Battery Powered v Mains Power Suppy

[BillMC]

I have a certain device (portable CD/mp3 play) which like many other devices can be powered from either battery or mains power supply. When I first used it I used batteries. I then found a mains power supply and used that, because the batteries power ran out. The device continued to work without any problems. I later used a new set of batteries, but for some strange reason the device would not work, yet when I plug the power supply back in it still continued to work.

Can anyone explain the reason why it no longer works with the batteries? BTW I tried a few different sets of batteries and I even tested them wil a meter and they all have a full charge.

[tunk]

The power supply contact in the CD player most likely has
a switch that disconnects the battery when the power supply
is plugged in. It could be bad.

[BillMC]

So I take it then that this switch must have permently switched off the contact for the battery input?

[coppercone2]

trace the battery circuit to the circuit board piece by piece and see if there is a circuit when you are probing a battery because consumer device battery cavities are often utter trash and the problem is compounded by small amounts of corrosion damage from burping cells etc. A dodgy switch is also a common problem.

its methodical probing that will find this problem most likely.