AR3500 Modification Follow-up: Battery Backup

By:  W8CWE

Overview

The AR3500 microprocessor provides five memory locations in which frequently used frequencies can be entered and stored. Unfortunately, the original engineering of this feature never took into account the occasional removal of power from the radio (such as is the case with base operation with an external power supply). This modification describes how to add a backup battery to the radio in order to preserve the memory locations when power is completely removed from the radio.
Parts Required

The following parts are required to complete this modification:

• Nickel-Cadmium (Ni-Cd) Battery, Standard 9V type - Radio Shack 23-299, $9.99 ea.

• 9V Battery Snap Connector - Radio Shack 270-324, $1.89 for 5 pk.

• 9V Battery Holder - Radio Shack 270-326, $.99 ea.

• 12V, 40-60mA Microlamp - Radio Shack 272-1092, $1.49 for 2 pk.

• Two feet of hook-up wire and shrink tubing (to lengthen the 9V battery snap connector leads)

• Modification Instructions

1. Solder a 1-foot length of hook-up wire to each of the 9V battery snap connector leads.

2. Solder one of the 12V lamps to the same joint.

        NOTE: The lamp provides a load for the Ni-Cd battery to prevent overcharging.

3. Thoroughly insulate the solder joints using shrink tubing.

        NOTE: Always use Shrink Tubing to insulate soldered wires inside any electronic device. Electrical tape mastic breaks down quickly over time and may
                     eventually allow exposure of the bare solder joint. Failure to comply may result in damage to the radio.

4. Remove the top and bottom radio covers.

5. Locate D-74 on the main board. This is found on the component side of the board, right-front portion of the radio.

6. Route the + (red) lead from the battery connector to the solder side of the board. Solder it to the banded-end of D-74.

7. Route the - (black) lead from the battery connector to one of the grounded stand-off screws securing the main board to the radio chassis. Loosen the chosen screw and clamp the ground wire underneath it. A drop of solder to ensure the electrical connection is recommended.

8. Locate an appropriate area of the top cover to mount the 9V battery clip. I found that the area of the cover which corresponds to the space just behind the front face assembly works well. Mount the battery clip to the top radio cover. I used epoxy to mount mine as I didn't want to mar the top cover with a screw.

ADDITIONAL NOTES:

1. Once charged, the battery will provide a couple of weeks worth of memory backup if power is removed.

2. If the radio ever locks up and will not allow you to reset it, disconnect the battery and then reset the radio. On some radios this may happen often. A switch can be added to the outside of the radio to allow disconnection of the battery for reset purposes.

You can also perform this modification using a regular alkaline battery, but an appropriately rated diode must be added to prohibit the battery from charging from the radio. Failure to use a diode may result in the alkaline battery heating up and possibly exploding.