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mini T h eremin Mk.2 PART 2: By JOHN CLARKE B Last month, we described the features of our new Theremin and gave the full circuit details. This month, we show you how to build it and describe the adjustment procedures. 102  Silicon Chip UILDING THE MINI THEREMIN is straightforward, with virtually all parts (except for the loudspeaker and switches S1 & S2) are mounted on a PC board coded 01207061 (188 x 103mm). This is housed in a plastic UB2 utility case measuring 197 x 113 x 63mm. Fig.5 shows the assembly details. Begin by carefully checking the PC board for any defects (eg, shorts between tracks or broken tracks). Check also that all the hole sizes are correct for the various parts. In particular, siliconchip.com.au Fig.5: install the parts on the PC board and complete the wiring as shown here. Note that the pitch antenna lead should initially be connected to point “X” (near T2), so that the equalising coil is bypassed – see text. check the hole sizes for the 6.35mm jack sockets, the DC power socket, the IF coils and the potentiometer mounting holes and redrill the holes siliconchip.com.au to a larger size if necessary. Note also that the corners of the PC board need to have cutouts as shown in Fig.5, to clear the four corner pillars in the case. If necessary, these can be cut using a small hacksaw and carefully filed to shape using a rat-tail file. Begin the board assembly by installAugust 2006  103 Table 1: Resistor Colour Codes o o o o o o o o o o o o o o o o No. 1 10 1 2 4 1 4 6 2 1 2 1 3 1 3 Value 330kW 100kW 47kW 22kW 10kW 4.7kW 2.2kW 1kW 680W 330W 220W 150W 100W 39W 10W ing PC stakes at the eight external wiring points, test points (TP1 & TP2) and the GND point (adjacent to VR2). In addition, PC stakes are used to terminate the leads from the equalising coil (L1). That done, you can install all the wire links using 0.7mm tinned copper wire. Note that the wire links all have a 12.5mm lead spacing. The assembly will be much faster if you cut a 12mmwide metal or wooden jig and use this as a spacer to bend the link leads. Follow the links with the resistors, using the colour codes in Table 1 as a guide to selecting values. It is also a good idea to use a digital multimeter to check each resistor before it is installed, as some of the colours can be confusing. The ICs, including the 4-pin optically-coupled LDR (OPTO1), can go in next, followed by the diodes. Make Table 2: Capacitor Codes Value 220nF 100nF 47nF 22nF 10nF 1nF 470pF 330pF 68pF μF Code 0.22µF 0.1µF .047µF .022µF .01µF .001µF    NA    NA    NA EIA Code 224 104 473 223 103 102 471 331 68 104  Silicon Chip IEC Code 220n 100n 47n 22n 10n 1n0 470p 330p 68p 4-Band Code (1%) orange orange yellow brown brown black yellow brown yellow violet orange brown red red orange brown brown black orange brown yellow violet red brown red red red brown brown black red brown blue grey brown brown orange orange brown brown red red brown brown brown green brown brown brown black brown brown orange white black brown brown black black brown sure that all these parts are orientated correctly – OPTO1 goes in with the dot on its body orientated as shown in Fig.5 (note: the numbering shown on the pinout diagram in Fig.4 last month is incorrect). The capacitors can then be installed (see Table 2) but watch the polarity of the electrolytics. The JFETs and transistors are next on the list. Note particularly that Q6 is a BC337 while Q7 is a BC327. The remainder are all 2N-5484 or 2N5485 types – be sure to use the correct type at each location. Once those parts are in, the trimpots can all be installed. Orientate the top-adjust multi-turn types with the adjusting screw at top, as shown in Fig.5. The IF coils can then go in. Make sure that the black-cored can goes in the T4 position. The three white-cored cans go in the T1-T3 positions. Now install the two 3-terminal regulators (REG1 & REG2). As shown, REG1 (7809) is mounted horizontally, with its metal tab secured to a small heatsink and to the PC board using a machine screw and nut. To do this, first bend REG1’s two outer leads down by 90° about 8mm from its body and its middle lead down by 90° about 6mm away. It can then be installed along with its heatsink on the PC board and secured using an M3 x 6mm machine screw and nut, after which its leads can be soldered. Note that the leads are soldered last of all. Do not solder the leads before fastening the regulator to the PC board, 5-Band Code (1%) orange orange black orange brown brown black black orange brown yellow violet black red brown red red black red brown brown black black red