Test and Calibration Procedure
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  1. Remove the plastic cover from the instrument. Apply power by connecting the 9 volt battery and setting the power switch to the "ON" position. Connect the instrument's ground to earth ground. The earth ground connection to the instrument may be made via the "sleeve" contact of the LINE OUTPUT JACK, J1, or its metal support bracket.

    Keep test leads and other objects away from the circuit board and antennas, as they will adversely affect the calibration.

    Refer to the One-page Schematic and the Circuit Board Component Locations for TEST POINT locations.

  2. Place both potentiometers in their central positions.

  3. Connect a DC voltmeter (10v DC scale) to the circuit as follows:

    Meter NEGATIVE lead to TPG (circuit ground).

    Meter POSITIVE lead to TP8 (output of the voltage regulator).

  4. Observe +7.50vdc +/-0.35vdc.

  5. Connect an oscilloscope (50mv/div. vertical sensitivity, AC coupled; 5uS/div. horizontal sweep rate, internally triggered) as follows:

    Oscilloscope GROUND LEAD to TPG.
    Oscilloscope PROBE to TP8 (output of the voltage regulator).

  6. Observe less than 40mv peak-to-peak power supply noise.

  7. Connect the oscilloscope PROBE to TP1 (output of the PITCH REFERENCE OSCILLATOR). (Oscilloscope set for 2v/div. vertical sensitivity, DC coupled; 1uS/div. horizontal sweep rate, internally triggered.)

  8. Observe a sine wave form: 4v peak-to-peak with the upper excursion at 5.5v, and a period of approximately 2.8uS.

  9. Connect the oscilloscope PROBE to TP3 (output of the PITCH VARIABLE OSCILLATOR).

  10. Observe a sine wave form: 5v peak-to-peak with the upper excursion at 6v, and a period of approximately 2.8uS.

  11. Move your hand to and from the PITCH ANTENNA and observe that the wave form period increases slightly with decreasing hand distance. The total period variation will be approximately 75nS, corresponding to a frequency deviation of approximately 10kHz.

  12. NOTE: To obtain an audible output from the theremin, the two pitch oscillators must be very similar in frequency; it is their difference frequency, generated by the MIXER, that equals the audible pitch. Therefore, critical adjustment of these oscillators' frequencies is important.

    For uncalibrated instruments, the initial relation of the two pitch oscillators will not produce an audible MIXER output. Assuming this is the case, the next step is to adjust variable capacitor C7, and observe if the MIXER output shifts toward an audible frequency.

    Connect the oscilloscope PROBE to TP2 (output of the MIXER). (Oscilloscope set for 200mv/div. vertical sensitivity, AC coupled; 1mS/div. horizontal sweep rate, internally triggered.)


    REMINDER:  Keep test leads and other objects away from the circuit board and antennas, as they will adversely affect the calibration.

  13. Observe the wave form while slowly adjusting capacitor C7.

    Adjust C7 so that the AC wave form is absent with the hand away from the PITCH ANTENNA, and rising in frequency as the hand approaches the antenna. For a difference frequency of 50Hz, the MIXER output wave's amplitude will be about 80mv AC peak-to-peak. At this frequency, the wave will be pulse-shaped with significant asymmetry. As the frequency increases, the wave will become more sinusoidal. At 5kHz, the wave's amplitude will be about 40mv AC peak-to-peak.

    If the two oscillators' frequencies differ substantially, no AC wave form will be evident at the MIXER output. In this case, a selected value will have to be inserted in the PITCH VARIABLE OSCILLATOR's C14 position, or C16 will have to be replaced with a different value capacitor, or both.

    If the adjustment of C7 alone produces satisfactory results, proceed to step 22. If not, proceed with steps 14 through 21.

  14. A frequency counter is used to measure the specific frequencies of the two oscillators. Although the 145 Theremin oscillators are buffered, the IMPEDANCE BUFFER in FIGURE 2 is recommended to provide additional isolation between the counter and the measurement points. Make the connection between the IMPEDANCE BUFFER and the circuit short and direct (less than one inch of unshielded wire) to prevent excessive capacitive loading of the circuit.



  15. Connect the IMPEDANCE BUFFER's output to the frequency counter.

  16. Connect the IMPEDANCE BUFFER's input to TP1 (output of the PITCH REFERENCE OSCILLATOR).

  17. While slowly adjusting C7, observe the counter. Select a position which produces a median frequency. This approximately corresponds to the center position of C7's rotation, leaving sufficient margins for subsequent adjustments.

  18. Record the indicated frequency. A typical value is 360kHz.

  19. Connect the IMPEDANCE BUFFER's input to TP3 (output of the PITCH VARIABLE OSCILLATOR).

  20. Record the indicated frequency. A typical value is 360kHz.

  21. Determine if the PITCH VARIABLE OSCILLATOR's frequency is higher or lower than the PITCH REFERENCE OSCILLATOR frequency.

    If the PITCH VARIABLE OSCILLATOR's frequency is higher, install a 5pF capacitor in the C14 position, and return to step 13.

    If the PITCH VARIABLE OSCILLATOR's frequency is lower, replace C16 with a lower-value capacitor, such as 130pF, and return to step 13.

    Repeat the procedure as necessary, increasing C14's value by 5pF each time.

  22. When calibrated, the mixer's AC output wave form is absent with the hand away from the PITCH ANTENNA, and commences, rising in frequency, as the hand approaches the antenna. Use the PITCH NULL POTENTIOMETER to "fine-tune" the instrument's response to the pitch hand's position. Optimally, the onset of the MIXER output at TP2 will occur with the hand at about two feet from the antenna.

