a500 mouse problems

Hello to everyone!

I just dug out from the cellar my old a500.
I got from a friend a spare mouse since I lost my old one. It appeared to be 100% amiga (brand Commodore, two buttons, serial number a0081685).
However, a few minutes after I switched the computer on (I had plugged the mouse in the Joystick 1 port, I began to smell something strange. I switched off and opened the computer in order to see if anything was damaged.
The resistor r601 seemed to be partially burnt. I replaced it and everything seems to be back in order. What has happened?

Now I'm wondering what to do. Could it be a mouse problem (should I give it another try with the same mouse, perhaps through port Joystick 2? Or look for another mouse?) or a circuit problem (since in those few minutes, I observed that the mouse buttons seemed to be working properly, but the cursor on the screen hardly moved)?