brown yellow violet black brown brown red red black brown brown brown black black brown brown blue grey black black brown orange orange black black brown red red black black brown brown green black black brown brown black black black brown orange white black gold brown brown black black gold brown otherwise the PC pads may crack or lift due to stress as the screw is tightened. No heatsink is required for the 7808 regulator. It simply mounts vertically with its metal tab towards REG1. Next, install the 6.35mm jack sockets and the DC power socket. Note that we have specified high-quality 6.35mm sockets (Jaycar Cat. PS-0195) here. These have a shorter threaded bush than other commonly-available units, which will clear the side of the case when the PC board is installed. Note: standard sockets are not suitable since they have a longer bush. This would protrude through the side of the box, making it impossible to install the PC board. Potentiometers Before installing the potentiometers, it’s necessary to cut their shafts to length to suit the knobs. On the prototype, this meant cutting the shafts to 11mm. They can be cut using a hacksaw, with the end of the shaft secured in a vice. Having cut the shafts, break off the locating lug on each potentiometer. The pots can then be soldered in turn to the PC board, taking care to use the correct value at each location. The speaker/headphone volume potentiometer at the top of Fig.5 is the only logarithmic type. It will have an “A” marking (A = log curve) on its body (eg, “A10k”). Don’t confuse it with the 10kW linear pot used for the Skew control (curve B). siliconchip.com.au This is the fully-assembled PC board, ready for installation in the case. Note the earthing link that’s used to connect the bodies of the pots together and to the ground stake on the board. As shown in Fig.5, the pot bodies are all wired together and connected to circuit ground. This is done by soldering a 140mm length of tinned copper wire along the top of the pot bodies and then running a short link back to the GND PC stake. Note that the anodised coating on each pot body will need to be scraped away at the soldering points. Earthing the pot bodies helps prevent tuning changes due to capacitance effects when your hand touches the adjusting knobs. The plastic knobs also help to prevent tuning changes so be sure to only use plastic knobs. Once the pots are in, LED1 can be installed. Take care with its orientation – its anode lead is the longer of the two). Note that the LED should be mounted at full lead length so that it can later be bent over to pass through its matching hole in the front panel (this hole is in line with the potentiometer shafts). Coil winding The last component to mount on the board is equalising coil L1. It comprises a bobbin and two core halves and siliconchip.com.au is wound using 300 turns of 0.25mm enamelled copper wire. The coil is wound onto the bobbin after which the two core halves are fitted, along with a 2.5mm Nylon spacer. This spacer provides an air gap which lowers the effective permeability of the core. This lessens any variations in the inductance of the equalising core with respect to temperature. Winding the coil is straightforward – it’s simply a matter of winding on the 300 turns of wire and then twisting the leads together to prevent the coil unwinding. That done, trim these leads to about 20mm and clean the enamel from their ends, so that they are ready to solder to the board. Important: equalising coil L1 must be wound so that its self-capacitance is as low as possible. In practice, this means that the windings should be jumble-wound by hand without regard to neatness. Do not wind each layer with each turn neatly placed adjacent to the next. The core can now be fitted to the bobbin, as follows. First, insert an M4 x 25mm Nylon screw through one core half and install three M4 Nylon wash- ers (these effectively form the 2.5mm Nylon spacer). That done, install the second core half in position, fit the assembly to the PC board and secure it using an M4 Nylon nut – ie, the screw protrudes through the PC board and the nut is placed on the underside. Finally, solder the two coil leads to their adjacent PC stakes. Cut-outs & hardware If you buy a complete kit, the case will probably be supplied pre-drilled and with screen-printed lettering on the front panel. Alternatively, if you’re starting from scratch, you will have to drill the plastic case as shown in Figs.6-8. Basically, this involves drilling the following holes: • eight holes in the front side of the case for the pot shafts (7) and the LED; • holes in the lefthand end