Replacing the Real-Time Clock (RTC) Battery
The socket for the RTC battery (BTI) is located on the auxiliary PCB at grid location 5D. Typical life of the
clock battery is 5 years.
Caution
: After replacing and RTC battery, switch the instrument, on then immediately switch it off; this
action will prevent possible damage to the RTC voltage circuit.
1. Disassemble the N-20P (see "Disassembly Procedure")
2. Using a thin flathead screwdriver, gently pry the RTC battery from its socket.
3. Insert a new battery into the socket, observing the polarity indication (socket's clip and battery's flat side are
positive)
4. Re-assemble the unit.
5. Switch on the instrument, then immediately switch it off (the RTC voltage circuit requires this step to prevent
possible damage to the unit). The instrument is now ready for normal operation.
6. Reset the clock (see "Setting Date and Time").
Replacing the PROM
A one-time-programmable PROM memory chip contains the system operations and diagnostic software. This
chip (U15) is located in a socket on the main PCB. If software updates are made available from the factory, or
if a problem with this chip is suspected, contact Nellcor's Technical Services Department of Nellcor's local
representative for information about a replacement chip.
1. Disassemble the N-20P (see "Disassembly Procedure").
2. After noting the chip's orientation, use a PLCC extraction device to remove the chip from its socket on the
main PCB.
3. Carefully insert the replacement chip into its socket.
Ensure the chip is correctly oriented.
4. Re-assemble the unit.
Note the alignment marks:
a small dot at the top of the chip and a small triangle at the top edge of the
socket should match up. Another key to ensuring correct orientation of the chip and socket is the notched
corner of the PROM is to be located at the top left of the socket.
Replacing Fuses
Two fuses (F1 and F2) are located on the auxiliary PCB. Fuse F1 may open to protect the CPU and its
associated components from damage if the power supply malfunctions. Fuse F2 may open to protect the printer
from damage due to excess voltage if the printer head jams or has been physically damaged. Refer to the
auxiliary PCB schematic for the locations of F1 and F2.
12-3