Schiller AT-102, Service Handbook

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2.4
AT-102 Service Handbook
MK 18 - 1 Main Board
Power Supply
The mains supply is full wave rectified to produce an unregulated dc supply of
approximately 30 V (+U). This voltage is used by a switched voltage generator to produce
+UD (13.5V). +UD charges the battery when mains is connected. When mains is not
connected, +UD is the battery voltage.
An ON/OFF control logic swUD to three voltage regulators. The unit is switched on
directly from the keyboard and then held on from the CPU. Detection of overvoltage on
either the 5.2 V or 24 V supplies directly switches the unit off. Similarly when an
undervoltage is detected on +US (indicating overcurrent) the unit is directly switched off.
The mains LED is lit directly when mains is connected. The same circuit also monitors the
switched dc supply (+US) and activates BATT when the unit is switched on and
mains is not connected (i.e. the unit is running on battery power).
A Battery low signal (BLOW) is set to logic 0 when battery voltage (+US) falls to 11.3 V. A
circuit compensates for voltage drop when the printer stepper motor is active and the BLOW
signal is active only at 10.3 V.
The battery voltage is also monitored directly by the CPU which switches the unit off when
the voltage falls below approximately 9.4 V.
Program and ECG Memory
A FLASH EPROM (electrically erasable) contains the unit software (512 kByte) and the
stored ECG data. The two memory blocks can be independently erased. It is possible to
update the software via the RS-232 serial interface.
The a serial EEPROM stores the unit base settings.
Thermal Print Head Controller
The Thermal Print Head is controlled by a print head controller and a CPU timer circuit. The
print head controller serialises the data for the print head and the timer circuit controls how
long current is applied to the head, and thus the intensity of the printout.