Task: Perform a Diagnostic Test on a Non-Working Commodore 64 Computer
Needed: Diagnostic Test Cartridge or Program, Commodore 64
Time: 10-15 minutes
Introduction
Nothing is more frustrating than a Commodore 64 that is not functioning properly. Commodore used its own diagnostic software to troubleshoot non-working C64s. This software is available now for anyone to use. It can test the RAM, ROMs, the various ports, and even the main ICs. Of course, this particular test does require the computer to boot and be very much functional. Also, it is important to note that it is not perfect, and some problems can appear to be due to a specific non-functional chip but actually due to another issue creating a conflict further upstream or downstream on the circuit board. However, it can provide some clues for diagnosis when used in combination with the awesome pictorial fault guide that allows you to match what you are seeing (or not) on the screen.
Instructions
There are two easy ways to use this diagnostic test. First, you can purchase a pre-made cartridge online for about $25 to $50. These are available from multiple sources including eBay. I purchased this one for $30 that included both the diagnostic test mentioned here and the dead test that allows the computer to boot without some of the key chips. I will cover that one separately in another post. This one also came with a nice 3D-printed case making it convenient to use. Other sources include Retro Rewind and Core i64. I also made some and put them on my Etsy store for $30. Second, you can download the binary file and place it on an SD-card device such as the Kung Fu Flash. I confirmed this works great. The main version circulating is 586220. There is a ++ version that extends to some other Commodore models.
Before you perform the test, you should take a look at the instruction manual. There are a lot of good tips for what to expect and how to interpret the results. You might also watch this informative YouTube video from The Retro Channel.
To run the test, simply boot your C64 with the diagnostic cartridge plugged in or select and run the program from the Kung Fu Flash or similar SD-card device. The test will start automatically and cycle through each of the checks. Be sure and match up any faults with what you are observing using the pictorial fault guide.
Note that the BAD results shown above are because I do not have a diagnostic test harness plugged in that sends the right signals from the various ports back to the cartridge. These are available on eBay for around $40. I will do a post about these when I decide to purchase one. You can test these ports manually by hooking up joysticks or a cassette drive and manually testing them.
Comments
A very useful device and worth getting if you have multiple C64s or plan to do repairs.