Re-activating my old Mac Classic

A bad logic board battery may or may not lead to startup difficulties and video issues, depending on the exact Macintosh model.

As you already know, work inside a compact Mac (such as the Macintosh Classic) can be dangerous because of the voltages involved. Dangerous areas include the CRT (which acts as a big capacitor) and other capacitors/components/circuits at high or line/mains voltage levels. Charges can remain for a long time after the computer has been switched off and disconnected from the mains. Repair or service work must be carried out by someone with the necessary expertise only.

The book Mac Classic & SE Repair and Upgrade Secrets (ISBN 1-56609-022-9) by Larry Pina could perhaps be of interest to you.

It is still possible to find old SCSI drives (for example, via an online auction site), but they are often expensive and the condition may be bad. An external SCSI drive, such as a Zip 100, could be connected to the Classic. There are also some modern alternatives/projects (Floppy Emu, SCSI2SD, etc).

First note: remove the battery asap if you are unsure of the condition. You don't want it to leak all over and ruin the logic board. New ones can be purchased on Amazon for a fair price (I got 5x for around $10)

Does the Macintosh start up at all and just not boot, or does it not power on. If it powers on to a floppy icon, you will definitely need a new hard drive or boot floppy. If you have the original installer floppies, the cheapest solution would be to get an external scsi drive that is less than 2gb in size and reinstall the OS from there. As for getting new software on to it, you will need a device that can read/write the Mac OS floppy format. I use a USB floppy drive and a Mac OS 9.2 system.

If the mac does not power on, or has strange patterns on the screen, it will likely need to be recapped. You can either do this yourself or find someone to do it, but I am uneducated in both of those areas.