VEB Robotron K7637 Replacement Controller and Firmware


13 Feb 2021, 20:15

Dear fellow nerds,

this is my first post on Deskthority. I'd like to present you my newest keyboard controller project. Well, actually my first controller project, although I have previously built and hacked converters. I'm sure you will find it interesting and I hope that somebody can help me to resolve some of my remaining problems.

We are talking about the VEB Robotron K7637 keyboard of course.


I got obsessed with this keyboard. For several years I again and again tried to build a converter and while this basically worked, I somehow managed to fry my hardware. Perhaps it was the EPROM which was exposed too long to sunlight. But even after replacing it with a compatible EEPROM and replacing one of the obviously broken logic gates, it still would not run. And the original serial protocol is too limited anyway. It cannot even discern make from break codes.
Getting a new board also wasn't an option because they (like all vintage keyboard stuff!) tend to be very expensive nowadays.
So eventually I decided to build a replacement controller. I ordered a Teensy 2.0++ because of its number of pins and 5V capabilities. This turned out to be a fortunate coincidence since the Teensy has the exact same size as the UB880D processor and guess where you can most conveniently access all of the matrix lines...

The result is this project:
You can watch a demo here: ... sp=sharing
Please excuse the Bigfoot-video-like quality. Even the melodies sound worse than in real life. It seems my equipment is not good enough to capture the high fidelity of 1988 GDR tech.

Firmware is based on TMK. Sorry, I didn't yet get accustomed to QMK (the new hype). When I last tried to build it, it seemed bloated and "modernish" to me. Required some Python stuff that wouldn't even install on my Ubuntu, even though I do run a LTS. And TMK gets the job done as well and I'm already familiar with it, so why even bother?

Regarding my particular board: I did not yet thoroughly clean the keys. Still waiting for a key remover tool. I mostly used IBM Model Ms and Fs before and their caps come off easily. While I'm at it, I should definitely lube the switches. Again, I have never before done anything like that.
I could use some advice. Which lube would you recommend for Hall Effect keys? But I won't buy Krytox. It costs 17€ for 10g in Germany. That's too expensive. What about Super Synco Lube DE 21030?
Should I completely remove and disassemble all the switches?

Furthermore, while the original board does not have a solenoid, I decided, I couldn't live without one. My plan was to drive it with a standard Arduino-compatible relay module since it already contains a transistor and diode. I will try to use a small 5V solenoid that I can (hopefully) directly drive from the USB power source. I ordered this one from China but it did not yet arrive. Of course, you could just as well drive it directly with a transistor and diode but I like the sound that the relay does all on its own. (Unfortunately, you cannot really hear the relay in the demo video.)
I will furthermore install a flyback diode to secure the relay from any power spikes the solenoid might cause when retracting.
Do you have any further advice on installing the solenoid?
How do solenoids usually trigger on Beamsprings? Do they get activated and released after a given time regardless of how long you press the key?
For the time being, I've implemented a different algorithm: When you press the first key, the solenoid/relay is activated until you release the key. When pressing or releasing any additional key, the solenoid/relay will be released and activated again after 50ms. This IMHO more closely emulates a clicky key mechanism. But we will see how well it works with a real solenoid. Also, the sound will differ depending on whether you press the first or any additional key. (Perhaps we need an array of 10 separate solenoids? Just kidding.)

Perhaps somebody would like to convert his/her board as well. We could also add support for different K7637 variants or closely related Hall Effect keyboards.

PS: I'm open for Robotron deals. I could use some replacement keycaps for instance. Or I'd be willing to convert your board for something in exchange, etc.

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