q801 is removed by turning OFF the television. As outlined in Figure 9,
any one of the following detection blocks can trigger the latch.
• Over voltage protection (O VP) Block (Refer
to Start-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
The thermal shock detection block triggers the latch
if Q801’s internal temperature e xceeds 150°C.
B+
1
Z801
Out
3
R883
Q862
R864
Q862
R884
C891
Figure 8
Oscillator Control
4Cont.
Q801
Figure 9 Latch
9
Over Current Protection
165V from D801
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 develops at the over current protect (OC) terminal pin
10. Resistor R870 and C874 detect the current
flow through the LC circuit. Resistor R866 samples
the voltage and applies it to pin 10. Capacitor C867
is a filter to reduce ripple. Depending on the input
voltage on pin 10, the over 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.
By reengaging the soft start, the main oscillator frequency increases, reducing the current flow through
the LC circuit. The soft start continues to operate
until the voltage on pin 10 drops below 2V.
R872
OC
10
C866
Q801
OC
5
Css
C867
OSC
Control
RC
11 RC
C872
R866
To pin 14,
Q801
R870
OSC
T862
C874
C870
(2) OC High Threshold Voltage: +2.5V
If the input voltage at the OC terminal exceeds
+2.5V , the oscillator frequenc y increases to its maximum frequency and C866 discharges rapidly.
When the Css terminal voltage decreases to 0.7V,
the circuit resets and C866 charges again. The main
oscillator frequency decreases gradually. If this
condition continues, the latch engages and oscillation stops.
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. While capacitor C866 charges,
the switching frequency increases to reduce surge
current through the MOSFETs. Once C866 is fully
charged, the switching frequency goes to its normal operating frequency (approximately 70-80
kHz).
Figure 10
Over Current Protect and Soft Start
Resonance Correction
By monitoring the current flow through pin 11, The
resonance correction (RC) block prevents the
oscillator’s switching frequency from dropping
below the LC resonate frequency. As the switching frequency 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.
10
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)
To
From
C869
CD
Q801
TSD OVP Start
6
Delay
Out
R861
to VB
Vcc DRI Vcc
Latch
Ref.
Logic
From
DRI Vcc
15 VB98
A
1
V
14
Out
IN
Pin Descriptions
Pin Name Vdc Description
4 CONT 7.2 Oscillator control terminal
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
From
R866
10
OC
OC
Css Cont. DT GND
From
C866
Control
From
R864
OSC
OSC
From
R867
11
RC
B
RC
354 7
From
C872
1VIN165 Half bridge power input
3 GND 0 Control unit ground
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
12
COM.
11
Waveforms
Q801 Internal
Internal Q801
Oscillator
Dead Time
1.5Vpp
Internal Q801
MOSFET (A)
Internal Q801
MOSFET (B)
Pin 14, Q801
Across C870
On Off On
Off On Off
160Vpp
140Vpp
12
Troubleshooting Flowchart
Caution:
Before removing or adding fuses,
remove all power from the
television and always use an Isolation
transformer when troubleshooting.
Yes No
Check R810
Start
Open fuse F470.
Does relay
SR81 close when the
Power button is
pushed?
Check the relay drive,
microprocessor and standby
power supply circuits.
No Yes
Using the isolated
ground, check the
voltage on the power
supply side of fuse
F470 with the fuse
still removed.
Yes
Connect a 100W
light bulb to F470’s
supply side.
Does the
power supply make
a soft "tick-tick"
sound?
Does a
constant voltage
appear at F470’s
supply side?
Check D864,R871 and C868.
No
Turn to next page.
No
Check Q862, C870, R865,
R870, C866, C869, and Z801.
Does the
voltage regulate
at 125Vdc?
13
Yes
The power supply is
operating normally.
Caution:
Before removing or adding fuses,
remove all power from the
television and always use an Isolation
transformer when troubleshooting.
Voltage appears then drops.
Continued from the
previous page.
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.
Check Q862, C870, R865,
R870, C866, and C869.
Yes No
Change IC801
Voltage is present.
Is pin 14
or 15 of Q801 shorted
to pin 12?
Using the
live ground, check
for 165V on pin 1
of Q801.
Check D876, R861, C869,
D873, D862 and D875.
