Sony HDW-750 Maintenance Manual page 89

. Power supply voltage measuring circuit
The power supply voltage measuring circuit (IC811 and
IC812) uses the DA output data from the FP CPU (IC801).
The two types of DC signal are output from the DA of the
FP CPU (IC801), and are used to change the voltage at
IC811 (pin-6) of the FP-121 board so that the voltage
measurement ranges are switched between the range of +9
V to +14 V, and the range of +12 V to +17 V. The result
of the voltage measurement is sent to the SS CPU (IC1) in
the form of a voltage value where the remaining amount of
battery and remaining data for display are calculated by the
SS CPU (IC1) on the SS-92 board. The created data is sent
back to the FP CPU (IC801). The FP CPU (IC801) shows
the remaining data at every serial communication event.
. PWM power supply circuit for FAN/FP circuit
The PWM control loop (Q813: Open Loop) precisely
controls the supply voltages by using the switching power
supply circuit. Because the voltage input at the primary
circuit of the power supply circuit is supplied from battery,
the input voltage changes significantly. Because of the
significant change of input power voltage, the cycle of the
PWM signal is changed in accordance with the result of
battery voltage measurement so that the FAN power
supply voltage is kept constant. The camera CPU (IC27) of
the AT-143 board that measures the CCD temperature also
determines the FAN control voltage. It sends the com-
mands through the SS CPU (IC1).
The switching circuit of the PWM circuit is shared so that
it can act as a power supply circuit for the FP microproces-
sor (so that the FP CPU (IC801) can work during power-
off) and the power supply circuit for the peripheral circuits.
The consumption current is reduced so that the +6 V
power is first generated by the PWM circuit. After that, the
series regulator circuit is activated. When the main power
is turned on, the +5 V power from MB-898 is sent to the
input circuit of IC805 to IC807 of the FP-121 board. When
the main power is turned off, +6 V power is input. The
output signal from IC805 and the diode D809 are connect-
ed as an OR gate so that the power supply of +5 V - 2 is
supplied all the time.
The output signal from CN107 (pin-38) of the FP-121
board is used to drive the fan. The FP CPU (IC801) can
operate during power-off from the power supplied from
CN107 (pin-39 and pin-40) of the FP-121 board.
. Soft control circuit at power-off
The FP CPU (IC801) detects the switch information. Only
when the FP CPU (IC801) and the SS CPU (IC1) of the
SS-92 board become ready to turn off, the main power can
be turned off. The FP CPU (IC801: pin-90) accepts the
input signal from the POWER switch. The FP CPU
(IC801: pin-91) supplies the output signal for power-off.
Only when this signal and the H_POWER_HOLD (IC801 :
pin-91) signal goes "L", the main power can be turned off.
However, the main power can be turned on simply by
turning the POWER switch on.
HDW-750/750CE V1
. FP CPU backup circuit
The FP CPU (IC801) is connected to a coin battery so that
the data in the internal RAM is kept saved even after the
power is turned off. By saving the system data, the FP
CPU (IC801) can start up very quickly when the power is
turned on.
IC802, IC808 and IC841 on the FP-121 board are identical
+2.5 V threshold voltage detector ICs. Because IC802 has
resistors R803 and R804, the detection voltage is +3.05 V.
Since IC808 has the delay circuit consisting of R983 and
C842, detection by IC808 is delayed after IC841.
The delay of detection is used as follows. A drop in the
voltage that is supplied to the CPU is detected by IC802
first and the processing to enter the STANDBY mode
finishes. Next, IC841 creates the hardware STANDBY
state (clock signal stops). Lastly, the FP CPU (IC801)
enters the reset state by IC808 waiting for the next power-
on. When the power is turned on, the STANDBY is
released, the clock signal oscillation starts and then reset is
released.
. Backup lithium battery voltage measurement circuit
The FP CPU (IC801) measures the output voltage of the
backup lithium battery BT101 (BP-33 board). When the
battery-end is detected, the low-voltage information is sent
to the SS CPU (IC1) of the SS-92 board. Because this
battery is used as the backup power for the watch IC
(IC809) of the FP-121 board, measurement is performed at
the low rate of once every minute to reduce battery
consumption together with the FET switch (Q804).
. Independent operation of FP CPU (IC801)
The power supply circuits are grouped so that the HDW-
750 can be operated even after the main power is turned
off. The power supplies of +3 V, +3.2 V, +5V-2 and +12
V are supplied even after the main power is turned off.
However, +12V-2 and +5 V are not supplied after the
main power is turned off.
. Automatic insert detection at the REAR XLR connectors
The LED drive signal that is used for the automatic insert
detection at the REAR XLR connectors, is supplied from
CN107 (pin-26) of the FP-121 board, and is sent to the
AXM-24 board. The return signal from the photo sensor on
the AXM-24 board is input to CN107 (pin-28 and pin-29)
on the FP-121 board. When the detection is set to ON by
the menu setting (section 4-6: MAINTENANCE menu,
VTR-MODE1 display: REAR XLR AUTO), the REAR
XLR input is selected regardless of the position of the
AUDIO IN SELECT switch.
1-26. Circuit Description
1-26-5. System Control
1-73