What's a safe and easy way to desolder the CPU from C64?

Question

I have probably broken my CPU by connecting 5 V from Arduino (500 mA) to the SENSE Pin of the Casette Port, So now I wonder what would be the safest way to desolder the processor? I have a cheap soldering Iron and a cheap desoldering pump. My soldering skills are Ok.I thought about soldering a IC socket and then plugging a new CPU in the socket after removing the old one. Is this a good idea?

Answer 1

The easiest way to desolder a dead chip is to cut the legs with your snips then remove each pin one at a time. Use a solder pump or solder braid to clean the holes. I would use an IC socket when replacing the chip as there is more of a chance of damaging the 6502 if you solder it directly. You can get low profile sockets if head space is an issue...

Answer 2

For destructive desoldering @Tim Rings method is usually the fastest: clip all pins, remove the chip and desolder each pin separately. Just be careful when clipping the pins. Use a small tool and try to cut each sideways (cutting flat), as large pliers and frontal cutting may twist the pin, which could damage traces or even the board. Clip all pins before starting to desolder.

Remove each pin separately and don't worry about removing the solder at this step. Free up the holes in a separate run, preferably with a solder pump, as now you can heat the hole on one side and suck with maximum efficiency from the other.

To keep the chip undamaged, I prefer to use a hot air gun. The tool should have a nozzle of about ~1cm (~1/2 inch) to direct the heat without damaging other components. Grab the CPU with an IC-pull-tool, heat the soldering side with your blower and gently move the IC to pull it out. After that, clean the holes as before.

Depending on the board style and density it could be helpful to build a little cardboard box and shield around the soldering holes (half way to other components and at ca. 1cm high, to be inset in a sheet) so excess air isn't affecting other components much. It all depends on your skills here.

Addendum: If this machine is to be used in further experiments, I'd go ahead and desolder all IC and add sockets. Then desoldering is a matter of minutes with a hot air gun - and future replacements will be quite easy, not to mention add on components :))

Answer 3

I was just working on restoring an original c64 breadbin model and ran into the same issue you have: how do you safely desolder a multi-pin dip chip. I also happened to have a black screen of death failure. For a 40-pin chip it's gonna be nearly impossible to remove the chip with just a soldering iron and solder wick or a solder sucker. After I struggled to remove a bad dram chip using a soldering iron, solder wick and a solder sucker, I decided it was time to use the correct tool. You should use a vacuum desoldering iron, they run over a hundred dollars for one that will actually get the job done. I used one at the maker-space in my area. I would look for a maker-space or a university lab you might have access to before buying the tool. Using the correct tool I was able to remove the malfunctioning chips in a matter of minutes with minimal effort.

Since you mentioned that you have a black screen of death failure, you should realize that it can be caused by literally any chip on the motherboard going bad. It's more likely you damaged a CIA or PLA than the CPU given what you described. Since original 6510 CPUs are hard to come by, I would perform some serious debugging of all chips which could be malfunctioning before you proceed to destroy a possibly working CPU. In the case of my black screen fail I was able to isolate the failure to two bad dram chips. Once I desoldered them, installed sockets and replaced the chips the system booted up perfectly. You can easily test if your CPU is working by using a "Dead Test Cartridge". If you plugin that cartridge and you see nothing on the screen then the cpu is bad. If the cpu is good you'll see periodic flashes of white on the screen indicating memory cells that are bad, if memory is mostly good you'll see a graphical diagnostics screen. These cartridges are continually being re-made and sold for ~$15 on ebay. If you don't want to go that route you can always use a logic probe to test the clock signal going in and out of the cpu and the address and data lines. If the cpu is good they should all be toggling. Good luck with your repair.

Answer 4

First of all, be careful of bad advice. Even from me. Second, if you are not 100% sure the CPU is dead, then do NOT clip the leads to make de-soldering easier. No offense, but if you can't properly de-solder a chip using either a solder-sucker or a de-soldering vacuum station, then I would pass the job on to someone else. They aren't making 6510's any more.

Also, why are you assuming you fried the CPU with a 5V signal? What indications do you have the CPU is dead? I'm not saying it isn't...I just wonder how you got to that conclusion.

Finally, for any IC (not just CPU), the proper way to de-solder is to first make sure you're set at the right temperature. Too hot (or too long) you will lift the pads or worse. Use a good solder-sucker if you don't have a de-soldering station. Take your time and do it right.

It's sometimes a good idea to add a little fresh solder to loosen things up. Then, melt the solder and quickly use the sucker.

Don't use solder wick for the bulk of the job. It's not really designed for that. It's better designed for pulling up solder from flat areas.

People are always so quick to recommend cutting stuff off. If it's a $1 part that is easy to find, still produced, etc. then I can see that being OK. But never a vintage part unless you REALLY know it's dead.