No voltage.
Check F860,C810,
D801 and R810.
14
Shutdown Guide
15
Summary of Shutdown Circuits
Toshiba incorporates an elaborate shutdown circuit 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 remote power switch operates the TV. Refer to Figure 1. Although the standby po wer supply 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 shutdown circuit, the AC power cord must be unplugged. If the power LED blinks, but the relay
still operates or remains closed, the television is not
AC Line Plug
in a shutdown condition. In this scenario, the blinking LED indicates that the problem is related to the
microprocessor’s serial clock and data lines. If the
relay doesn’t energize or remains closed and the
power LED remains steady , the tele vision is not in
shutdown. The main indicator of a shutdown condition is that the relay immediately disengages after closing, and the power cord must be unplugged
and plugged back in for the relay to reengage.
Beware! The relay may disengage so quickly after closing, a person may only hear one “click” and
not realize the relay disengaged. To determine if
the relay is engaged, check the voltage drop across
the relay solenoid. An 11V drop across it engages
the relay .
Because Z801 is supplied by the stand by 5V,
the TV must be unplugged to reset shutdown.
SR81
Stand-By Power
Supply
Main Power Supply
5V
12V
11V drop
activates relay.
Relay Drives
Q843 and QB30
5V TV ON
0V TV OFF
Figure 1.
Shutdown Operation
Microprocessor
(QA01)
15
16
The shutdown circuit
bypasses the microprocessor
to turn OFF the relay drivers.
+125V for main power
Shutdown Curcuit
(Z801)
16
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 drivers 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 unplugged to reset Z801’s internal latch. Refer to
Figure 2. Three inputs to Z801 monitor for shutdown. 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.
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.
The second input to Z801 is an x-ray protect between pins 11 and 13. Pin 11 is the reference voltage and 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-response (or min-max) meter is required for pinpointing the momentary cause of the shutdown. A peakresponse meter measures and holds the highest
voltage that occurs at a test point.
Figure 2.
Block Diagram of Shutdown Circuit
+ 36V OCP
+ 18V OCP
- 18V OCP
R7782
Q768
R7750
Q759
R7765
Q762
5V from standby
power supply
D7701
OVP = Over Voltage Protect
OCP = Over Current Protect
UVP = Under Voltage Protect
Protect out to
relay driver QB30
0V = Shutdown
+ 5V = Normal
1
16
15
14
Z801
D315
D370
2
11
13
+ 125V OVP
X-RAY REF.
R889
Q340
Q341
R370
Q370
+ 200V UVP
+ 35V OCP
D428
D429
R470
25V from main
power supply
D471
X-Ray Protect
+ 9V UVP
+ 35V UVP
17
No Peak-Response Meter
If a peak-response meter is not available, try using
an oscilloscope on the dc setting. The scope reacts
quicker that the digital voltmeter, and the chang e
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
make it difficult to get an accurate dc voltage reading. Therefore, the peak-response meter is the preferred method for measurement.
Caution: Alw ays use an isolation transformer when
troubleshooting televisions.
Monitoring Circuits
T o help f ind the cause of a shutdo wn condition, it
is necessary to know the operation of each monitoring circuit and the conditions that trigger shutdown. The following circuit explanations describe
the operation of each monitoring circuit, give a test
point for each circuit, and provide troubleshooting
tips to help in the repair of the TV. Use the troubleshooting flowchart at the end of this section to help
determine which monitoring circuit is causing shutdown. Please take note: the troubleshooting tips
and flowcharts in the following sections are intended as a troubleshooting guide, NOT an answer
to all situations.
Warning!!
Toshiba does not recommend disconnecting the
shutdown circuit for troubleshooting. Disconnecting the shutdown increases the possibility of a failure damaging the television.
18
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 activates shutdown. To determine if x-ray radiation
protection is causing a shutdown condition, connect 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 voltage is most likely the cause of the shutdown. Another way the x-ray protect can trigger shutdown,
is by a loss of the 25V reference on pin 11. This
voltage is developed by the main power supply and
used for the audio amplifier and horizontal start-up
circuit. The 25V on pin 11 should also be checked
using the peak-response meter .
