Rework a MC68000 CPU on an Amiga computer motherboard - Page 1
Forum section: electronics / repair

author: Mamejay
author_rank: Contributor
timestamp: July 27, 2015, 11:57:23 pm
content: Hey guys,I need some help and advise please.I am wanting to rework a MC68000 CPU onto an Amiga 600 motherboard. I have a hot air station but I mainly use this for removing small surface mount parts and have not tackled anything like this.I suspect a problem with the CPU connections as I purchased a ram expansion module that clips over the CPU. I started getting some error codes and the computer not starting so started to investigate.I found that the HALT pin on the CPU was not connecting to the pad. I did manage to fit this just using some flux and got the system running again but as soon as I try to do anything once on the OS the PC freezes.I do not know where the other connections go so cannot follow traces to determine if other points are not contacting correctly.The CPU looks like the attachment I have below.Any tutorials on doing something like this? Any assistance would be much appreciated as I don't want to let this one go to motherboard heaven.

author: divelectservices
author_rank: Contributor
timestamp: July 28, 2015, 12:13:57 am
content: If you don't care about the chip, hot air will do it along with a lot of patience!A preheater on the bottom of the board will make the job much easier.

author: helius
author_rank: Super Contributor
timestamp: July 28, 2015, 12:51:04 am
content: With an inspection microscope or a magnifier, you can look for cracked or cold joints, although inspection is much harder for a PLCC like that, than for a QFP. What you're looking for are dull joints where the solder doesn't evenly wet the pin. For electrical testing you need a meter with a LV Ohms mode. The voltage it uses needs to be <1.5V so that it doesn't turn on any transistors or cause damage to ICs. If you can see an exposed pad that's part of the same net as each of the pins, test for conductivity on LV Ohms between the top of the pin and that pad.It can make it easier to do that test if you take a PLCC socket and press it inverted onto the top of the chip. Then you can use grabbers on the socket's pins instead of pressing probes onto the PLCC's pins.

author: Anks
author_rank: Frequent Contributor
timestamp: July 28, 2015, 12:12:59 pm
content: First thing is dose the Amiga work without your expansion? If it works without (and this is a common problem with expansions that sit on top of the cpu) either the expansion is bad but more than likely not sitting correctly on top and shorting pins or not getting good connectivity.

author: McBryce
author_rank: Super Contributor
timestamp: July 28, 2015, 12:22:05 pm
content: I assume you're trying to get an ACA-630 or similar to work. Because these connect by just plugging a standard PLCC socket over the CPU, usually the instability is due to bad connections between the PLCC pins and the CPU pins. Clean the outer surface of all the CPU pins first and see if that improves things.McBryce.

author: BloodyCactus
author_rank: Frequent Contributor
timestamp: July 28, 2015, 01:01:53 pm
content: the ram expansions you push down on the plcc socket, are known to be finicky and have to push SUPER hard to get proper contact. look on amibay / eab.abime.net / etc and see all the people who couldnt get it working because they didnt connect the two together properly.

author: Kleinstein
author_rank: Super Contributor
timestamp: July 28, 2015, 07:05:12 pm
content: The extra connections of the expansion to the CPU can also mess up line impedance and degrade signal quality. So unreliable operation is no suprise.