Difference between revisions of "User:Deriss"

Latest revision as of 12:08, 4 April 2017

Bought a 464 or 6128 from eBay ? Broken or not working right ? Read on for repair tips and where to buy spares..

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. Equivalent ones are nowadays frequently physically much smaller than the 1980's originals. So don't be surprised at that.

4: Examine all resistors for signs or damage or discolouration. Check resistance with a basic multi meter and replace as needed.

5: Examine all the chips for any corrosion, damage or discolouration. Virtually all the components are still available to buy online if you search thoroughly.

6: Check for damage to the tracks at the edges of the board. these can often be fixed with a few wires soldered to nearby points, to re-route the voltage around broken tracks and back where it was supposed to go. Remember to use wires that are the equivalent thickness or greater, of the broken track.

REPLACEMENT PARTS :

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. 85c rated parts are fine. 105c is also fine but excessive.

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 actually the same product. So shop around first and check the specs/pinouts before buying one that is over priced. Compare the model numbers printed on the chips on these boards: http://www.cpcwiki.eu/index.php/Mainboard_Versions to see what will and wont work. For example, the same CRT chip is made by three different manufacturers.

1980s DRAM (that makes your 64k memory banks) is surprisingly easy to source from eBay as there are a surprising number of UK suppliers who still have stocks of authentic 4164, 4264 and 3764 dram chips. When changing the memory, change the four 74LS153 chips as well. These support the memory operation and so may have also been damaged. Don't forget to buy your 16pin DIP sockets as well. We don't solder in chips.. Sockets are far easier.

Do not buy from microchips anywhere except the UK, US or Germany. There are too many eastern European and Chinese fakes around to be worth the minor savings.

The Gate Arrays, either Amstrad 40007 or 40010, are the only expensive parts at between £8 and £15. It's a suppliers market for these unfortunately.. Look at eBay in France or Germany too, as they were big CPC markets back in the day. ( My last 40010 chip came from southern Germany). The two chips are NOT interchangeable btw, as they have different pinouts. Although some boards made potential allowance for either chip in their designs, only one type is ever used.

For the floppy control circuitry, the NEC 765 floppy disk controller can still be acquired but the SEC9420C buffer is a hard one to track down. Luckily these are virtually never needed as you are likely to have replaced everything else before getting round to that anyway. The floppy controller chips are not required for the machine to boot btw. So they would never be the cause of a core failure to boot.

All the other various chips labelled as 47LSxxx are easily bought from eBay or RS, Farnell etc. Even the older ones.

The last versions of the 464/6128 (1989/90) had a more modern integrated SMD chip that removed the need for a gate array. They were known as the "cost-down" boards. These chips physically cannot be swapped out and so would render any such board as scrap, if it turned out to be the SMD chip. Avoid any major expenditure on these boards that you could not recouped back via eBay, should the board be beyond repair.

REMOVING CHIPS:

A subject often treated with a fear that is unwarranted. Just observe the following:

Firstly make sure you are properly earthed with a £2 anti static strap that is plugged into a proper mains socket (via the earth pin, obviously..) Then get setup on a nice clear desk with a good lamp, preferably one of those with a large magnifying glass to look through, like jewellers use. Then get yourself an Antex 25w or 15w soldering iron with a smallish soldering tip on the end. You will also need a stand for it with a sponge for wiping off crud from the tip. A simple de-solder pump with plastic tip is also needed. Finally get some good quality, fine nosed wire cutters. Maplin sell all of these, as do eBay.

Now you are ready. Clean of any dust from the chip in question and carefully snip the legs off the chip at their highest point, closest to the chip itself as possible. Remember that we don't care about the chip as it is duff anyway. The mainboard and its tracks are what we have to protect.

Once you have snipped all the legs, the chip will just fall off, leaving a bunch of legs or pins sticking up. Now use a pair of tweezers and your soldering iron to pluck out each pin in one piece. Hold the pin with the tweezers, melt the solder with the tip of the iron and pluck the pin out of its hole. Like plucking eyebrows, or so I am told. Then use your de-solder pump and soldering iron to suck out the solder from the holes where the pins were, ready for a chip DIP socket to be installed.

This is a slow process but be calm and methodical and you wont go wrong. Its all worth it when that Amstrad logo comes up again !