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Icom IC-751 (IC 751 IC751) How to replace the lithium battery in your ICOM radio
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Note: All this information is based upon the IC-751; if your radio is a
different model the board references may be different.
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Go to Radio Shack or your favorite supplier and buy a lithium battery
to replace the one in your radio. The actual type used is a BR2325 (3.0
volt 165 mAh), which Radio Shack does not carry. You can order this one
from Digikey. I bought a CR2450 at Radio Shack (3.0 volt 500 mAh), which
was the largest capacity unit they had in about the same size. I had to
solder some short leads to this battery (quickly to avoid overheating),
before I could attach it to the RAM board.
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Find a 5 volt power source to run the RAM board during the transplant.
I used a modular plug-in unit rated at 4.5 volts @ 60 ma. I connected a
500 Mfd. capacitor to this supply and then added a resistive divider to
reduce the voltage to 5 volts. You may need to re-adjust this divider to
supply 5.0 volts after connecting it to the RAM board. Connect several
feet of wire to the divider output, which will go to the RAM board.
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Remove the covers from your ICOM radio to expose the plug-in RAM board,
which I found on the bottom of my radio.
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Turn the power on and locate the 5.0 volt and ground pins, which bring
power to the board. In my ICOM radio service manual I found a board overlay
for the 'RAM Unit', which shows the two connectors and labels the
connections. While looking down at the board note two connectors on
opposite sides of the board. Both connectors are closer to the same end of
the board. The pin closest to the end of the board on the longest
connector (J1) is pin 12, which is the 5 volt input. The pin closest to the
end of the board on the shorter connector (J2) is pin 1, which is ground.
If you connect a voltmeter between these two pins you should measure 5.0
volts. These are the pins you will connect your external power to.
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Turn off the power to your radio and remove the one samll screw that
holds the RAM unit to the main board (you need a jewlers' screwdriver).
Now you can remove the RAM board from the radio; remember from now on
you do not want to touch any tools or other metal items to the board or
it's components.
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With the board removed you now need to solder the two wires from your
5 volt power source to the board. You can solder to the small metal piece
that is on the side of the connector toward the inside of the board;
this will keep solder out of the connector pins. First make sure that
your soldering iron does not have any direct ground connection to the
board power source.
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After the external power source is connected, turn it on and measure
the voltage being supplied to the board. You may need to adjust the
values of the dividers to get between 4.5 and 5.5 volts. Disconect the
power while making modifications.
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Now, with the 5 volts connected and on, you are ready to remove the
old battery. Use an insulated tool to pry the battery off while heating
the solder connections on the backside of the board. Only touch one of
the battery connections at a time. Here you need to be careful not to
touch any of the board traces or component leads.
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Once the old battery is removed, you can solder the new one in place,
being sure that the polarity is correct. Now carefully inspect your job
to be sure that the battery is connected to the correct pads on the board.
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Now you can disconnect the external power and unsolder the leads from
the board.
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The final step is to re-install the board in your radio and continue
to use it for many more years.
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