Troubleshooting Advice

Please read our Hazard Warning page.

Safety Warning:

Anyone attempting repair or adjustment of an electrically live RCA Theremin is at risk of burns and potentially fatal shocks. While the RCA Theremin is engineered to be safe under normal operating conditions, a number of factors can compromise this built-in safety. Numerous points in the instrument operate at dangerous voltage potentials.

Power and speaker cords on RCA theremins and vintage speakers are now more than 80 years old and insulated only with cloth. To prevent electrical shock prior to use, always inspect all power and speaker cords (high voltages can be present at the speaker jacks) for exposed bare wire and/or breaks in the insulation, and have them professionally replaced if needed, using correct fabric covered reproduction wire; stranded 16 AWG for the AC power cord and litz wire with correct pin style ends for the speaker.

The AC plug (including the plug of the RCA 106 loudspeaker) should also be inspected for loose wires and stray strands. Dress any loose strands safely away from opposing prong or screw, tighten screws and re-inspect before before plugging into the mains. When unplugging, always grasp the outside of the plug. Do not unplug by pulling on the wire. If using a power extension cord, always be certain it is properly rated to handle the current load.

No Sound or Weak Sound

Assuming that the upper instrument chassis is correctly wired to the SPU (power supply), tested vacuum tubes are all in their proper sockets, and the theremin is correctly connected to the speaker and power, absence of sound can be caused by:

  1. Power switch is turned on, on the RCA 106 loudspeaker, but has been left off on the theremin (or vice-versa). Turn both switches on.
  2. Defective power or mute switch. Temperamental toggle switches are a known issue with 80+ year old electronics. For the power switch on both the theremin and the RCA 106 loudspeaker, the pilot lamp (located between the theremin control knobs) is your indicator of power on at the chassis.

    To test the mute switch, unplug the theremin, withdraw the shelf just enough to access the mute switch lugs, and test the switch for continuity.

  3. A defective speaker. Test for a resistance reading across the speaker wire tips, with tips removed from the jacks. Unplug the theremin prior to removing the speaker pin plugs.
  4. Incorrect speaker. See that the speaker impedance is in the 1,000 to 2,000 ohm range. An 8-ohm or modern speaker, if connected directly into the pin jacks on the SPU, will work with very low volume or not at all, and can result in damage to the theremin.
  5. Defective vacuum tube or tubes. Test all vacuum tubes prior to analyzing for further problems. Tests should be made on a mutual conductance type of vacuum tube tester if possible. The vacuum tubes that are most involved with the audio are the type 27 pre-amplifier, type 71A (both oscillator and output functions), and the UX-120. If either pitch oscillator vacuum tube (also type 27) is bad, or not making good contact in the socket, this can also result in no sound, or a very faint pitch that's affected by hand capacitance.
  6. Volume oscillator trimmer (on chassis rear panel) improperly adjusted.

To test the volume control oscillator section, you can bypass the safety interlock switches (note warning elsewhere on this site), electrically or by holding down the two spring-loaded levers with the instrument turned on. With room lighting dimmed, watch for the filament glow in the UX-120. With the hand away from the volume loop antenna and away from the volume resonant coil, the filament should be moderate to brightly lit. When bringing the hand close to either the loop antenna or coil, you should notice the UX-120 filament dim and extinguish.

Vacuum Tubes Not Lighting

See section 1 and 2, above. Note that it's normal for the vacuum tubes to not light when the cabinet doors are open, assuming that the safety interlock switches are connected and working properly. Opening the doors, for the safety of the user, automatically shuts off power.

Also note that the glow of most of the vacuum tubes used in the RCA theremin is normally quite dim, and can be hard to see. Additionally, the metallic coating (getter) on the inside of the glass on some vacuum tubes can make filament viewing difficult. To see if the vacuum tubes are lit, follow the protocol described under section 5, above. Viewing in a dark room is essential, and it may be necessary to use a dental mirror to view the filament illumination on the vacuum tubes that are located behind the tall resonance coils. Avoid contact between the metal of the mirror and any part of the theremin components or chassis.

Finally, if the vacuum tubes test good, and some but not all are lit, make sure that the vacuum tube pins are clean and bright to ensure best electrical contact with the terminals inside the socket. Damaged, loose or corroded terminals inside the vacuum tube sockets can prevent the filament and other vacuum tube elements from making proper contact.

Sudden or Jagged Changes in Pitch (Skip Effect)

There are three primary causes of sudden, unintentional jumps in the pitch, either up scale or down, instead of a smooth, sliding tone when you move your pitch hand:

First, make sure you have good electrical contact at the pitch antenna (clean & bright, snug fitting antenna end), and good contact at the wire that connects the pitch antenna socket to the pitch resonant coil, with clean, tight connections at both ends. If the antenna is loose in the socket, carefully spread the legs of the plug end of the antenna, just enough to make a snug fit in the socket. Do not force the spread more than a millimeter or so, or you risk breaking off a section of the antenna end!