and rear side for the antenna brackets; • holes in the righthand end for the two jack plug sockets, the DC power socket, the two rocker switches and a banana socket; • holes in the bottom of the case to attach the metal baseplate and a ¼-inch August 2006  105 Fig.6: this diagram can be copied and used as a template to mark the hole positions in the ends of the case. Also shown are the details for the pitch and volume antennas. Tee nut (which is used to attach the stand); and • holes in the lid of the case to mount the loudspeaker and to let the sound escape. Note that the holes for the switches can be made by first drilling a series of three holes for each and then filing them to the required rectangular shape. The banana socket requires an elongated hole and this can be done by first drilling and reaming it to 6mm and then elongating it using a rat-tail file. As shown in the photo, the metal baseplate is attached to the underside (base) of the case and is secured using M3 x 6mm screws and nuts. The ¼-inch Tee nut goes in the centre 106  Silicon Chip and is glued to the baseplate and box using epoxy adhesive. Alternatively, you could use builders adhesive (eg, Selleys Liquid Nails). Note that an eyelet and short length of green hook-up wire is attached to the front left corner baseplate mounting screw – see Fig.5 Finally, the lid of the case requires four 3mm holes to mount the loudspeaker plus a pattern of 13 x 6mm holes directly in front of the loudspeaker cone. The antennas The pitch and volume antennas are both based on towel-rail end brackets and lengths of 16mm chromed steel tubing (or towel railing) – see Fig.6. The pitch antenna is cut to 375mm long, while the volume antenna is 125mm long. Plastic end caps are used to cover the exposed ends of each antenna while the opposite ends fit into the end brackets which in turn are attached to the case using M4 x 10mm screws and M4 nuts. Final assembly Now we come to the final assembly. The PC board simply clips into the integral slots inside the case but before doing this, it’s necessary to first remove the top screw that’s used to secure the mounting bracket for the Volume antenna In addition, it will be necessary to remove some of the ribbing inside siliconchip.com.au Fig.7: these are the drilling details for the front and rear sides of the case. The 7mm holes can be made by drilling small pilot holes first and then carefully enlarging them to side using a tapered reamer. the box adjacent to the DC socket and around three of the potentiometers, so that they sit flat against the sides of the case. siliconchip.com.au You can quickly determine what has to be removed by trial fitting the board inside the case. A sharp chisel can then be used to remove the ribbing. Next, solder the green earth wire from the groundplane to the GND stake on the underside of the PC board (ie, to the same stake that earths the pot August 2006  107 bodies). A second 150mm-long green earth wire should then be soldered to the GND stake on the top of the PC board for later connection to the banana socket. Don’t do this after the PC board is in the case, as heat introduced into the GND stake could melt the soldered joint for the groundplane lead without you knowing. You can now fit the washers to the pot shafts and insert the PC board into the box. This is done by angling it so the pot shafts enter the holes in the side of the box, after which the board can be clipped into the integral side mouldings. Once it’s in, secure each pot with its nut and fit the two jack socket nuts. The leads on the LED can then be bent down by 90° so that it goes through its matching hole in the front panel. All that remains now is to fit the two switches and the banana socket and complete the external wiring. Note that the connections to both the volume and pitch antenna brackets are made via crimp-connection eyelets which are secured under the nuts of the mounting screws. Initially, the pitch antenna wire should be connected directly to the PC stake marked with an “X” (adjacent to T2), to bypass the equalising coil (L1). It’s later connected to its correct PC stake after the setting-up procedure has been completed (more on this later). The connections to the two switches can either be soldered or made via spade connectors. The final wiring connections are to the loudspeaker and to the banana socket from the GND stake. Initial checks Fig.8: here’s how to drill the bottom of the case and the aluminium base plate. Make sure that the holes for the Tee nut are accurately positioned. Right: the threaded Tee nut is pushed through its holes in the baseplate and secured in position using epoxy adhesive. It accepts the threaded tip of the microphone desk stand. 