Troubleshooting Tips:
• A shorted secondary winding of the flyback trans-
former or distributor block can cause an increase
in the high voltage. A ringing check may indicate
a bad flyback transformer. However , replacement of the flyback transformer or distributor
block may be the only way to determine for certain if they are 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, would change and may increase the
high voltage. However, this is very uncommon
with T oshiba televisions.
• A shorted audio amplifier may pull the reference
25V to ground causing the voltage on pin 11 to
drop below the voltage on pin 13, triggering shutdown. Pin 11 must be checked with a peakresponse meter because the main power supply
is not operational after shutdown.
Problems with the horizontal output’ s resonance capacitors, flyback transformer, deflection yokes, anode caps, loss of the 25V-reference voltage, or a
shorted CR T may trigger the shutdown.
• The CRTs are the most likely culprit with an x-
ray protection shutdown. Each CRT can be disconnected separately by disconnecting the drive
PC board. The television can operate with one
of the CRT s disconnected without damaging the
remaining CR Ts or television. A CR T may intermittently arc and cause intermittent shutdown.
LIGHTL Y tapping on the neck of the CR T may
duplicate the symptom. T ake caution when tapping. T apping too hard can damage the CRT .
• The sealant around an anode cap on one of the
CR Ts may become leaky . If this occurs, the sealant has to be removed, the area on the CRT
cleaned with denatured alcohol, and the anode
cap must be replaced. Also check the other end
of the anode lead for proper contact with distributor Z450. The anode lead may bend inside
the distributor block. The anode lead can be unplugged, straightened, and tined to reinforce its
integrity; otherwise, it should be replaced. This is
covered in service bulletin TV9610.
19
D885 R889
R472
T461
25V Reference
D471
C471
22V Normal
> 25V Shutdown
Figure 3.
X-Ray Protection
11
Z801
13
+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 increases. If the voltage drop rises to or abo ve 1.5V,
Z801 engages the shutdown. The normal operating voltage across R470 is about 0.2V, and it can
have approximately a 1V drop at turn on due to
surge current. T o test this cir cuit, connect the peakresponse meter directly across resistor R470 and
measure the voltage drop at shutdown. Because
of the physical location of the resistor, it is easier to
take the measurement from R470 rather than Z801.
Troubleshooting Tips:
Many things can cause the 125V OCP to trigger shut-
down. A shorted horizontal output, flyback transformer, or the horizontal output’ s resonance capacitors can pull excessive current through R470. Another possibility is improper power supply regulation.
The main power supply may produce too much current and cause the shutdown. Finally , R470 can change
value and increase the voltage drop across it and cause
shutdown.
• The over current sensing resistor R470 can increase
in value and cause a false shutdown. R470’s value
may increase only slightly and cause a shutdown condition intermittently or when the high voltage first comes
up due to surge current.
Over Current
Monitor Point
0.2V Normal
>
1.5V Shutdown
T862
D883
D884
R470
R479 R471
C472
2
1
To
Load
Z801
• T o check the main power supply , refer to the Main
Power Supply troubleshooting chart within the
main power supply section of this module.
• A shorted horizontal output is the most likely cul-
prit of this problem. However, the output might
be shorted because of a shorted flyback transformer, arcing in the CR T s and anode caps, or a
shorted yoke. A ringing test may indicate a
shorted yoke or primary winding of the flyback
transformer. Nevertheless, replacement of the
yoke and transformer might be necessary to determine the failed part. For tips on troubleshooting the CR T, resonance capacitors, and anode
caps, refer to the x-ray protect circuit in the previous paragraphs.
Figure 4.
+125 Over Current Protect
20
+36V Over Current Protect
Troubleshooting Tips:
Figure 5 is the circuit diagram for the +36 over
current protect. Resistor R7782 is the over current
sensing resistor. It monitors the current flo w to the
conv ergence pump-up circuit. An increase in current increases the voltage drop across R7782.
During normal operation, Q768 and Q758 are
turned OFF and Q757 is ON. Because Q757 is
turned ON, the voltage at its collector is 0V. A
slight increase in voltage across R7782 turns ON
Q768 and increases its collector voltage. Then,
Q758 turns ON, and its collector voltage drops to
ground and turns OFF Q757. The emitter-collector current of Q757 stops, and the voltage on the
collector rises to a logic HIGH (approximately 2.1V
or higher). The logic HIGH is applied to pin 14 of
Z801 through D7701, and shutdown takes place.
