Amazing Score

The week before last, my wife and I took a trip to the Missoula, Montana area to do some dog-related activities (my wife competes with our German Shepherds in Canine Agility and Sheep Herding). Along with that, I had recently re-connected with my old High School computer science teacher, whom I hadn't seen since 1978 (I graduated in '77), who lives in the Missoula vicinity. When we arrived at my teacher's home, it was like we picked up right where we left off, even though many years had passed. It was so wonderful to see him again. After we'd settled in and visited for a while, he took me down to his basement, and there sitting on a table were two IMSAI 8080 computers! Back in my junior year of high school, I pestered my teacher to purchase a computer kit to be built by the advanced computer class as an educational project. I researched the available 8080-based computer kits (Altair and IMSAI) and was convinced that the IMSAI was a better choice due to its more robust design. My teacher didn't have the money in his budget to buy the kit, so instead, he bought it with his own money - a clear indication of his commitment to education. He was an amazing teacher, and I credit him for stirring up, then cultivating my interest in computers. We received the kit late in the school year, and got started on building it, with the cardinal rule being that anything that was done had to be explained and understood by our teacher. We got through building the power supply, chassis, and front panel, but didn't complete the CPU and memory board (ROM/1K RAM) during my junior year. The machine was finished up early in my senior year, and with a few minor issues (LED installed backwards, a couple of resistors that were swapped, and a few bad solder joints) it worked. It was a great learning experience for the team of five kids, including myself, that built it. The school year after I graduated, a second kit was purchased, and the same process followed to build it, as well as a some additional boards (memory, video) purchased for the original machine to augment it. The two machines sitting on the table were those two machines from the school. My teacher had rescued them and took them home when he left the school district, , and kept them all these years. The machines were obsoleted in short order when the school district purchased a bunch of Apple II's for a more standardized computer curriculum across the district. When he showed the IMSAIs to me, he said "They are yours, take them home with you". I was stunned, but also extremely happy and thankful. The machines were a bit hodge-podged. One was stuffed with boards, and the other was minimalist. The first machine, the one that I was involved with building, had a the original CPU, ROM/RAM, Front Panel, and 3P+S I/O board, along with a Processor Technology (PT) Video Board, PT CUTS cassette interface, a 32K DRAM board, and two 16K RAM boards. The second machine had only the Front Panel, original CPU board, and a 64K DRAM board. The first machine was in nice shape, but three front panel toggles were missing. The second machine had some scuffs on the front panel, but all of the switch toggles were there and in good shape. Along with the computers, there was a big stack of original documentation for them. When I got home with these treasures, I focused on getting the machine that I'd help build running. I first went through the power supply, and found that even after all of these years, the big caps were still good, and it was delivering clean unregulated voltages within specifications. Then the front panel, CPU, and the ROM/RAM board were installed, and it was slowly powered up with a variac. It came up properly, and after a RESET, it was clear that there were some issues. The front panel was "twitchy", operating erratically. Setting an address in the front panel switches and operating EXAMINE would result in some bits being on where the switches were off. The problems were fixed by reseating all of the ICs on the front panel in their sockets...they were all secure, but the sockets were not the best quality, and there was probably a little oxidation here and there that caused some problems. A couple of the switches were also a little flakey, which were traced to "less than optimal" solder joints that were easily cleaned up. After a little tinkering, I could reliably deposit data into the 1K memory, and examine it back accurately. A little memory test program verified that the memory was working well, and that the CPU was working as it should. I then added more memory, first installing the 32K DRAM board and verifying it worked, then adding another 16K board and making sure that it worked. I then put the PT video board in, and could poke data into the display memory from the front panel, and it too worked great. Given that three switch toggles were missing, I tried putting the front panel from the other machine in, but it was quite messed up, so I just stole three of the toggles from that front panel to make the original front panel complete. I have yet to get the CUTS board installed and checked out, as well as getting the 3P+S board reconfigured to use RS-232, as it was configured to use current loop for an ASR-33 teletype that was used on the system originally. Then, I am going to dig into the second system and see if I can get it running, and find some kind of serial I/O board. Needless to say, I'm really happy to have these great old machines. It is really special to me that I'm reunited with the system that I instigated and helped to build so many years ago. Rick Bensene The Old Calculator Museum (along with some old computers, too) http://oldcalculatormuseum.com