either the TA8435HQ stepper driver IC needs to be electricaly isolated from the heatsink or the heatsink needs to be grounded
otherwise a faulty IC will damage other ICs mounted on the same heatsink !

take a close look at the discoloured contacts
Click image for larger version.  Name: TA8435HQ socket.jpg  Views: 265  Size: 423.7 KB  ID: 20061

check the socket for heat damage that may of softened the spring contact
so it will no longer grip the new IC's pins as tight as it should


John