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Model 890412 Service
HOW
TO
REPLACE THE MEMORY BACKUP BATTERY
Description
The battery that backs up non-volatile memory has
a
limited lifetime. It should be replaced
every five years or whenever it shows
signs
of deterioration. Replacing the battery must be done
carefully
so
as to not interrupt the contents of the RAM. If the RAM looses its contents, the entire A2
Digital Assembly will have to be replaced because its serial number and option information will be
lost.
Replacing the backup battery consists
of
unplugging and freeing the A2 Digital Assembly, wiring a
power supply and resistor to RAM power supply pin, removing the old battery and soldering in the
new one, removing the power supply backup, and re-assembling the instrument.
The normal precautions against electrostatic discharge must be followed but
do
not place the
A2
Digital Control Assembly
on
a conductive metal
surface,
which could short the battery or the
RAM
power supply pin.
The
substitute power supply
must
be floating
if
the
solder
iron
has
a
grounded
tip. (All soldering
is
done with
the
HP8904A
completely disconnected
from
the power
line.)
Do
not apply too much heat to printed circuit connections when soldering.
The traces on the multilayer board are easily damaged.
Do
not overstress
the
interconnecting cables.
Tools
and Supplies
Desoldering Tool Resistor, 10 kR
Needle Nose Pliers Solder Iron
Power Supply (Floating)
or
3V Battery Torx Driver
Parts
If
the instrument has serial prefix 2942A and below order a 2.8V battery
(HP
1420-0281) and three
insulators
(HP
3050-0990).
If
the instrument has serial prefix 2942A and above order a
2.8V
battery
(HP
1420-0338).
Procedure
1.
Switch
LINE
to
OFF
and disconnect the power cord.
2. Remove the instrument’s top cover. The cover may be tightly secured in the groove of the bottom
cover. If the top cover does not pull
off
easily after backing out the screw on the rear edge, back
out the screw on the rear edge of the bottom cover also.
rev.
15DEC89
Troubleshooting (Memory Backup Battery) 8-48.1