Task: Troubleshoot a Nonworking Commodore 64 by Testing the Continuity of the Circuit Board Traces
Needed: Multimeter set to continuity mode
Time: 30-60 minutes or longer
Introduction
I recently had a nonworking Commodore 64 with a 250407 board (Rev. C) in a breadbin case that I fixed by replacing a bad 74LS257 logic chip. Once I had it working, I decided to socket all the chips on the board and use it for testing / experimentation for future repairs and projects. I also added an RF replacement board for S-video out. After I added the new RF module, the board became unstable with memory errors deteriorating to a back screen that would not boot the dead test. I retested all the ICs and they checked out fine. I tested all the electrolytic capacitors and found several out of range. I replaced those using a kit from Console5. This did not bring it back to life. After some other testing I decided that I must have damaged a trace or solder pad when I desoldered the logic chips and added sockets to the board. After continuity trace testing, I discovered that pin 10 of the 74LS373 logic chip was not connected to the ground plane as expected. I soldered a connection to the ground plane and that restored the computer to working order. I could not see the damaged trace because that connection is on the top side of the board now hidden under my newly installed socket. A good reminder to very carefully check for damage after desoldering chips and before the sockets go in. I somehow missed this one. Below are a few tips and a nice online resource that I used to identify the traces.
Instructions
First, you will need a multimeter set to continuity mode. Here is a short video on how to do this. Be sure and toggle on the sound so you can hear the beep when continuity (i.e. no resistance) is present. I do this on my Kaweets by pressing the function key (upper left red key) once in the right mode. You can see the sound symbol appear on the screen.
Second, you will need a map of the connections of all the chip pins to each other via the circuit board traces. One way to do this is to find the schematics and follow the connections from chip to chip. This can be very tedious. Fortunately, there is an interactive map on GitHub from bwack. Scroll down and click on the interactive BOM link for the 250407 board. From here, you can click on the pin of any chip and all its connections will get highlighted in red. You may need schematics of your board if not this version. Zimmers.net is a good place to find those.
Finally, simply follow the traces from your pin of interest to other points on the board and use your multimeter probes to test each pair and listen for the continuity beep. The lack of a beep told me that pin 10 of the 74LS373 logic chip was not connected to the ground plane as it should.
Comments
Finding and repairing broken traces is tedious, but a really important skill to have in your arsenal.
Conversely, this approach can also be used to find a short between two parts of a circuit board that should not be connected.
I was not able to get this Commodore 64 working until I found that ground pin issue and repaired it. Not an obvious fault, especially given it was covered by a socket.