Bought a 464 or 6128 from eBay ? Broken or not working right ? Read on..
Firstly, make sure you have a 5v regulated power supply and that your monitor / tv are working ok. (You'd be surprised...)
Make sure your power supply provides a minimum of 2A continuous current. Some PSUs can be very "optimistic".
On the 6128, 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. It's a 30 year old machine remember...
Now carefully remove the case.. Be VERY careful with any keyboard membranes. You CANNOT buy these anymore.
eBay is The best source for spare chips and other replacement components that I have found, btw.
The obvious things on the board to look for first are:
1: check the 5v power and 6pin display sockets for dry solder joints. A dab of fresh solder will fix this.
2: check the rest of the board for dry joints too. This can be tedious but is worth it in the long run. Never touch the soldering iron tip to a joint for longer than 2 seconds to avoid damaging sensitive components.
3: Check for any damaged or leaking capacitors. Check the ceramic ones too, in case a previous owner has knocked and damaged one. Remember to replace any capacitor with a product of equal or higher voltage rating. Equal ones are now frequently physically smaller than the 1980's originals.
4: examine all resistors for signs or damage or discolouration. check resistance with multimeter and replace as needed.
5: Examine all the chips for any corrosion, damage or discolouration. All the components are still available to buy if you look hard enough.
REPLACEMENT PARTS :
The Z80 cpu can be easily replaced for as little as £3 new, as can the 8255AP-5 interface chip and the AY-3-8912 audio chip. The CRT controller UM6845R is about £4.50. They are all made by several different manufacturers, with very slightly differing model numbers for what is the same product. So shop around first before buying one that is over priced.
Capacitors and resistors are too cheap to mention. FYI: buying a bag of 5 is frequently little more expensive than buying just 1 or 2. Then you have spares as well.
The gate arrays, usually either Amstrad 40007 or 40010 are the only expensive parts at between £8 and £15. It's a suppliers market for these unfortunately.. They are NOT interchangeable as they have different pinouts.
The last models of 464/6128 had an integrated SMD chip that removed the need for a gate array. these cannot be swapped out and so would render any suck board as scrap. In that case cannibalise it and sell the parts.
One common fault is probably someone having plugged a 12v PSU into the 5v socket. This usually fries some or all of the ram (memory) and may sometimes damage a few ceramic capacitors along the way too.