Over Current Protect - Toshiba TP43H60 Service Training

+35V Over Current Protect
As outlined in Figure 8, +35V develops at the cath-
ode of diode D302 during normal operation. That
voltage is a supply for the vertical output Q301,
vertical blanking, the DPC, and high voltage regu-
lation 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 col-
lector 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 turn-
ing Q370 ON, an increase voltage drop across
R370 is the cause.
+35 V
D302
T461
C310
Q370
0V Normal
>
9V Shutdown
Figure 8.
+35V Over Current Protect,
+9V Under Voltage Protect, and
+35V Under Voltage Protect
R370
Load
R372
R371
D428
(30V)
D429
(30V)
R373
To pin 14
of Z801.
D370 D371
Troubleshooting Tips:
The vertical output Q301 and the high voltage regu-
lation 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 cur-
rent 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 com-
ponent is drawing excessive current. Also wiggle
the PC board. It is possible the female connec-
tor (part number: 23902863) on the deflection
board is bad. Try to resolder or clean the con-
nector; 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 inopera-
tive. With no regulation, the picture height and
width change with picture brightness.
• Check the + 9V and +35V under voltage protect
circuits.
+9V
+35V
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