The keyboard is based on a rather typical J-wire style keyboard. 49 J-wires are sequentially pulled to ground by means of a counter/decoder circuit on the keyboard assembly itself. This is synchronized by several signal lines with the keyboard assigner circuit on one PCB below the left hand control panel.
Circuit description
For all circuit descriptions I will use my redrawn schematics attached to the post. The part designators are randomly chosen as none of the original PCBs has component designators of any kind.
The whole scanning of the keyboard including the storing and assigning of key data is clocked by a 555 timer IC running at 14.6kHz with an almost symmetric output. This clock is fed to the assigner circuit by the grey wire connected to pin 6 of the multi-pin connector. Simultaneously it clocks a cascade of two SN7493 4-bit counters.
They are arranged that 6 outputs define 64 time slots of 68.5µs, so a whole scan cycle takes 4.4ms. The lower three bit are connected to the ABC inputs of 7 SN7445 BCD decoders, while the upper three drive another SN7445 which in turn enables one out of the 7 other 7445. As the Q0 output of the latter 7445 is unused, the actual keys are mapped to the time slots 8 to 56.
The last decoder output, which goes low in the 63rd time slot and is used for synchronization purposes via the blue wire on pin 9 of the connector. Additional timing information is derived from the 6th bit of the counters (U3 pin 8) which, via the white wire on pin 1, indicates whether the lower (key 1-24) or higher (key 25-49) half of the keyboard is currently scanned. This is obviously necessary to allow for split assignment.
U3 pin 11, the 7th bit which is not used for the keyboard itself, is also sent via pin 2, brown wire, to the assigner circuit. This plays an important role in the control circuit for the key memories as the actual key data is only interpreted in every other scan cycle, so the real scan time becomes 8.8ms rather than the previously mentioned 4.4ms. The pink wire on pin 7 carries the actual key data, it becomes low for every timeslot in which the associated key is pressed.