Sony S75F Service Manual page 36

positive and down negative, so the secondary coil produces a reverse electromotive force. It is the
stage of storing energy in the coil because D902, D904 are closed. But while being given a high
level, The primary coil produces a EMF with its down terminal positive and up negative, and the
secondary coil appears a reverse EMF at the same time, then D902, D904 get through
accompanied with a voltage output of 12V and 5V through C904, C909 which are used to filter.
R901, C918 contained in the RC circuit is used to absorb the surge voltage produced by D902,
while R909, C907 is use to absorb the surge voltage produced by D904.
b. Inverter
Input interface section: 1.12V DC input, offered by power section; 2. ON/OFF enable signal, offered by Pin
68 of U201 in the main board with its value of 0V or 5V. When OFF=0V, the inverter doesn't work, while
OFF=5V, it works under normal situation. 3. DIM signal for luminance modulation, offered by Pin 98 of
U201 in the main board with its range of 0~5V. The Inverter offers different currents to the loads
because of the different feedback of voltage to the feedback-terminal of PWM. The smaller the value of
DIM, the smaller the current output of Inverter, the darker the brightness.
Voltage start-up circuit: It consists of a PNP and a NPN transistor with two working stages. The first stage:
When the ON/OFF enables voltage is 0V, the Q201 is closed and Q202 is closed too. The DC voltage
at collector of Q202 can't be applied to PIN9 of U201, so there are no pulse output at PIN10 and PIN7.
Hence, the Inverter don't work. The second stage: ON/OFF is high level, Q201 gets through and base
of Q202 is pulled down, thus Q202 gets through. Then 12V voltage is applied to PIN9 of U201 resulting
in the operation of IC. Hence, U201 produces pulse output to control switch transistor, and the Inverter
which applied high voltage across the backlight works well.
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S75F Sony