Paia Keyboard

I’ve always been a big fan of Paia gear. I built several of their things from plans in Radio Electronics magazine. The Oz keyboard was one of the first things I ever made. My version was made using a circuit board from Paia, and parts from my own suppliers. The original Oz circuit board was for a 1½ octave design, but since I had a 2½ octave keyboard from an old organ, I added a second octave – and so it became a sort of a super-Oz or mini-Organtua depending on how you look at it. A couple Organtua boards and a Stringz ‘n Thingz chorus board from Paia were added later for additional ranks. It eventually ended up getting scavenged for parts for other things.

Nowadays you can get old Stringz ‘n Thingz and Organtua keyboards on Ebay from time to time. I picked up a pretty beat up Stringz ‘n Thingz cheap, and to my surprise, it included not only the Stringz ‘n Thingz circuit boards, but also a couple of those Organuta boards too! So combined with my boards from the super-Oz (see above) still in my junk box after all these years, I made a new case and rebuilt the whole thing into a hybrid unit that actually has the functionality of both a Stringz ‘n Thingz and an Organtua on steroids in one unit. Pretty cool. And if that wasn’t enough, I even added a MIDI interface to drive other stuff (more on this later).









By the way, the keyboard bushings - the little rubber bumpers that stop that annoying “thump” when you press and release keys - were old and dried out so I got a replacement set from Archive Sound. Easy to install, and highly recommended for restoring or rebuilding these old units.

The Stringz ‘n Thingz uses a very different keying circuit than the Organuta. The Organtua uses a MK50240 top octave generator to create individual notes as square waves and then connects those individual note outs through mixing resistors and the keyboard switches to the output. The Stringz ‘n Thingz uses a diode switching scheme to trigger, sustain, and decay notes. To make them compatible, I added a 37 note diode switching circuit to the Organtua outputs so the same key signal that triggers the strings also triggers the organ:



The 12V key switches are also sampled and used to trigger the MIDI processor to generate MIDI data. There’s no velocity sensitivity, but note on and note off information is sent out from the keyboard. A split of the data across MIDI channels 1, 2, and 3 is also performed based on the control panel switch settings. So even though the keyboard is small, it is zoned to allow simultaneous control of multiple MIDI devices.



Here’s a detailed picture of the card rack at the right side of the unit. The 3 blue boards at the top are the Organtua ranks 1, 2, and 3. The next board is the diode switching board. The next two beige Organtua boards are ranks 4 and 5, which are keyed through a decaying version of the diode switching circuit shown above and are fed to the second chorus board. The final board closest to the keys is the MIDI interface, based on a PIC16F877A microcontroller.



Here is a view of the control panel artwork. Controls are provided for all the funtions including tuning of the two master organ oscillators, tuning of the strings section, split point for the strings (cello vs. violin), octave selection of each of the 5 organ ranks, modulation and attack rate for the organ ranks, independent chorus control of the strings and the rank 4/5 outputs, sustain times, individual level controls, and MIDI modes.