I took the power supply from the old Nokia phone.
Power circuit on the stabilizer L7806. The output is a stable voltage of 6 volts.
Details
Everything in the assembly should fit in the compartment where the two batteries should be.
We solder the wires to the two batteries and insert them into the standard sockets. We twist the lid and pre-drill a hole for the wire on the side.
The power of the device is 2.6 watts.
The whole alteration of the device took 2 hours.
He will no longer need batteries for the rest of his life. Thank you all for your attention.