Because Q758 and Q757 are also controlled by
Q759, the collector of Q768 should be used as the
test point. A voltage of 5V or GREATER at this
point indicates the transistor is turning ON and activating shutdown.
+36V
R7784
R7782
C7770
Load
R7783
Stand by
+5V
• If excess current is pulled from the power supply ,
check the convergence output ICs (Q752 and
Q751 located on the convergence output PC
board) and the surrounding biasing resistors. The
digital convergence board can cause Q752 and
Q751 to work too hard and pull excess current.
If this is suspected, remove the digital convergence board from the television with the television unplugged. Plug the television back in and
turn it ON if necessary . If the television comes
ON*, the digital convergence board may be bad.
If the television still shuts down, Q752, Q751, or
their surrounding biasing circuits may be bad.
* The television can power up without the digital convergence board in place, but the television will be out
of convergence. The raster bows in from all sides
because the horizontal and vertical scanning are not
going all the way to the edges of the CRTs. Do not
let the television run for an extended time in this condition. If left in this condition long enough, it can burn
the phosphorous. If additional testing is required in
this condition, turn the contrast and brightness all the
way down to reduce the risk.
• The over current sensing resistor can increase in
value and cause a false or intermittent shutdown.
Make certain the current sensing resistor is the
proper value.
Q768
R7785
0V Normal
>
5V Shutdown
R7742
D7701
R7745
To pin 14
of Z801.
Q758
Q757
From
Q759
Figure 5.
+36V Over Current Protect
21
+18V and – 18V Over Current Protect
To pin 14
of Z801.
1.5V Normal
-5V Shutdown
>
Load
Q762
C7763
R7764
R7763
R7765
R7758
R7742
D7701
-18V
Stand-By
+5V
Q761
Q757
R7757
R7745
The +18V and –18V over current protect operates
in a similar fashion as the +36V over current protect. Refer to Figures 6 and 7 for the circuit diagrams.
+18V
R7751
Q759
R7747
0V Normal
>
5 V Shutdown
R7750
R7749
C7760
R7745
Q758
From
Q758
Figure 6.
+18V Over Current Protect
Load
Stand by
+5V
R7742
D7701
To pin 14
of Z801.
Q757
Figure 7.
-18V Over Current Protect
22
+35V Over Current Protect
Troubleshooting Tips:
As outlined in Figure 8, +35V develops at the cathode of diode D302 during normal operation. That
voltage is a supply for the vertical output Q301,
vertical blanking, the DPC, and high voltage regulation circuits, and it is monitored by the +35V over
current protect (OCP). During normal operation,
current flows through the current sensing resistor
R370. If the load current becomes excessive, the
voltage drop across R370 increases and turns ON
transistor Q370. When Q370 turns ON, the collector voltage increases towards the +35V supply .
Zener diode D370 conducts and delivers a voltage
to pin 14 of Z801. Use the peak-response meter
on the collector of Q370 for a test reading and the
“process of elimination” to determine if the load is
drawing excessive current. If Q370 is being turned
ON, check the +9 under voltage protect (UVP) and
+35 UVP. If neither of the UVP circuits are turning Q370 ON, an increase voltage drop across
R370 is the cause.
+35 V
D302
T461
C310
R370
Load
R372
The vertical output Q301 and the high voltage regulation IC Q483 are located on the deflection PC
board. Either of these ICs or the dynamic pincushion
(DPC) circuit are the likely causes of excessive current draw from the + 35V line.
• If the vertical output fails, usually , pins 1 and 2 or
1 and 6 short together.
• If the DPC board is suspect, look for discolora-
tion or cold solder joints on the PC board. The
discoloration may give an indication of which component is drawing excessive current. Also wiggle
the PC board. It is possible the female connector (part number: 23902863) on the deflection
board is bad. Try to resolder or clean the connector; otherwise, replace it. Refer to service
bulletin TV9625
• IC Q483 is part of the high voltage regulation
circuit. Pins 1 or 9 of the IC can draw excessive
current and trigger shutdown. These pins can be
disconnected for troubleshooting. The television
can power-up with the regulation circuit inoperative. With no regulation, the picture height and
width change with picture brightness.