The vacuum tubes of an RCA Theremin, with the 27 vacuum tubes highlighted

The vacuum tubes of an RCA Theremin, with the 27 vacuum tubes highlighted.

Second, if all electrical connections as noted above are clean and secure, try simply switching the two pitch oscillator vacuum tubes. If this fails to correct the condition, try substituting each pitch oscillator vacuum tube with the audio preamplifier vacuum tube, which is the same vacuum tube type as those used for the pitch oscillators. The pitch oscillator vacuum tubes will work fine in the pre-amp socket. These vacuum tubes are highlighted in the adjacent illustration.

Additional Causes of Trouble

Open sections of the ceramic voltage divider resistor (located in the upper instrument chassis) are a common culprit when an RCA Theremin isn't working properly or at all. In this case look for missing voltages. One of the most common problems is caused by incorrectly adjusted trimmer capacitors on the back edge of the chassis that someone has monkeyed with. Maladjusted trimmers result in mismatched pitch oscillator frequencies (compressed or reversed playing arc and limited octave range), poor or no volume response, volume hand affecting the pitch, etc. Correct setting of the oscillator frequencies is key to a properly playing instrument. Even if an RCA Theremin has 100% perfect components, it will not work right if the trimmers are adjusted wrong, so it's important to determine the trimmer settings before looking to replace any parts.

Other common problems are caused by tarnished or bent vacuum tube socket contacts (and/or tarnished vacuum tube pins), breaks in the #38 AWG wire windings on the tall resonance coils (especially where they span open areas beyond the turns), oxidized or loose connections on the SPU terminal strip and wiring harness lugs, wire strands broken at a lug (or shorting to an adjoining lug), possible open wire-wound card resistor in the SPU, loose or tarnished connections at the UX-120 filament induction-coil terminals on the volume resonance coil form, or even an open or intermittent connection in the litz speaker wire and pin plugs, which can appear intact but be open internally. Another common issue is intermittent toggle switches—See section 2, above.

An easy thing to try is simply to remove the vacuum tubes and reinsert them in their sockets, one at at a time so you don't mix them up. You can fix a mildly compromised electrical connection this way. An RCA Theremin that was repaired in the northeast was found to have some instability two years later, and the problem was completely cleared up simply by pulling and reseating the vacuum tubes. Humid climates can exacerbate this problem. Naturally, the vacuum tubes must first test good.

For all issues where an RCA Theremin is partially functional, but not working right, the 1929–30 RCA Service Notes (Red Book) should be consulted, as it has a nicely organized troubleshooting section prepared by RCA engineers. This information is readily available on line.

Testing Vacuum tubes

See section 5, above. There are many makes and models of vacuum tube testers. We prefer the Hickok 532, 534, 539 series of tester, or the military model TV-7 series. In our own shop we use the Hickok 539C. Note that there isn't a test on the 539B and 539C for the UX-120 on the obsolete data charts. For this vacuum tube, set up the tester for a 99 (filament: 3 volts, selectors ER-3200-0), and set the bias control for 22 volts. A micromho reading of about 325 or higher is good. If it has been many years since the vacuum tube has had power applied, it can take 15 - 30 minutes of warm-up time before a stable reading can be taken.

There are certain protocols for obtaining accurate vacuum tube tests, including the condition and calibration of the vacuum tube tester itself. After a warm-up of at least a minute or two, vacuum tubes should be tested for shorts and secondary emission, in addition to the primary tests. When checking for shorts, gently tap or thump the vacuum tube a couple of times for each position of the shorts switch. Tapping the vacuum tube this way from two different directions can sometimes reveal a short, when tapping from a single direction or angle will not.

Obsolete vacuum tube data for the Hickok 539B & 539C can be found on the manuals page.

To avoid accidentally burning out the vacuum tubes, always check to make sure the tester's filament switch is correctly set. 20s and 71As can get expensive.

Tuning for Range

Many RCA Theremins that have not been restored will require touching up of the trimmer controls on the rear skirt of the chassis, in order to achieve a zero-beat over a usable portion of the front panel pitch control knob, and to yield an appropriate octave range. RCA Theremins are commonly stated to have a 3 1/2-octave range, but the authors have found that a range up to 5 usable octaves is possible with careful adjustment of the pitch oscillator frequencies. In some cases, the range will be limited to the lower figure, depending on the length of the pitch resonant coil windings and the condition of the frequency-determining capacitors in the pitch circuits.

Detailed directions for adjusting the pitch oscillator trimmers can be found on page 10 of the RCA Theremin Service Notes. Adjustment of the volume trimmer can be found on page 9. When adjusting the trimmers, never force them tightly closed (fully clockwise). If the settings cannot be achieved by following the proper procedure, the trimmers might be damaged, or other components may be out of tolerance. Refer further servicing to qualified personnel.

checking power & 120/110 switch

The SPU Voltage switch, shown switched to the 120 volt setting

Make sure the switch is thrown to the right, as in this illustration.

