Project: Restoring A SONY WM-EX652 Walkman - Contact Cleaning For "Gum Bar" Battery.

I was trying to measure the amount of Play time having both batteries installed on the Walkman, as I saw some information saying that it should reach almost 40h.
In my case it was no longer than 10h, so at some moment I assumed the charger is not working and the "gum bar" is not charged.
I tried to use it without the AA battery extension and it was not working at all.
After measuring the "gum bar" battery I realized that something else is causing this low amount of Play time, maybe a broken or corroded contact.
I removed one of the face plates of the Walkman, as you can see below:

After removing the screws marked with red color I was able to reach the battery housing:

After a closer look I saw the corroded contact:

In order to clean it I used an old tooth brush and contact cleaner, below you can see the result:

Now the unit is working on both batteries, used together or separated. What is left to be done is to count how many tapes I will be able to listen without charging the batteries.