Bought a 464 or 6128 from eBay ? Not working ? Then read on..
Firstly, make sure your 5v power supply and monitor are working ok. (You'd be surprised...)
Check it with a multi meter and make sure it provides a minimum of 2A continuous current.
Some modern switch mode PSUs are very optimistic in their "ratings".
You don't need a 12v feed for anything except the 3" disk drive. So it can be omitted for testing.
Earth yourself to prevent static damage or better yet, get an anti-static strap. 30yr old machine remember...
The obvious things to check first are:
1: check the 5v power socket and DIN6 display socket for dry solder joints.
2: check the rest of the board for dry joints too. Never touch the soldering iron tip onto a joint for longer than 2 seconds. This is to avoid damaging components.
3: Check for any damaged or leaking capacitors. Check the ceramic ones too, in case a previous owner has knocked one.
4: Examine all the chips for any corrosion, damage or discolouration.
The most common fault is probably someone having plugged a 12v PSU into the 5v socket and damaged
This usually fries some or all of the ram (memory) and may sometimes damage a few ceramic capacitors along the way.