Service manual
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If there is no valid external DC line supply applied to the monitor, the monitor will default to use the battery.
DC from the rechargeable battery arrives on thermal fuse FS1, from where it is routed to the source of TR2,
a p-channel FET under the control of TR3. The battery supplies power to VRAW via transistor TR2 and
diode D28. When operating from a battery, VRAW will be in the range 10.4 - 13.5 V depending on the
battery charge. A sample of the battery’s terminal voltage is taken by R1 & R2 and applied to the ADC input
of the PIC (DC_BATTERY). From this, the PIC can determine the charge state of the battery.
Diodes D24 and D25 provide a power supply line VDC_OP which is present whenever a supply is available.
This supply is used exclusively for IC16, a programmable voltage regulator with internal comparator. IC16
performs two functions. First, its internal comparator compares the sampled line input voltage DC_INPUT to
an internal reference level. When the DC_INPUT is below approximately 10.4 volts, IC16 turns off TR14 to
indicate to the PIC that there is no valid external DC input. The second function of IC16 is to generate a
precision voltage supply from VDC_OP, set by R5 & R9 to 4.5 volts. This voltage is connected to the
BATT_BACKUP line by D13, D23 preventing the voltage from entering the NiCd backup battery. This
battery, B1, has a nominal terminal voltage of 3.6 volts and is trickle charged form the +15.1 volt supply
when the system is powered.
The VRAW supply, which is nominally 14.8 volts when operating from an external line voltage or 10.4 to
13.6 volts when operating from battery, is used to generate the other power supply lines. The monitor’s
power on switch is routed directly to the PIC, which controls the set up sequencing of the power rails. The
PIC switches on the +5VD line then the +VDISP line, the 12-volt supplies are controlled by the system
processor which turns them on after the +5VD supply is stable.
VRAW enters IC7, a step down DC-to-DC converter, which is used to produce the +5VD digital line. The 5
volt logic supply IC7 is controlled by the PSU_INH signal from the PIC. VRAW also connects to IC17 to
produce the +4.5 volt LED display supply +VDISP, which is controlled by the PIC signal ENABLE_VDISP.
The 12 volt line and other supplies are controlled by the 12V_ON signal from the system processor on the
Main Board. When this pin is at a high logic level, TR9 is turned on, as is p-channel FET TR10. This causes
the VRAW supply to enter IC8, a boost-switching regulator. R41 & R42 set the output voltage of the flyback
converter to 14.5 volts, which is further regulated by linear regulator IC9 to produce the pump and valve
supply +12VV line and linear regulator IC18 to produce the analog supply +12VR line. The output of TR10
also feeds the inverting switching regulator IC10, which produces a -14 volt supply to linear regulator IC19
that in turn produces the analog supply -12VR line. A sample of the +12VV line is taken by R44 & R45,
producing the PUMP_V signal, while a sample is taken across +12VR and -12VR lines by R47 & R48
producing the ANALOG_V signal. These are measured by ADCs inside the PIC to confirm the operation of
the power supplies.
2.4.7 Audio Amplifier
User warning sounds are generated by the power supply board PIC as a pulse width modulated digital
signal AUDIO_FREQ. This signal is fed to all the inputs of the quad bilateral switch IC14, their outputs being
connected together via a resistor ladder to produce a switched gain-summing amplifier IC6. The gain
elements are operated by the PIC signal A_GAIN0-3, together providing a 4-bit (16 level) volume control.
The output of the summing amplifier IC6 also applies band-pass filtering to the audio signal. The circuit
around IC6b sets the upper breakpoint of the filter, while the circuit on the input to the final amplifier IC15
sets the lower breakpoint and provides amplification for the speaker.
2.4.8 Alarm and Communication Interfaces
The remote alarm signal originates from the system processor on the Main Board, from where it is logic
ORed with the FAILSAFE signal, inverted and sent as the REMOTE_ALARM signal to the power supply
board. The signal controls n-channel FET TR11 to drive the coil of the relay RL1/2. The switch side of the
relay contains both normally open and normally closed contacts, which are separated from the monitor
electronics via an isolation barrier. A dual pole dual throw relay is used, but its switch elements are
connected in parallel to provide a single pole dual throw action with increased current capacity.