Keybed scanner

Building the Keybed scanner point out to be more interesting than expected, since the original behaviour and hardware should be maintained as much as possible. Maintaining the electronics include keeping the resistive wires from the tonewheel connectors to the switches and find some electronics which is able to sort out all the cross connections when multiple keys are pressed. In contrast to a regular switch matrix this wiring does not include any diodes etc.
Additionally the whole construct was planned with rather low voltages (some dozen milliVolts) in mind which source low impedance (~16 Ohms) terminations.
Working with voltages of 3.3V or 5V would result in a current of 100-150mA per contact (16+16 Ohms from source to ground) which seems to be inappropriate for those materials besides the fact that it easily sums up to one Amp if all nine contacts of a single key are closed..
 
The final implementation uses a voltage source of about 200mV which sequentially feeds each tonewheel input which creates a current of about 6mA per contact. The 74 tonewheel contacts are connected to two 40 channel demultiplexer boards which nicely fit to the back of the switch enclosure.
The picture on the left was taken during the cabling of the demux boards which are directly wired to the tonewheel contacts. The smaller board on the left (with the MIDI connector for test purposes) contains an ATXMEGA32E5 controller which contains a 16 channel ADC with an internal gain stage, so it easily handles the low voltage levels on the bus, which are 100mV max, with full resulution.

Since we are refurbishing a 40 year old rock'n'roll unit we are not done with just adding some smart electronics. Several parts at least need some cleaning before re-assembly.
The base was treated with a rust converter to avoid further growth of the existing rust. The key bars and caps were just cleaned individually.
It really was fun to assemble the whole unit again, small alignment studs automatically pulled everything into place without any efford. Just as hardware has to be.
The complete unit looks like it was designed for exactly this purpose, as you see below.