Introduction
I purchased this untested Commodore 64 from a local friend. I was not surprised to find that it booted to a back screen. On the plus side, the machine was clean and in good condition. I detail the repair below along with some comments about a mistake I made.
Case Study
The Dead Test cartridge gave white flashes suggesting a RAM problem. Upon opening it I discovered two immediate possible faults. First, the motherboard was loaded with eight Micron Technologies (uT) 64Kx1 DRAM chips (lower left of photo below). I also discovered a MOS 7709 logic chip (lower right). Both the uT DRAM and the MOS logic chips are prone to failure and highly suspect as the cause of the black screen. I decided to start with the DRAM first. I had an important decision to make. Do I try to determine which DRAM chip(s) is bad or replace them all? Replacing them all is the right thing to do and will leave the next owner a much more stable computer. Unfortunately, only the SID and the VIC-II chips were in sockets which meant I would need to desolder the eight DRAM chips and install sockets which is what I did. The next photo below shows the removed uT RAM followed by another photo of the new sockets. I installed eight NEC 256Kx1 DRAM chips and was able to get this c64 back up and running after an unfortunate rabbit hole I will describe in the comments below.
Comments
This should have been a pretty straightforward repair with some time to install eight new 16-pin sockets. However, after installing the new DRAM the machine still did not boot properly and displayed the screen shown below. The pictorial fault guide suggested the multiplexers and some logic chips. I went down the rabbit hole of socketing and replacing those chips including the MOS 7709 which did not fix the computer. I had set this computer aside and when I came back to it decided to inspect the new DRAM again. I discovered that the U20 DRAM chip had a pin bent underneath that was not in the socket. I straitened it out, reseated it, and rebooted. Success! I had accidently bent that pin when seating the new DRAM. Many, if not all, of the replacement DRAM you will buy are harvested from other computer and the legs can be a bit weak and sometimes even break off if you are not careful. A good lesson in seating chips carefully and slowly with observation of all the legs to make sure they go in the socket correctly. I also decided to replace the PLA with a new GAL PLA to further future proof this computer for the next owner.
