I have been interested in vintage computers for years, and I have a large collection of Apple II hardware, but I had never ventured into the world of the PET. Hardware was just too expensive. Then, in the winter of 2009 I remembered that a family friend had given me the main board, upper case and keyboard to a PET 2001-8 when I was about 6 years old. Amazingly, I have saved all of the appliances and electronics which I dismantled at that age, so I was able to (relatively) quickly find the board and case. That is when I discovered/remembered that I had had lots of fun pulling all of the chips out of the board! Besides one ROM, one RAM, and a PIA chip, which with many mangled pins, the board was bare.
Over the next few weeks I searched dozens of 20 gallon plastic bins of parts of stuff I had taken apart at that age and I eventually found all but one RAM, one ROM, the CPU, and the PIA. For all I know, I never had these missing chips. So began my attempt at a “restoration.”
After straightening all the bent pins and replacing the broken ones with carefully cut pieces of copper sheet I put what chips I had in the board. A little Internet research told me that the video system should work without the CPU (I later found this not to be quite true) I planned to power it up and see if I could get a screen full of “random characters” which would tell me that the video system was working. I found a schematic on line which told me where to connect the power supply, and the video out pins are marked on the board, so I was set to go. Connecting the vertical sync, horizontal sync, and video lines from board through 0.1uf capacitors to an old Apple Monitor IIc and applying ~7.5v to the board revealed a very distorted screen full of a single random character. I was not sure if this meant that it was working or not, but it was at least progress. I should mention here that this was my first real venture into digital electronics. Before this I had mostly repaired analog circuits, like old TVs and radios, so I had to learn as I went. Careful study of the schematic showed that on every 8th clock cycle the next location in video memory is read and its value used as the first 4 bits of the address sent to the character ROM, the output of which is then sent as serial data out the video line. Each such cycle corresponds to one character worth of one scan line. Three other bits of the address sent to the character ROM increment according to the current scan line in each character. Understanding this made it clear that the screen should be filled with many different random characters, not just one. Some oscilloscope work quickly revealed the problem. Clock pulses are passed through the CPU to the clock lines on the video RAM so without a CPU present the video RAM output lines do not change with the address lines and only one character is printed. Installing a jumper to pass the clock pulses gave me a screen full of garbage. Perfect.
I knew that I would not be able to go farther without the missing ROM. Doing lots more Internet research revealed that the MOS 6540 ROM chips are not available, and no modern chip is a drop-in replacement. I read about several people who had built adapters, but I could not find a schematic of one, so I decided to build one myself. The result is here.
Note that this post was copied from the old 6540rom.com. The publication date has been modified to reflect the original posting date.