108  Silicon Chip Now for the initial set-up procedure. Here’s what to do, step-by-step: Step 1: set trimpots VR9 & VR10 fully clockwise, VR8 & VR11 fully anticlockwise, VR12 fully anticlockwise and pots VR1-VR7 to mid-position. If necessary, now is also a good time to remove the indicator buttons from the knobs and replace them so that the pointers are vertical. Step 2: check that the power LED lights when power is applied. If it doesn’t light, check the polarity of the DC plug from the plugpack. The centre pin should be the positive. If this is correct, then the LED might be installed the wrong way around. siliconchip.com.au The PC board is installed in the case by first angling it down at the front so that the pot shafts pass through their respective holes in the front panel. The back of the board is then lowered and clipped into the integral slots. Step 3: check the supply rails to the ICs. IC2 should have 9V between pins 4 & 11, IC3 should have 9V between pins 4 & 7, IC4 should have 9V between pins 4 & 6, and IC5 should have 9V between pins 4 & 8. Step 4: connect a multimeter between test points TP3 and TP GND. Apply power and adjust transformer T4 for a reading of about 1.5V. Step 5: connect the multimeter to TP4 and adjust VR14 for a reading of about 7V, regardless of hand movement near the volume antenna. siliconchip.com.au Step 6: check each oscillator for correct operation by measuring the DC voltages on the drains of JFETs Q1, Q3 and Q5. These should each measure between 3V and 6V. Step 7: check the voltages on the remaining JFETs and transistors. Q2 and Q4 should have around 0.3-0.4V on their source (middle) pins; Q6 should have about 1.4V on its base (middle pin); and Q7 should have about 7V on its emitter. Step 8: vary VR3 and check that the voltage at TP1 ranges from about 0.7V to 7V. Similarly, varying VR4 should vary the voltage on TP2 over the same range. Step 9: set VR2 to its centre position and adjust the slug in T2 until a lowfrequency sound is produced from the loudspeaker. Check that the pitch of this sound changes in response to hand movement near the pitch antenna. This should start off at a very low frequency with the hand away from the antenna and rise in frequency as the hand is brought nearer to the antenna. Step 10: attach the microphone desk August 2006  109 increases as the hand is brought closer to the pitch antenna. Frequency changes with hand move­ment should now be quite linear for each octave up to 2048Hz. Note that if the antenna gives very strange results or the frequency does not appear to change, the resonance setting for the equalising coil is probably incorrect. In that case, adjust T2’s slug slightly anticlockwise and readjust T1 until the effect disappears. Volume adjustment The microphone desk stand screws into the Tee nut that’s attached to the underside of the case. stand to the unit and readjust transformer T2 carefully until the results are correct. Check that VR2 alters the tuning frequency. Equalising coil If this is all working, it is time to tune the unit with the equalising coil connected. Here’s the step-by-step procedure: Step 1: set pot VR2 (Pitch Range) to its mid-position. Step 2: wire up the test circuit as shown in Fig.9 using insulated wire. Note that the equalising coil lead must be lifted from point X and connected to the far left side of the board via a length of hook-up wire instead. Note also that the pitch antenna lead is moved to its correct location. Step 3: turn T2 slowly and find the point where the voltage is lowest. In the prototype, the voltage dropped from over 2V down to around 0.4V at the dip. The idea of this test is to find the frequency where the antenna and equalising coil resonate, as indicated by a dip in voltage as T2 is adjusted. Step 4: move your hand so that it is about 10cm away from the antenna. The voltage dip should change by about 50mV. By contrast, if you incorrectly set T2 at a position away from the dip setting, the measured voltage will vary by more than 200mV on this test. Step 5: verify this dip in voltage by placing the lid on the box (as this affects the reading) and placing your hand about 10cm away from the antenna. 