There is a toggle switch on the forward right side of the SPU next to the type 80 rectifier vacuum tube. Be sure that this switch is always thrown to the right (120 volt position), to ensure longevity of the power transformer, other components in both chassis, and to extend the life of the vacuum tubes. Operating the theremin with the switch in the left, 110-volt position causes the power transformer, voltage divider resistors and vacuum tubes to run hotter and puts unnecessary strain on these valuable parts. Most municipalities today maintain line voltage above 110 volts, most of the time. It has been established also that a 10% reduction in the operating voltages of vacuum tubes (below their rated filament or heater voltage) can extend vacuum tube life as much a 100%. While operation in the 120-volt switch position may not reduce these voltages by as much as 10% below ratings, the same basic philosophy applies.

AM radio trick

Here’s a way that you can check to see if the pitch and volume oscillators are working in your theremin, even if you have no sound out of the speaker. This will help you narrow down where a problem may be originating from. (For demonstration purposes on a working instrument, the same test can be conducted with the mute switch closed, i.e., down position of the toggle switch on the front panel):

Volume Oscillator Check

Place an AM radio near the theremin, and tune the radio dial to the vicinity of 780 to 850 KHz on the AM dial. If the oscillators are working, you will hear a heterodyning whistle or raspy pitch, as you approach this part of the AM dial. What you’re hearing through the AM radio speaker is the 2nd harmonic of the theremin’s volume oscillator. The theoretical correct frequency of the theremin’s volume oscillator is 420 KHz, so in a perfect world you would come into and out of zero beat (of the volume oscillator) at 840 KHz. However, due to normal aging of the components and the setting of the theremin’s volume oscillator trimmer, you’ll likely hear this signal near, but not on the 840 AM dial position.

You can test to see that theremin’s volume oscillator is responding to space control, by moving your hand near the volume loop antenna. This will result in a change of pitch, in the speaker of the AM radio because you are changing the frequency of the volume oscillator circuit, which is exactly as it should be, if everything is working right. So even though it’s the volume side of the theremin, it’s normal to hear a pitch change in the AM radio speaker as your hand moves in and out of the volume antenna field.

As you sweep across the AM dial you will hear multiple places where you can pick up the volume oscillator, but these are more distant harmonics of the volume oscillator, and will be correspondingly weaker. There will be one AM dial position that you will find to be louder than the rest, and this is the actual 2nd harmonic of the volume oscillator. Note that if you’re correctly tuned in to the volume oscillator, you can change the pitch coming from the radio speaker by moving your hand near the theremin’s volume antenna, but you cannot change the pitch by moving your hand near the pitch antenna.

Pitch Oscillator Check

You will find the 3rd harmonic of the theremin’s pitch oscillator at about 525 on the AM dial. Some AM radios don’t go down this low, so alternatively you can hunt for the 4th harmonic at about 700 on the AM dial, although it will be weaker. As with the volume oscillator, drifting component values and trimmer settings will affect the exact position on the dial where you will pick up the theremin’s pitch oscillator.

To confirm that you are in fact tuned into the pitch oscillator, the note heard in the AM radio speaker will be affected by your hand position in relation to the pitch (as well as to the volume) antenna.

If you are not able to successfully perform the tests outlined above, you may have a defective oscillator stage, caused by a poor vacuum tube, compromised contact between the vacuum tube pins and internal socket contacts, open section of the voltage divider resistor or other defective parts.

These tests are not affected by the position of the mute switch.

Neon light trick

Another simple way to confirm that your theremin’s oscillators are working (a favorite quick-check method of Howard Mossman's) is to touch a miniature neon lamp, such as a type NE-2, to each of the theremin’s antennas. If the oscillator is working the neon lamp will glow. No connection to the neon lamp wires is needed. If you have a number of neon lamps to select from, choose the one that has the closest spacing between the electrodes inside the bulb. See additional details below.

Volume Oscillator Check

Once you’ve established that the neon bulb will glow when placed on or very near the volume antenna, you can check that the theremin’s front-panel volume control is functioning by turning the volume knob while the neon bulb is touching the antenna. Your result will somewhat depend on the particular bulb you use, as some bulbs are more sensitive than others. The glow in the lamp will be diminished or extinguished when the knob is turned counter-clockwise, and the glow will increase when the knob is turned clockwise. This is a visual analogy to what is happening in the filament of the UX-120 volume vacuum tube inside the closed cabinet. If the neon glows the same regardless of the control setting, you can obtain the variable-glow result by holding the bulb a fraction of an inch from the volume loop (for example with a layer or two of handkerchief between antenna and bulb).

Pitch Oscillator Check

Repeat the tests with the neon bulb in contact with the pitch antenna. In some cases (again, depending on the sensitivity of the bulb), you might not obtain a glow. The glow in the neon will not be affected by the pitch knob setting or the position of the mute switch.