Macintosh Classic board is very prone to battery leak. Leaking battery can cause substantial damage to the metal chassis…
…and the PCB
Fortunately, the surroundings of the battery cradle are rather limited in PCB traces, so there is high probability fixing the board will be successful. First remove the components, which are in the battery fire range. These very likely includes UA2 RTC chip and the nearby sound chip.
Clean the PCB using a fine-grained sandpaper and cleanup IC pads from solder.
Clean even more all the surroundings of the damaged space, identify broken traces with a multimeter and fix them using a bodge wire. It can easily go through PCB vias.
Reinstall missing ICs and capacitors
Cover the polished parts of the PCB with a PCB varnish and install a separate battery cradle.
Kingston Module (KTA-1/Classic)
Apple Module (820-0405-01)
Unidentified Module (P/N 10105)
Macintosh Classic is the only Macintosh coming with OS in ROM – it can be booted by pressing CMD-OPT-X-O keys together and rebooting machine (try to do this in one person) – it is pretty cool!
Power Supply/Analog board
If your power supply gives no sign of life and the fuse is ok (meaning there is voltage on the primary winding of the transformer but not on the secondary), the first thing to replace is TDA4605-3 control IC. Prior to replacing this chip, it is a good idea to recap the analog board, as broken capacitors may be the reason for TDA4605 to burn. It is best to install a chip socket and have some spare chips handy, so in case they break again, it is easier to replace them.
If there is a short circuit in the PSU, which causes the fuse to blow on power on, it is most probably because of shorted QP2 IRF-BC40 switching transistor. You can detect it with multimeter doing a diode check between terminals of the transistor. A broken transistor will be likely conductive between any two pins and both directions. Removing the transistor should stop the fuse blowing (of course the PSU will not work yet).
Floppy Disk Drive
If the FDD is not reading the disks, inspect the distance between the upper head and the disk surface. If there is space, the heads have to be removed (together with the harness that is moved by the stepper motor) and it is possible to adjust the strength of a spring pulling the heads together from the bottom of the harness.
Hard Disk Drive
40MB Quantum ProDrive LPS
SCSI terminator (branded by Apple!)
A battery from 1991, that is not leaked at all! (although someone mounted it in a wrong direction)
And the infamous Maxell battery bomb