Why did my mp3 player die after battery replacement?

[Mp3]

I have a mp3 player where the charge port broke off. Also the internal battery was getting weaker, since i used it every day for the past 5 years.

I opened it and found the battery was a 3.7v LiPo, 700mah. Power consumption around 70mA.

Since I had some 18650 holders and a couple 3000mah 8A limit batteries. So i removed the LiPo and wired the 18650 holder to the battery connections on the PCB. (please note, I did check the battery voltage periodically, and never let it go below 3.9v)

It worked great for a few days until i noticed it was slowly getting quieter and quieter, and occasionally had a sound of static coming thru the headphones - ESD damage potentially?

Today i went to use it and noticed the volume, when put all the way to maximum - which is usually quite loud, enough to damage some earphones - barely audible.

I went to check solder joints and swapped the op-amp, but no improvement.

Did i kill it with ESD?

[rs20]

Far too little information to come to such a definitive conclusion. If you're advanced enough to be swapping out op-amps, I'd consider debugging the problem from scratch (i.e., checking the signal strength at the op amp, DAC outputs, etc) and figuring out where the problem lies. Then you'll know what IC is dead (or, what other sort of PEBKAC might be going on). After that, we could start to explore how likely the theory of ESD damage is.

PS/ I do congratulate you for considering the theory of ESD damage being the culprit -- the number of people who confidently say "I've never killed anything with ESD" astounds me; you can only really say that if you've spent your entire life in an ESD-safe lab, or every device/IC failure/malfunction you've encountered has been 100% definitively proven to be a non-ESD cause.

[Dan_Ohm]

did you make a silly little mistake like put the old battery back?
Mixing up the old and new batteries would not be the first time.

Try changing the batteries again and check the voltage on the batteries with a volt meter.