102 Minimum Theremin Troubleshooting Guide

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(Back to "Assembly Instructions")

If your 102 Minimum Theremin doesn't work, first follow these basic steps:

  1. Make sure that the battery is fresh, and that the red lead of the battery connector is soldered to the "9V BAT+" point and the black lead to the "9V BAT-" point on the printed circuit board (PCB).

  2. Make sure you're using the antenna assembly supplied with the kit, or its equivalent, and that the antenna plug is firmly inserted into the antenna jack. Make sure that the wire between the antenna jack and the "ANT" pad on the PCB isn't pinched by mounting hardware, and that the wire is not excessively long. The wire length should be just enough to go between the jack and the board, plus a little slack. If you have provided your own metal enclosure, be sure that the wire between the antenna jack and the PCB isn't pressed against a metal surface.

  3. Make sure that the "BIAS ADJ" potentiometer (smaller of the two screw-adjusted parts) and the "ZERO CAL" potentiometer (larger of the two screw-adjusted parts) on the PCB are adjusted as described in the "Calibration Instructions."

  4. If you are troubleshooting the complete kit (product 101-KIT), carefully check that all the wires between the PCB and the off board components are connected between the correct points and soldered properly. Make sure that adjacent lugs on the potentiometers, OUTPUT jack, and POWER switch are not bent in a way that would cause them to short-circuit, and that they are free from excessive solder that may "bridge" between them. Make sure the wires to the output jack are not reversed. Make sure that the ON-OFF switch is properly connected. One of the wires to the ON-OFF switch should go to to its center terminal. Make sure that the wires that connect the SPEAKER terminals to its paper cone haven't been broken.

  5. If you are troubleshooting just the PCB assembly, make sure that your user-supplied amplifier-speaker system is working correctly. Check for a faulty connecting cord between the OUTPUT jack and the audio amplifier or a problem with the amplifier, especially if a 60 or 120 Hertz "hum" is evident. If you have a second amplifier and connecting cord available, we recommend that you try them for comparison.

  6. If your theremin functions, but has poor fidelity, the cause may be interference from a nearby source such as an LED or fluorescent lamp, computer monitor, or appliance. You can determine if the problem is being caused by interference by moving your theremin to another location, away from electrical devices, and noting if there is an improvement.

Next, look at the circuit board:

  1. Verify that all the parts are in their correct positions.
  2. Verify that the POLARIZED parts (C5, C7, C8, C9, C10, C11, C13, C14, CR1, U1, U2, U3, and VR1) aren't put in backwards.
  3. Make sure all 14 pins of each U1 and U2, and all 8 pins of U3 have been pushed through the board.
  4. Make sure that none of the wire leads have broken off their component bodies, and that none of the components look excessively discolored or charred from too much soldering heat.
  5. Make sure that none of the conductive traces on the printed circuit board have been burned away by excessive heat or abrasion from soldering.
  6. Check the kind of solder you used, and make sure that it is one of the recommended types, or equivalent, to those described in "Choosing the Type of Solder" in the Assembly Instructions. Improper solders may leave a conductive residue that prevent correct operation.
  7. Look at all the soldered connections carefully with a magnifier, under a bright lamp, to make sure there aren't any fractured solder joints or "solder bridges" between adjacent points. This may happen where points are close together, such as with U1 and U2, or the two closely-spaced points for electrolytic capacitors C5, C7, C8, C9, C10, C11, C13, and C14.
  8. Large "globs" or "peaks" of solder on a connection may hide the fact that the solder has not adhered to both the protruding component lead and the board's conductive pad. Good solder joints will have a thin application of smooth solder that conforms to the contour of the component lead and flows smoothly onto the pad. The contour of the component lead should be clearly visible under the solder, and there should be no "voids," or little air holes, in the connection. For conventional tin-lead alloy solders, the connection should have a bright, smooth appearance with no cracking. The appearance of connections made with lead free (tin) solders will have a duller, somewhat coarser appearance, but should otherwise have similar characteristics to connections made with tin-lead solder.
  9. Improper soldering, especially the application of too much heat, may result in the discoloring of the solder pad's surface and/or the component lead. In such a case, the solder, itself, may also look dull and have a darkened, charred appearance. This discoloring results from oxides on the metal surfaces, and may prevent a good electrical connection between the component lead and the circuit board pad. If such discoloring is evident, then it is advised that all the solder is removed from the connection with Soder-Wick® or a vacuum desoldering tool. Then the metal surfaces should be gently scraped so that the oxidized surface is removed, and fresh metal is exposed. This may be done with a fine-blade hobby ("X-ACTO®") knife, however, take care not to completely sever the thin metal traces that connect to the pad. Once the pad and lead are cleaned, the connection may be resoldered.
  10. Inspect the top (component side) of the board for solder globs between adjacent leads, especially the close-spaced leads of U1, U2, and VR1. Such excess solder may cause short-circuits, and must be removed with solder wick.
Almost all problems with the 102 Minimum Theremin result from improper soldering.

If you still experience problems, please contact sales@harrisoninstruments.com for assistance.

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The 102 Minimum Theremin design is used by licensed agreement with its owner. Specifications may change without notice.

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