SCM Cogito 240SR calculator (1965) - Schematic Mapping & Repairs - Page 1
Forum section: electronics / repair

author: richfiles
author_rank: Regular Contributor
timestamp: June 27, 2014, 05:59:46 am
content: Years ago, back the the 1990s, I'd successfully managed to map the schematic of a TI-85 calculator main board using a multimeter and a lot of patience. Inspired by someone else's discussion here on mapping unknown circuits, I was inspired to share this terribly daunting project that I have been delaying starting. Not sure if I'll honestly ever get around to it. I DO want to do it, at least for the sake of preservation, anyway. The LEAST I could do is to at least scan the PC boards... I might get on that some time. It'd give me a chance to clean all the wiring contacts too.I have a 1965 SCM (Smith Corona Marchant) Cogito 240SR calculator that desperately needs servicing. The machine won't load the accumulator output results (I think) into any register, even though I can manually load data into any register using the keyboard to load data into the "K" register, and the swap keys to manually swap the data between registers. The fact that it recalls ANYTHING at all suggests the delay line is okay. The machine's primary logic consists of 6 large square (about 9.25x10 inches on a side or about 23.5x25.4 cm) PC boards that consist of almost NOTHING BUT resistors, diodes, and transistors (and possibly some hybrid modules). No chips at all! There are 4 smaller PC boards that appear to be associated with the CRT display, and a small pair of PC boards that are related to the magnetostrictive delay line memory. All those boards, and it's wired with board to board plug in wires and cables. There is no simple socketed back plane. it's ALL BLACK WIRE!!! WHY can't things be simple!Fortunately, if it's any consolation... the boards appear to have all the components very nicely laid out in a grid. No clue about the traces beneath.I would love to map the schematic to this thing out. Therearepatent documents that describe the basic operation of the machine. Patent 3,518,629; Filed Feb. 6, 1964 and dated June 30, 1970, for Stanley P Frankel. I, and others, have yet to find any trace of actual service manuals or documentation. Only the very segmented diagrams included in the patent. I would LOVE to take my machine apart and map out the entire circuitry of the machine... It is... Most daunting...A bit of trivia, Stan Frankel was involved with the Manhattan project, but lost his security clearance in the 1950s during the "Red scare" because of his views on making nuclear technology academically accessible for civilian development... The military was NOT keen on that idea. This barred him from the majority of computer related jobs, since nearly every computer application in the 50s was government or military connected. That led him to work with Marchant, who made mechanical calculators back then, to develop the Cogito 240SR electronic calculator.Still... wow... I am utterly amazed by the skills of those computing pioneers, people who managed to come up with this stuff at a hardware level, and using such primitive technology!I don't doubt Icouldmap it out, given enough time and unfathomable levels of patience to do it. It's that it only uses about 3 or 4 major part types to perform logic! Even AFTER mapping out the schematic, I'd have to actually figure out and organize what diodes and resistors are grouped together, and what logic function they perform! I have no idea if there is any way to test the boards live either... They are not situated in a means that appears to make that simple. Hybrid modules are also troublesome... They are like chips with no datasheets. Joy.At least it is still an amazing piece of history, and quite the sight to look at, even if it is not functioning at 100% anymore.Quite frankly... I am hoping that it's just an oxidized connector that only needs to be cleaned and reseated! LOL, I can have hopes and dreams!...I was considering using a program like Eagle (or something better, if it's free... I'm as broke as my calculator) to try to create an approximation of each of the PCBs, and see if it can generate the schematic off of that. Not sure if the process can go in reverse. I suppose, another option would be laying out all the part symbols exactly as they lay on each of the PC boards, and then making connections based on the actual PC board layout. That might work too, and might be quite straight forward even. I'd have to remember to mirror the back side though. Anyway, once the nets are made, I imagine a person could simply play around with arranging them in a more logical arrangement. Just start with a part, move it, and then move the parts the rats nest connects to before or after said component, or beside it, based on where the parts connect between each other. Eventually, recognizable logic gates and structures could potentially be spotted, and could be further arranged logically.I guess I'm curious if anyone here has messed with anything of the sort before. Part of me is scared to start. If I were to do something dumb, like tweak something in the delay line, for example, the thing might never work again! It's just such a huge and daunting project to take on. I know there can't be many of these old machines left... I was SO SADDENED the day i turned it on and realized it wasn't functioning correctly... I know another owner of one of these machines, and likewise, he's quite nervous to tear it apart as well. If something irreplaceable gets harmed... there really is no going back! That is a terrifying thought!...Oh dear... I have my work cut out for me.Pics of the inside of my Cogito 240SR:Made a video of me trying, and failing, to do much with the machine. I copy-pasted most of this for the description... Easier for me at least, than typing something new.