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Paia Keyboard

I’ve always been a big fan of Paia gear. When I was a teenager, I built several of their products from plans in Radio Electronics magazine, and I made a keyboard using circuit boards from Paia and parts from places like Jameco and Digi-key. It included a couple Organtua boards and a Stringz ‘n Thingz chorus board. Somewhere along the way, it got dismantled but I saved all the parts.

Nowadays you can get old Stringz ‘n Thingz and Organtua keyboards on ebay from time to time. I managed to score 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 the parts saved from my old keyboard 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. To bring it into modern times, I even added a MIDI interface.

Here's a couple pictures of the finished unit.  It's slightly bigger than a stock Stringz 'n Thingz, but it still preserves the same vintage Paia aesthetic including the catches, slip-hinge removable cover, and suitcase handle.
Front Panel

Here is a view of the controls and below, the artwork used for the panel.

All the original Stringz 'n Thingz controls are still there, plus 5 Organtua ranks.  Ranks 1 to 3 are identical to a stock Organtua, and ranks 4 and 5 are a special set that are run through a polyphonic VCA circuit to make them more bell-like and then through a second Stringz 'n Thingz chorus board.

Controls are provided for all the functions 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.
Rear Panel

The rear panel provides MIDI and mix outputs as well as individual outputs for the organ 1-3 ranks, organ 4-5 ranks, strings, and electric piano signals.  I use the four individual outputs to drive the four inputs of my Chameleon synth processor.

Here are some pictures to show what's going on inside the case.  There are also a couple boards under the control panel. 

The bottom photo is a 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 individual diode VCAs and then are fed to the second chorus board. The final board closest to the keys is the MIDI interface, based on a PIC16F877A microcontroller.  Schematics for everything are shown later on this page.

When I got my Stringz 'n Thingz from ebay, the keyboard bushings - those little rubber bumpers that stop the “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.

Here's a drawing to help visualize the layout of everything in the case.
Panel Wiring

There is a lot in this little case, really need a good roadmap to put it all together!  Note the colored circles that identify sources or destinations for all the connections.

Strings Schematic

I apologize for the really bad quality of this schematic, it's my only photocopy of the Radio-Electronics article schematic from 1978!  What's important to me now is the interface connection points shown by the colored circles.
Organ Schematics

Here you have the original Organtua organ schematics, amended to show all my panel connections and additions. 

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.  This circuit is captured in the bottom photo of this section.
Chorus Schematics

As I mentioned earlier, there are two chorus boards - one for the stock Stringz 'n Thingz and one for my custom organ ranks 4 and 5.

Recently I decided to implement
Stefan Vorkoetter's excellent noise reduction modification for both of my chorus circuits.  I recommend it highly - it makes quite a difference in the sound quality.   This mod circuit is spliced into the chorus boards at the places shown with the blue boxes.  The actual mod circuit is the third photo in this section.
MIDI Interface Schematic

The 12V key switches that are used to key the voices 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.
Mixer Schematic

This is a pretty basic mixer and output buffer for the 4 sound sources.  The piano output from the Stringz 'n Thingz gets its own noise gate instead of being mixed through the same one as the strings sound. 

I was getting some hum interaction between the organ and string ranks, but a couple audio isolation transformers (shown in the lower middle part of the schematic) solved that problem.
Power Supply

Paia's design has a +/-12V supply sharing space on the chorus boards.  Since I have two chorus boards in my unit, I use one to power the strings and MIDI interface and the other to power the organs.