DIMI 1106/1107 sandwhich face side down on my desk.

DIMI 1106/1107 sandwhich face side down on my desk.


This time I will tell you about the first part of DIMI I have finished. This is a unit of two boards sandwhiched together. Top PCB is board 1107 which is the visible touch pad of DIMI. Straight underneath is PCB 1106 which I like to call the bikini board. This is easy to understand by looking at the PCB etch mask. 1107 does not have any electronics in it. Its function is simply to provide interface for the instrument. 1106 has lamp driver ICs for the number plates, pullup resistors for the keyboard, diode matrix for mapping number plates to 4 bit bytes and DIL sockets for three DIP16 cables that will connect the board to main logic board. Drivers for remaining indicator lamps are located in the main logic board. Similarly keyboard tracking is done in the main logic board that is found underneath 1106/1107 boards.

1106 board etching mask.

1106 board etching mask.

DIMI has eight different PCBs (four boards are identical) and I have made EAGLE CAD versions of all of them. I scanned original PCBs, edited images in GIMP, imported them to EAGLE and then did lots of manual tracing. As the original schematics and etching masks are lost, I needed to do everything from scratch. I will probably make a tutorial about this process at some point. Most of the boards I have etched by hand with help from Olli from Olegtron, but all the files are ready to the factory print as well.

Original DIMI-A board from summer 1970. 1106 board is lifted and 1107 backside revealed.

Original DIMI-A board from summer 1970. 1106 board is lifted and 1107 backside revealed.

As seen from this build footage of the original instrument from 1970, I’m proud to say that the end result is almost identical to the original.

Indicator lamps on DIMI are telephone slide lamps. I didn’t have a clue what they were and I still don’t know exactly where and how they are used. I would be happy to add a picture of an actual device where those are used here. But what I do know, is that they are used either in telephones or telephone exchange and that you slide them to some sort of socket. On DIMI they are hard wired straight to the board, so no sliding here. There are many different brands making these, but the company who manufactured lamps installed on original DIMI happens to still exist, so I ordered bunch of these from Taunuslicht.

Installing laboratory hose bits on 1106.

Installing laboratory hose bits on 1106.

Telephone slide lamps radiate light around them so on DIMI Kurenniemi’s design was to use laboratory tubing bits as a shade. I have to admit that it works nicely and gives the light orange color that the original has as well. I found out that you have to use quite tight tube as the lamps are located fairly close to each other. Lamps barely fit inside and I came up with technique of boiling them to soften them up a bit and then using water and electric spray as lubricant. That allowed me to install the hoses without breaking too many lamps in the process.

Boiling laboratory tube bits.

Boiling laboratory tube bits.

Another funny design solution that gives a nice touch is the magnet holding stylus connector on top of the keyboard. Tiny plate on top of the instrument is actually a keeper plate of small magnet hiding underneath the board. Model railway mini banana plugs are installed on keeper. They function as screw-in connector for stylus leads. Most likely Kurenniemi happened to have only one magnet lying around that he decided to use here, as the other DIMI (the one now in Stockholm) had to do without.

Original keeper plate from Helsinki University Studio DIMI-A.

Original keeper plate from Helsinki University Studio DIMI-A.

Sourcing original vintage magnet turned out to be difficult, but I ordered modern replacement from the same company. Keeper plate of contemporary version is much more boring and does not have the cool Eclipse logo the old one has.

magnets

I could easily go on for several pages about the details on this board but I feel it is time to wrap this post up. But at least now it is clear that to actually build a new instrument instead of simply documenting one helps to produce optimal documentation. Already now I have been spending quite a few hours fixing digital files of the bugs and flaws found during the build. If I would build another one, it would be much closer to the original.

To test the board, I hooked it up on Arduino MEGA, did a bit of programming and here you are:

1106+1107 board demo with Arduino MEGA.

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