Technology splits roughly into the following categories, which will determine how difficult the conversion is. The list is, from least to most difficult
1) 2000's and later instruments - Fully MIDI capable, plug it in and go, with some minor annoyances. I have found instruments from the late 1990s have quirks which are very annoying. An example is a 1998 Johannus Sweelink 30 that sends an "all notes off" message on each piston.
2) 1980's instruments - These instruments have digital key action and scanners, but have analog tone generation systems. The most effective way to convert these instruments is to use the scanners that are there, but replace the control computer with a MIDI capable microcontroller. This is my Op. 4 conversion.
3) 1970's instruments - I'm about to start on a project with a Rodgers of this era (schematics are actually from the late 1960s). Here, the tone generation is analog, and the key scanning is fully electronic, though not digital. In this case, all of the wiring for the keyboards does go to one place, which allows you to use the wiring that is there, but all of the electronics must be replaced. So the wiring task is simplified, but the amount of electronic work is increased.
4) 1960s and before instruments - Here, the key action is electromechanical, and the only option is to strip it down to the switches and wire each key itself. This is the Op. 3 instrument.
There were a number of goals for the Op. 3 instrument. It is an instrument that happens to be going to my new teacher, Tom Mueller, but that's somewhat beside the point. I wanted to experiment with a few things after a less than successful conversion of my Op. 2 (Zimmer) instrument. One of the key things was to try to get a real moving stop action in place without spending a fortune on SAMs. On eBay, I picked up two sets of Allen stop tabs. The first one was from an older instrument. These were more like traditional SAMs. There's a coil for ON, a coil for OFF, and two terminals for a switch. Add all of your own stuff to make it work. It turns out I didn't use these, and they are waiting for another project (along with the Zimmer SAMs). I got a second set from an 80's vintage Allen. The SAMs for this system are interesting:
There's only one control input, high for off, low for on. The problem is that the coil is always energized if the power is supplied, So you need to control the power applied. If you have 60 SAMs, each being 30 ohms (not to speak of the inductive load), and require 24V to drive them, you are talking about almost 50A surge. So some careful design is require to control the power. I have a 20A 24V power supply and some big capacitors. The SAMs act quite quickly - on the order of 50ms, so the energy amount is manageable. I have my controller programmed to provide 125ms pulses when there is a change, which seems more than adequate. The power supply crowbars relatively quickly down to 15V output.
I ended up designing and building my own stop drivers. The existing ones all had issues, and were really expensive. It was also a good experience doing the firmware and hardware development. It took forever, but now that it's done, I can use the same design elsewhere.
The board I built initially on one panel was two boards, each having 48 pulled up inputs and 48 outputs. The outputs are designed by board population option to either have series output 300 ohm resistors to interface with the "new" Allen SAMS, or to have integrated output drivers (ULN2803A) which conveniently include the base current limiting resistor and the buck diode. What I figured out is that it is pretty easy to put the scan chains together and have a single processor with 96 inputs and 96 outputs (of which I used 64)
So the system includes keyboard and pedal scanners from midi-works.com. The stop driver and scanner as well as the scanner for all the pistons is from my board.
Incidentally, I've eschewed the mini display panels. In my recent use, I've found myself using the monitor more and more, (probably because I have an instrument that won't fit on my console), but I've also found the mini display panels really difficult to get working properly. More trouble than they are worth. Plus, this goes to a home installation, which will be far better served by a monitor anyway.
On to the 1970 conversion!! Some day, I'd like to do an Allen. I suspect they might be more interesting because they kept the moving stop action way longer than Rodgers did. Plus, I already have the driver hardware!
