Circuit Description
GB 87
A10PTV2.0 NTSC
9.
Figure 9-7
9.5
Full Power Supply
When the set is turned on, the Main Switch Mode power
supply is turned On This supply produces the +130 volt, +32
volt, -32 volt, two +15 volt, and Audio supplies. When the On/
Off line goes low, transistor 7309 is turned off, turning
transistor 7300 on. This switches relay 1305 on. Startup
voltage from the neutral side of the AC mains will charge
capacitor 2303 to 14.5 volts, which will overcome the
undervoltage lockout of IC 7302. After the Soft start
capacitor 2443 charges; the oscillator inside the IC will turn
on. Each cycle of the oscillator will set the flip-flop, which will
cause Pin 3 to go high. This will turn the FET switch 7301 on.
Voltage is applied to the Drain of 7301 through Pins 4 and 8
of 5300 from the Power Factor Correction circuit. Current
through sensing resistors 3308 and 3309 will develop a
voltage, which is applied to comparator “B” connected to Pin
7. When the voltage on Pin 7 reaches the reference voltage
on the inverting input, the flip-flop will be reset. The voltage
on the inverting side of comparator “B” is limited to 1 volt.
Therefore, the ramp voltage on Pin 7 will not exceed 1 volt.
The circuit will continue to operate until the charge on
capacitor 2303 falls below 9.4 volts shutting the IC Off. Each
time 7301 is turned on; energy is stored in transformer 5300.
Voltage from the hot secondary on Pin 10 is rectified by 6301.
When the output of this circuit has sufficient energy to
maintain 2303 above 9.4 volts, the IC then operates in steady
state.
When IC 7302 develops a normal steady state operation, the
130-volt supply is sampled by resistors 3324, 3323, and
3322. This sample voltage is then sent to Shunt regulator
7303, which drives the feedback optoisolator 7303. The
feedback voltage on Pin 14 is then compared with an internal
2.5-volt reference by comparator “C”. Comparator “C” then
sets the reference voltage on the inverting side of comparator
“B” to control the On time of the drive at Pin 3. If the voltage
on Pin 14 increases due to the 130-volt supply increasing,
the On time of the pulse on Pin 3 will be reduced. If the 130-
volt supply decreases, the voltage on Pin 14 would decrease,
causing the On time of the pulse on Pin 3 to increase. This
is used to keep the 130-volt supply at the correct voltage.
Variable resistor 3322 is used to adjust the 130-volt supply to
the correct level.
The overvoltage protection circuit of the IC will shut the IC
down if the VCC level on Pin 1 exceeds 17 volts.
To troubleshoot this circuit, first check the On/Off line from
the Small Signal Panel to ensure that it is going low. Then
check for the presence of startup voltage on Pin 1 of the
regulator IC 7302. If the IC is working correctly, this voltage
will be changing from 9.4 to 14.5 volts. If the startup voltage
is not present, check the startup resistor 3300 and the bridge
rectifier. If the voltage on Pin 1 is changing, check the drive
signal on Pin3. Then check for signal on the gate and drain
of 7301. An excessive load on the secondary, a short on the
130-volt line for example, would cause the supply to pulse
with little or no voltage on the secondary.
CL 06532143_014.eps
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