Enjoying my breakfast and midday lunch (and often evening meal) watching crazy YT-videos, I watched through a series of LMNC’s videos. Today I watched the video about the T2000 repair and the Psych-Tone and was surprised I couldn’t find any info on the latter in here, so I decided to write a few notes, even though, my plans to build one were laid to rest in mid 70s.
My early history
In my younger days I regularly bought Popular, Practical and Radio Electronics and when reading about the Psych-Tone and when I finally ended up at the tech college in Linköping, Sweden, I managed to trace the PCB with curve tape and the round tape-pads and a friend who had connections to people at DATASAAB (SAAB had three divisions back then - cars, airplanes and computers!!!) got me a nice etched board that I drilled and then put aside, due to components (the rotary switches).
Today
Since then I have always been into electronics and always looking for cool musical or sound projects but it isn’t til now I have the time. Anyway, the Psych-Tone is a cool device and may be, as LMNC showed in the video used partially for other projects.
Sources
First of all, the journal, Popular Electronics, Feb 1971, can be picked up at World Radio History, where many many journals and booklets can be found.
I must say that building an ORIGINAL Psych-Tone won’t be easy as it uses RTL (Resistor-Transistor-Logic, a precursor to TTL - Transistor-Transistor-Logic) and those chips are either impossible to find or VERY expensive. Convertying the thing to CMOS is a better idea. Also, I guess some of the transistors are hard to find (I’ve got the transistor kit 
). But once again, what’s cool is the, what was it called? TUNE COMPUTER. The sound-making circuits can be replaced by a synthesizers VCO, VCF, ADSR and VCA.
Tune Computer
The tune computer is actually a slowly clocked PRNG - Pseudo Random Noise Generator - made up by a shiftregister with a feed back signal generated by EXOR-gates hooked up to the shiftregister’s outputs.
Here’s a typical noise generator made from this design (ETI 46000 synth):
Also the noise IC, used here and there (MOOG MG-1) - MM5837 - is based on the PRNG.
This circuit generates a string of 1s and 0s that sounds very much random - noisy. The chain of registers can be made longer and the longer the more “random” will the resulting noise signal be. There are also guidelines showing how and where to attach the feedback gates, but I just point you to some sources describing the principle:
Pseudo-Random Noise; Using it and How to Create it
Pseudo-Random Noise Generator
Pseudo Random Noise Source
Schakelingen: pseudo random noise generator Dutch
Digital Noise Generator
Further deep diving into the WWW-ocean may reveal yet more fun stuff and more info how to arrange the feedback for a larger chain.
Back to the Original
As noted above the ICs used - MC799, MC791 and MC724 - are hard to find and/or extremely expensive. Besides they run on an odd voltage of 3.6V.
The flip-flops ( 
) are used as shiftregisters and the *-gate that either is a NOR, a NAND or MAYBE-gate, constitute the EXOR-gate.
And On To A Modernized Version
A more up-to-date approach would be to use CMOS flips and flops - CD4013 or CD4027 - or go for a shiftregister like CD4094 (or why not a CD4006?!) and a 4070 EXOR-gate.
Here’s a drawing I picked up, signed PHOBoSapiens that shows a complete Tune Computer using more modern components.
This can definitely be another (simpler) random sequencer than for example the Thuring Machine or the 1l/lfinite Melody module I will add to my modular synth.
Hope this triggers some of you to go furter.