1)Unhook or unplug the battery box from the radio.
If soldered in one lead is fine.
2) measure volts. If 4.5 V the batteries & holder OK.
If not then the problem is the holder or bats.
3) If the voltage drops a lot when you hook up
the radio something is loading things down. Look
at the output IC or transistors first.
The radio may have crapped out then got tossed in a drawer
& the bats leaked......
If you like working on small stuff ( I do !) get some DC
output wall warts. They used to make them with a voltage
switch ( 4.5 - 12 V ), multi connectors & polarity switch.
ADD a pair of wires with alligator clips that are color coded
& you can sub it in to any low current device without
batteries involved. As for bats I only use duracells from a
known source to avoid Chi-Com junk & counterfits.
73 Zeno
