Step 1: Open the Player
Remove the screws. Pry the case of the player open.
Step 2: The Goal
The player's power jack is designed so that the connection to the batteries is broken and only external power flows to the player circuitry when the power supply's plug is in place. In time the power jack's internal connections loosened. The least movement while listening often caused the power connection from the external supply to be lost. That also caused my place in my audio program to be lost. It was frustrating.
The solution was to use the external power jack and supply to feed charging current to the batteries. Making that possible meant adding a shunt so the batteries are not locked out when the external power supply's jack is inserted. See the next steps for the circuitry and how to know which solder connections to shunt.
Step 3: How the Power Jack Works - Part 1
Here you see the circuit as it works without the external power plug inserted into the player. Note the closed contacts that are part of the negative side of the female power receptacle.
Step 4: How the Power Jack Works - Part 2
The power jack is center point positive. The inside of the jack is mostly plastic. But, there is a curved piece of metal that makes contact with the external part of the male power plug. See the black curved line. When the male plug is in the player's power jack, the battery is cut out of the circuit.
Step 5: The Shunt
After the player's circuit board is out of the player, use your ohmmeter to get a clear understanding of which solder joints go to which conductors in the player's power jack. Determine which solder joint on the circuit board connects directly to the negative terminal in the battery compartment. Which terminal connects to the negative terminal in the battery compartment when the male power jack is not inserted into the player, but is not connected when the plug is inserted? Connect it with a shunt to the solder joint always connected directly to the negative terminal in the battery compartment.
When soldering on the circuit board, be careful that no solder forms an unintended bridge to another solder joint and shorts something out.
My charging time is about 45 minutes. The batteries go down quickly when the number of bars in the power indicator on the display get below half full. The batteries I use were already used in another application, so I do not worry too much about overcharging them. I can get more.
It is possible to listen to the player while the batteries are charging, too.