110  Silicon Chip Now remove the lid, adjust T2 slightly, replace the lid and repeat the previous step (Step 4). Note: the Theremin must be away from metal objects or the results will be affected. Step 6: repeat step 5 several more times, adjusting T2 just a little each time (don’t forget to replace the lid after each adjustment). If the slug inside T2 has to be set at the top or bottom of its range to give the required voltage dip, you will need to change the number of turns on L1. Add a turn or two if the slug is at the top of its range and take off a turn or two if the slug is at the bottom of its range. If you cannot find the dip, check that the antenna has the same length and diameter as used in our prototype. The gap between L1’s cores also affects the resonance. A slightly smaller gap will give a lower resonant frequency and a wider gap will raise the resonant frequency. When you are satisfied that the resonance adjustment is correct, do not make any further adjustments to T2’s setting. Step 7: connect the equalising coil back into circuit by reconnecting it to point X, as shown on the overlay dia­gram (Fig.5). The two insulated wiring leads (shown green and orange on Fig.9) should also now be removed. Step 8: adjust T1 until the tone is at a low frequency, then put lid on. In necessary, readjust T1 so the tone starts off at very low frequency and One problem that you may have at this stage is heterodyning (ie, an audible squeal) in the audio output. This effect is an audible beat frequency which is produced by frequency differences between the volume oscillator and the pitch and reference oscillators. The solution to this problem is to adjust transformer T3 until the whistle disappears. Note that changing T3 may affect the voltage at TP4. If this drops below 6V, you may not hear anything at all and so T4 will need readjusting to bring TP4’s voltage back above 7V. Check again for spurious noises in the sound. Now connect your multimeter between TP3 and TP GND and adjust T4 so that voltage falls as your hand approaches the volume antenna. Set T4 so that the voltage on TP3 is around 1.6V. Note that if you set T4 so TP3 is at 1.3V instead, then the volume control range with hand movement will be less. This will cause the volume control to be more abrupt. Conversely, setting it at more than 1.6V will widen the control range with hand movement, so there will be more gradual volume control. Next, make sure that VR1 is set to its mid position, then set VR14 so that TP4 is at 6V when your hand is away from the volume antenna. That done, adjust VR1 so that the volume control range is nice and smooth. Waveform adjustments The waveform adjustment range is made by first setting VR3 fully anticlockwise and adjusting VR9 so that the reading at TP1 is 0.7V. Once that’s been done, set VR3 fully clockwise and adjust VR8 for 1.5V at TP1. The Symmetry range adjustment uses a similar procedure. First, set VR4 fully anticlockwise and adjust VR11 for 6.9V on TP2, then set VR4 siliconchip.com.au Fig.9: this diagram shows how to wire the unit for the resonance adjustments (see text). Don’t forget to restore the equalising coil wiring as shown in Fig.5 and remove the insulated leads after making these adjustments. fully clockwise and adjust VR10 for 0.9V on TP2. You can experiment with these ranges but be aware that with wider adjustment settings, the signal can drop out for particular settings of VR3 and VR4. Skew range The skew range is adjusted using VR12. This increasingly limits the skew range as it is turned clockwise and vice versa. You may wish to limit the skew range for several reasons. First, you will find that with too much skew control, the lowest frequency that can be played will be too high. This is because the pitch and reference oscillators lock together at high skew settings. The oscillators will only snap to a different frequency when pulled sufficiently by a large change in hand movement adjacent to the pitch antenna. Second, once a certain skew level is reached, further increasing the skew does not necessarily change the tone. Finally, too much skew will pull the reference and pitch oscillators so far off frequency that the pitch adjustment siliconchip.com.au This view shows the parts on the righthand end of the case. control will no longer be able to set the lowest frequency required. Adjustment of VR12 should be done with these points in mind. Earthing Finally, note that the Theremin will work best if the circuit earth connects to either mains earth or to your body. A mains earth connection is automatically made if the Theremin is connected to an earthed amplifier. However, if the Theremin is not earthed in this way, an earth strap can be used to connect between your lefthand wrist and the banana socket on the Theremin. A suitable (anti-static) strap is available from Jaycar – Cat. TH-1780. Alternatively, you could also run the Theremin from a 12V supply that has an option to connect the 0V rail SC to mains earth. August 2006  111