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)

^
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.
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