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The Pitch Board

- This website is about recapturing an original classic theremin sound -

The pitch & volume boards are connected together through the J2 TRS jack. 12 volt power normally goes to the volume board first then passes a regulated 9 volts through J2 TRS jack to the pitch board. The RF section uses 9 volts, the audio section 5 volts. All this from a single 12 volt AC transformer with ground. Amazon

The NPN MPSA42 transistors are preferred over anything else for a much fuller sound. The right side of this graphic below is the Harmonic Exciter that was designed for use as the EWS modification but the EWS use of transistors does not deliver what the 300 volt MPSA42 do. (I tried 1000v transistors but they would not oscillate)


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The voltage at TP2 should only be about .5v p-p for proper wave shape. It can be boosted to 1.5v p-p on the Volume Board.

The easiest way to dial in the oscillators is to use an analog AM Radio tuned to 900 kHz. Each oscillator will create a blank spot in the radio background noise. You want to tune the oscillators so their frequencies are on top of one another. When heterodyning begins the whistle sound will scream over the radio and at TP2. A digital radio will not work for this test.

For someone only wanting to build a simple pitch only theremin which you can expand upon in the future, then just build this one board at first.

T4-Null terminal for Pitch Null is modified so this board mount 1k potentiometer can be used with an extension tube for access when doing a simple theremin. Have Pot facing downward towards Pot-3.

Theoretical, needs more study:
Pot-3 is experimental and used for
thermal drift compensation. The first best way to compensate for thermal drift is to use a VOM meter that has the ability to measure a rough transistor Hfe or forward gain. Without touching your transistors so not to influence its temperature find two that have an identical Hfe number or very close. Use two matching transistors in the L1 & L2 oscillators. Extra MSPA42 are added to the project for this purpose of finding a match.

A properly biased transistor will take Hfe out of play but our interest is in thermal sensitivity. Adjusting current flow in either transistor cancels or balances out thermal drift behavior. Have Pot-3 set half way at first. If you have thermal drift adjust Pot-3 either way and see if it gets better or worse, worse then turn the Pot in the other direction. Not critical, go full either way.


 Mount these first to avoid confusion C7 = 100pf  /  Mount next C17, C19 = 39pf

 C2, C5, C8, C9, C18, C20, C27 =.1uf  /  C13, C16, C22, C23 = 47pf  /  C6, C21, C28 = 10uf  C4 = 10uf tantalum

 C1, C11,  C25, C14, C15 = 1uf  / C29 = 470uf  / C10 = 1000uf Not used if volume board used

 C3 not used, Output-2 for any optional use

  R3 & R10 on left side of Q3 changed to 100k




Phoenix 2020    300 Hz


1929 RCA Theremin Waveform

This is the authentic waveform, the theremin grail, neither Lev Sergeyevich or myself create it, rather we capture it.


I had to purchase ViewMate after the computer crash loss, still learning it.


Drill Sizes:   #22 - 4mm - .157"       #52 - 1.6mm - .0635"       #56 - 1.2mm - .045"       #65 - .9mm - .0354"

I encourage you to do home tin plating after etching to preserve it from corrosion. There is a trick to good plating.

Post your experience in my Chat Forum maybe others can eventually help one another.

4.17.19 Last update

 

 

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