video upload by Instrument Attic
via this eBay listing
"The 1966 Johnson Intonation Trainer is a rare and intriguing instrument developed by the E.F. Johnson Company of Waseca, Minnesota. Originally designed to assess pitch-matching and pitch-discrimination abilities, this device has garnered attention for its unique features and potential applications in modern music production.  
One of the standout characteristics of the Johnson Intonation Trainer is its full polyphony, allowing all 37 keys to be played simultaneously. Each key is associated with its own oscillator, enabling microtonal adjustments and the exploration of unconventional scales and tunings. The instrument offers four distinct voicing options, ranging from basic sine waves to more complex, reedy tones. Additionally, the “Scale Selector” switch provides a choice between standard tempered tuning and a tunable mode, with individual “Tunable Scale Controls” corresponding to each note on the keyboard for precise pitch calibration.    
Housed in a robust fiberglass case, the Intonation Trainer features a built-in speaker, though it can also be connected to external amplifiers or mixers via standard 1/4” outputs. Some units include a vibrato effect, though functionality may vary between models. 
In contemporary settings, the Johnson Intonation Trainer has found a niche among experimental musicians and sound designers. Its ability to produce rich drones and facilitate microtonal exploration makes it a valuable tool for creating unique sonic textures. The instrument’s vintage analog circuitry imparts a distinctive warmth and character that is highly sought after in modern music production.
Due to limited production and the passage of time, functional units of the Johnson Intonation Trainer are exceedingly rare. Finding one in full working condition, complete with original documentation, is a remarkable discovery for collectors and enthusiasts alike. The scarcity of this instrument, combined with its unique capabilities, underscores its status as a treasured piece of audio history."
via this eBay listing
"Made in 1966 in Waseca, Minnesota by the E. F. Johnson Company. Originally designed as an ear training and intonation device, but in practice it is a unique fully polyphonic analog synthesizer with extensive tuning control. Very few were made, and surviving examples in this condition are vanishingly rare.
