Switching Of Audio Signals; Monitoring Of The Usb Power Supply - Sony VPL-PX20 Service Manual

As shown in the figure, the QA board first switches between INPUT A (DSUB15), INPUT A (5BNC),
and INPUT B. The switching is performed by IC823 (RED), 824 (GREEN), and 825 (BLUE). SYNC is
performed by IC811. These ICs are controlled by the output of IC806 and IC807 (both DAC) controlled
by the CPU using the IIC bus.
Next the video signals input from the BA board and the RGB signals described above are switched by
IC801. This IC amplifies the signal switched by two times, terminates the signal at 75 Z in the later stage
board, and outputs it. This IC is also controlled by IC806.
As described earlier, SYNC is switched by IC811, but the H SYNC input from 5BNC has additional
conditions. When the signal input from 5BNC is 15 kHz, it is sync separated by IC805, and input to
IC811 in the later stage. If it is not 15 kHz, it is input directly to IC811.
The SYNC selected by IC811 is waveform-rectified by Q821 to Q826, IC812, and IC813.
It is then branched in three ways, output to the BA board, input to IC814 for monitor-out, and input to
IC802 for sync separation. After sync separation in IC802, it is switched with the SYNC from the video
block by IC803, converted to 3.3V, and output to the BB board. IC804 fixes the video clamp pulse to a
fixed pulse width.
The monitor-out outputs only signals projected to the screen from INPUT A (DSUB, 5BNC) and INPUT
B. IC808 to IC810 are amplifiers for monitor-out. IC814 functions as an output buffer for SYNC.

3-6-2. Switching of Audio Signals

Audio signals input are switched by IC819, and volume-controlled. The output of IC819 is separated into
that for the monitor-out and that for the speaker-out. The one for monitor-out is input to IC821, and
output from CN806 to the QC board. That for the speaker-out is converted to 3 channels by Q836 to
Q838. The L and R channel signals are then output to the KS board from CN810 while the mixed L and R
signals are output from CN809 to the K board. Q831 to Q835 compose the mute circuit.

3-6-3. Monitoring of the USB Power Supply

The USB download side must be supplied up to 500 mA at 5V. IC822 monitors this current amount. 5V
power is supplied to the download side from Pin 8 of IC822. When this exceeds 900 mA, the power is
stopped, and at the same time, the High signal is supplied from Pin 2 to the USB control IC (IC3303) in
the Y board.
IC822 also supplies power to the mouse driver (IC1201) for INPUT A on the QB board.
When a connector is connected to the upload side of the USB, a Low signal is input from the USB control
IC (IC3303) on the Y board to Pin 4 of IC822, and a 5V power is supplied from Pin 5.
VPL-PX20/PX30
3-11