The later Elka Synthex models had a small MIDI interface built in, consisting of not more than a serial interface device (Motorola 6850), some glue logic and the obligatory photocoupler. The original boards are quite rare, and therefore rebuilds showed up over the years.
I’ve also designed a small board, using SMD components where possible for the ease of assembly and availability of components. Since the firmware used is still the unmodified original, the modern MIDI board suffers from the same problems as the original does – more messages than note on/off would probably screw up the small CPU of the Synthex.

Enough requests provided, a replacement CPU board allowing to MIDIfy the whole synthex (except the LFO section, because it’s strictly analogue) would be a possibe future products. So come on guys…

Oh sure, you want some tech stuff. Here it is, small enough to hide below the 24 wire strip cable: