Finished Opus 5, 1970 Rodgers conversion

I finally finished this project.  As discussed in previous posts, this project was level 2 of 4 on difficulty.  Here the instrument used electronic key and stop action, but didn't use integrated circuits.  This means that unlike with an electromechanical stop action, there was only one wire per key, and essentially only one wire per stop.  However due to the lack of digital circuits, there are multiple voltages running around.  The design is actually really clever, given the technology available.  For the capture system, this instrument uses core memory!  87 bits of memory total, almost 11 bytes!

The key then is to isolate all of the voltages, and come down to a ground referenced switch.  This was easy for the key action.  All of the wires came to this box (red, purple, yellow for the three keyboards, green for the pedal).  All of the existing circuitry which interfaced with these wires had to be cut away, but it was really just one diode which needed to be cut out for each line to isolate it.  It was pretty simple to find the common for each keyboard and wire that to ground.  Ribbon cables which went to scanners then were straightforward to attach

The pistons were somewhat more complicated, as due to the clever design require in the absence of digital electronics, each of the switches had a different voltage reference.  Once all of those wires were isolated and connected to ground, then the pistons were digitized.

Most complicated were the stops.  These are lighted stops.  The on and off action separately interacted with the rest of the circuitry to gate the stops on and off in the original design.  Almost baffling, and quite clever, but it was all removed.  Also a challenge is that the lights draw over 100mA each, giving 8.7A if all of the stops are lighted.  Obviously my single board with darlington drivers wouldn't be able to handle this (with a .6V drop across the darlington, you'd end up with a maximum of 5W which there's no way I could easily dissipagte) so I had to figure out how to use the lamp drivers already in the stop action.  After a lot of staring at the schematic, I figured out how to make it work.

The drive voltage is 12V.  Keep the drive transistor, and eliminate everything else.  Notice that the switches are 12V referenced, and not ground referenced.  I tied them both together, and used a resistor divider to get them down to the 3V input for my scanner circuit.



The board is designed for some flexibility, but here the jumper is grounded, and with 1K in the RN54 slot and 3.3K in the RN50 slot, you get a 1/4 output, or around 3V.


On the output side, this flexible design can take either a darlington driver or a resistor array.  Put in 1K, and drive the base of the transistor in the existing lamp driver, and you're there.

The wiring for the stop action was tedious but not difficult.  It involved first cutting out 4 resistors and shorting across the SCR for each stop driver, moving the green ON wire to where the brown OFF wire was, and then putting the ribbon cable for the switch and another for the lamp drive in.

Here's the PCB used for the stop action part of the project.  Inputs on the right (note resistor arrays, ICs on the bottom) and outputs on the left.