    NOTE: The distance-to-pitch relationship may be reversed, the highest pitch evident with the hand away from the antenna, depending on the specific adjustment of C7 and the PITCH NULL POTENTIOMETER.


    REMINDER:  Keep test leads and other objects away from the circuit board and antennas, as they will adversely affect the calibration.

  23. Connect the oscilloscope PROBE TP4, (output of the VOLUME OSCILLATOR). (Oscilloscope set for 2v/div. vertical sensitivity, DC coupled; 1uS/div. horizontal sweep rate, internally triggered.)

  24. Observe a sine wave form: 3.5v peak-to-peak with the upper excursion at 5v, and a period of approximately 3.7uS.

  25. Move your hand to and from the VOLUME ANTENNA and observe that the wave form period remains constant.

  26. Connect the oscilloscope PROBE to TP5 (slope detector buffered output). (Oscilloscope set for 1v/div. vertical sensitivity, DC coupled; 1uS/div. horizontal sweep rate, internally triggered.)

  27. Slowly adjust C25 while observing the oscilloscope. Observe a waveform, centered at approximately 3.5 volts, that varies in amplitude as C25 is adjusted. The amplitude value of the sine wave should vary from approximately 0.8v to 3v peak-to-peak. Symmetrical clipping distortion will occur at approximately 2v peak-to-peak.

    If the VOLUME OSCILLATOR frequency differs substantially from the parallel-resonant frequency of L3 and C21 in the VOLUME PROCESSOR, an insufficient sine wave amplitude and amplitude variation will result at TP5.

    In this case, a selected value will have to be inserted in the VOLUME PROCESSOR C24 position, or C21 will have to be replaced with a different value capacitor, or both.

    If the adjustment of C25 alone produces satisfactory results, proceed to step 30. If not, refer to step 28, which provides a procedure for matching the VOLUME PROCESSOR's resonance to the VOLUME OSCILLATOR's frequency.

  28. With the oscilloscope PROBE still at TP5, set the vertical amplifier to AC coupling and center the trace. Increase the oscilloscope's vertical sensitivity until a wave form is visible.

    Start by inserting a 5pF capacitor in the C24 position in the VOLUME PROCESSOR circuit. Note the wave form's amplitude. If it increases with the addition of the 5pF capacitor, replace the 5pF capacitor with a 10pF capacitor and determine if a further increase is evident. Repeat this procedure with successively larger-valued capacitors until the greatest possible wave form amplitude is obtained.

    If the above procedure does not result in an increase in wave form amplitude, remove any capacitors from the C24 position, replace C21 with a lower-value capacitor such as 300pF, and return to step 27.

  29. With the oscilloscope PROBE still at TP5, adjust C25 so that the wave form amplitude is maximum with the hand furthest from the VOLUME ANTENNA, becoming smaller as the hand approaches the antenna. The wave form amplitude is greatest when the parallel-resonant frequency of L3, C21, and C24 in the VOLUME PROCESSOR equals the frequency of the VOLUME OSCILLATOR.

  30. Connect the oscilloscope PROBE to TP6 (VOLUME PROCESSOR output). (Oscilloscope set for 1v/div. vertical sensitivity, DC coupled; 1mS/div. horizontal sweep rate, internally triggered.) Observe a DC voltage level that varies from approximately 2v to 6v with the hand's proximity to the VOLUME ANTENNA. The DC voltage level at TP6 is lowest when the parallel-resonant frequency of L3, C21, and C24 in the VOLUME PROCESSOR equals the frequency of the VOLUME OSCILLATOR.

  31. Once an adequate response at TP6 is observed, connect the oscilloscope PROBE to TP2 (the MIXER's output). Adjust the PITCH NULL POTENTIOMETER so that a wave form is present regardless of the hand's proximity to the PITCH ANTENNA. This will provide a constant signal to further calibrate the volume circuit. The exact frequency of the wave form is not critical; any frequency around 400Hz will work.

  32. Connect the oscilloscope PROBE to TP7 (audio preamplifier output). Slowly adjust C25 while observing the oscilloscope. An AC wave form should appear within a segment of C25's adjustment range. Set C25 to the position where the wave form just appears.

  33. Move your hand near the VOLUME ANTENNA and observe that the wave form's amplitude increases as the hand's proximity decreases. NOTE: There are two different C25 positions that will satisfy this criteria, however, only one of the adjustment positions will provide the proper volume response. Correct calibration will be evident when the onset of the wave form at TP7 occurs with the volume hand approximately 12 inches from the VOLUME ANTENNA, increasing in amplitude as the hand becomes closer. "Fine-tune" the volume response with the VOLUME NULL POTENTIOMETER. Return the PITCH NULL POTENTIOMETER to its normal position.

    C25 may be readjusted to obtain an inverted (traditional) volume response, in which the output amplitude is highest when the volume hand is furthest from the antenna.

    DANGER:  The following step uses headphones. In the event of a circuit malfunction, dangerous volume levels may occur in the headphones. If the headphones have a built-in volume control, initially set the control for minimum volume. If the headphones do not have a volume control, be sure that the volume level is reasonable under all circuit conditions, before wearing them.

    Do not play this instrument at a high volume, especially when using headphones. Use headphones that have a built-in volume control, and adjust the volume control for a comfortable level. Hearing experts advise against the continuous, extended use of headphones.

  34. Connect headphones to the HEADPHONE OUTPUT JACK, J2 of the instrument. Place each hand in the proximity of their respective antennas, and observe an audible pitch that may be frequency modulated with one hand's proximity to the PITCH ANTENNA and amplitude modulated with the other hand's proximity to the VOLUME ANTENNA.


    Text and drawings ©2000, 2002 by Arthur Harrison

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