• Check the + 9V and +35V under voltage protect
circuits.
0V Normal
>
9V Shutdown
+9V Under Voltage Protect, and
R371
D428
Q370
R373
D370 D371
(30V)
D429
(30V)
Figure 8.
+35V Over Current Protect,
+35V Under V oltage Protect
+9V
+35V
To pin 14
of Z801.
23
+ 9V Under Voltage Protect
0V Normal
3.6 V Shutdown
>
R389
R347
Q340
R390
200V
+ 9V
D341
Q341
R346
R392
C340
D315
+200V Under Voltage Protect
Figure 8 also shows the +9V and +35V Under
Voltage protect circuits. An under voltage protect
(UVP) circuits monitors the +9V line which is regulated from the 12V supply. The 12V supply is developed at the secondary side of the main power
supply . Refer to Figure 8. If the 9V drops belo w
5V, zener diode D428 conducts and turns ON
Q370. When Q370 turns ON, the collector voltage increases toward the +35V supply. Zener diode D370 conducts and delivers a voltage to pin
14 of Z801. Use the peak-response meter on the
anode of D428 to determine if the +9V is present.
Troubleshooting Tip:
If the + 9V UVP is causing shutdown, check fusible
resistor R830 and the voltage regulation circuit consisting of transistor Q830 and zener diode D830.
Refer to the appropriate service manual for part numbers and schematic diagrams.
+ 35V Under Voltage Protect
The flyback transformer T461 produces about 200V
at the cathode of D406 (not shown). A voltage
divider drops it to 6.8V and applies it to the base of
Q340. Under normal operation, Q340 is turned
ON thus, keeping Q341 turned OFF with a collector voltage around 0 V. Refer to Figure 9.
If the 200V drops to about 160V , Q340 turns OFF,
and Q341 turns ON. When Q 341 turns ON, its
collector voltage goes to about 6 volts. The 6 volts
sends a logic HIGH to pin 14 of Z801 and triggers
shutdown.
Troubleshooting Tip:
The dynamic pincushion circuit (DPC) is a plug-in
module and the most likely cause of triggering the
200V UVP circuit. The 200V UVP is located on the
DPC module. It is possible the female connector (PN
2390286) that the DPC plugs into is bad. This would
result in a loss of 200V to the UVP . T ry to re-solder
or clean the connector; otherwise, replace it. Refer
to service bulletin TV9625.
The + 35V lines operates in the same manner as the
+9V UVP . Refer to that circuit’ s explanation for the
operation.
Troubleshooting Tip:
Check the high voltage regulation circuit: Q483, R497,
R485, D483, and D481. Q483 can be disconnected
for troubleshooting. With Q483 out of circuit, the
television picture fluctuates with brightness, but the
television should power-up and display a picture.
Refer to the appropriate service manual for part numbers and schematic diagrams.
Figure 9.
+200V Under Voltage Protect
24
Troubleshooting Flowchart
Caution:
Before removing or adding
fuses, remove all power from
the television and always use
an Isolation transformer when
troubleshooting.
No
Connect a 100W light
bulb between the power
supply side of F470 and
isolated Ground. Check
the voltage across it.
Start
Remove fuse
F470.
Apply
power to the TV.
Does it still
shutdown?
Notes:
This flow chart is to help isolate
the cause of shutdown. Refer
to the circuit explanations for
additional information.
With F470 open, the power
LED blinks whether the
television is in shutdown or not.
The indication of shutdown is
the relay disengaging after
closing, and the power must be
removed and restored to the
TV to re-engage the relay.
Key:
OVP = Over Voltage Protect
OCP = Over Current Protect
UVP = Under Voltage Protect
Yes
Check the +18V OCP
-18V OCP and + 36V OCP
(Section one)
Yes No
Remove light bulb and put
F470 back in circuit.
Check Q370’s
Collector with a
Peak-response meter.
No
Check +125V OCP, X-ray
Protect, and +200V UVP.
(Section three)
Does the
voltage regulate
at +125V?
Does
Q370’s peak
collector voltage go
to or above 9V?
Check the main power
supply.
Yes
Check +35V OCP, +9V UVP
and +35V UVP
(Section two)
25
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