Fullscreen HTML5Med Res (??MB)HTML4

Widgy Distortion effects for y Do you own a guitar but don’t have an overdrive (or distortion or fuzz) box yet? Well your prayers have been answered! This one sounds great, it’s cheap and it’s easy to build! By PETER SMITH I F YOU’RE A GUITAR PLAYER, then you’ll certainly know all about the various “effects” that can be used to enhance guitar sounds. Over the years, many great players have combined these effects with their own unique styles to create unmistakable signature sounds. Some of the most sought-after sounds are produced by deliberate harmonic distortion of the music content. Originally, this type of effect was produced exclusively by over-driving the output stage of valve amplifiers. About overdrive & distortion These days, distortion effects are generated by dedicated electronics equipment. Perhaps in an attempt to capitalise on the success of past 22  Silicon Chip legends more than anything, much of this equipment boasts valve-like distortion qualities. Valve amplifiers have a reputation for soft-clipping the output signal when they are overdriven, at least at moderate levels. If the signal is a pure sinewave, the peaks are simply round­ ed off, with a certain amount of wave shape compression occurring. These rounded peaks create predominantly lower-order harmonics. Essentially, this means that the harmonics are closely related to the fundamentals and therefore tend to sound quite natural. Perhaps we could say that they “resonate” or “ring” with the fundamental tones. Harmonics, by the way, are referred to as “partials” in the music world. They are simply some multiple of the original, fundamental frequencies. Once the input to any amplifier is increased well beyond its design limit, the output signal is either hard clipped or transformed into indistinguishable noise, depending on the amplifier’s overload characteristics. Unlike the rounded peaks of a softclipped waveform, hard clipping is characterised by flat, sharp-edged waveforms. This is due to the output stages driving all the way to the power supply rails, slicing the peaks off and compressing, or “crunching”, the signal. Hard clipping results in many higher-order harmonics of the fundamentals. The resulting sound is often described as “reedy”, “rather harsh” and “more metallic”. A side effect called “intermod­ulat­ ion distortion” occurs when all these harmonics inevitably mix. The product of two frequencies is both the sum and difference of the originals, and they may not necessarily be “musically” related to the content. Therefore, intermodulation distortion is unwanted noise that is quite easily detected by the ear. Ideal distortion? As far as we can discern, there is no easy way of generating the ideal siliconchip.com.au yBox your guitar distortion effect. Why? Primarily because it would be impossible to get broad agreement on what that sound is. It has more to do with music type, personal preference and playing styles than the pure technicalities. Many commercial distortion effects units combine both soft and hard clipping and user-accessible controls are included to provide adjustment between these two extremities, thus accommodating a range of music and styles. Some also include tone controls for increased versatility. The SILICON CHIP “WidgyBox” (like the name?) is based on these ideas. The design criterion was simple: it had to be uncomplicated, low-cost and easy to build. We think it will make a worthwhile addition to any guitarist’s basic effects line-up. Reproducing the sound Now for the $64 question: if valve amplifiers already produce the desired sound, then why bother trying to reproduce it? Why not just use a valve amplifier? Well for a start, valve amplifiers are expensive. In addition, they need to be over-driven to produce the effect. This means lots of volume, which can obviously be a real problem. In the words of one disaffected player, “I have good tone when I play loud but I get kicked out of clubs and bands”. Dedicated effects boxes (also known as “effects pedals” and “stomp boxes”) address these issues. They create the MAIN FEATURES • • • • • • Low cost. Easy to build. Battery-powered. Adjustable distortion. Three tone controls. Optional stomp switch. desired effect before the amplifier input, allowing the musician to play at any volume. They also allow easy experimentation for those in search of a unique sound. What’s more, you don’t need a valve amplifier – a (much) cheaper solid-state amplifier will suffice! How it works Fig.1 shows the details of our design – it’s based entirely around the TL07x series op amps. Like most effects pedals, the circuit is designed to connect directly in-line with the guitar’s output. A 47µF capacitor AC-couples the input to the first op amp stage (IC1a). This capacitor is much larger than you might expect in order to ensure low May 2003  23 24  Silicon Chip siliconchip.com.au Fig.1: the circuit uses three low-cost op amps (IC1-IC3) and operates from a 9V battery. Schottky diodes D1 & D2 provide the soft clipping function, while IC1b provides hard clipping, depending on the setting of VR1. noise performance. As with all the following stages, IC1a’s input is biased to one-half the supply rail voltage (+V/2), in this case via a 220kΩ resistor. The 1kΩ resistor and 10pF capacitor at the input act as a low-pass filter, preventing RF (radio frequency) signals from being coupled into the circuit. IC1a is wired in a non-inverting configuration with a gain of 4.9, as set by the 39kΩ and 10kΩ feedback resistors. The 150pF capacitor in the feedback path rolls off the frequency response above the audio spectrum. IC1a’s output appears at pin 1 and is coupled via a 2.2µF capacitor to Drive pot VR1. This pot controls the signal level into the next stage, for reasons that will become clearer shortly. The signal from the VR1’s wiper is in turn AC-coupled to op amp IC1b via a 15nF capacitor. This capacitor also acts with a 100kΩ bias resistor to form a high-pass filter, to provide a small measure of pre-distortion equalisation. This is necessary to reduce the effects of harmonics from the lower strings. Apparently, these low frequency harmonics tend to sound a little “fruity” during chord work. In addition, cutting the low end response may also help with guitar pickup equalisation. Effects Bypassing: The Different Methods Generally, it’s desirable to be able to switch effects in and out during a performance. A popular means of doing this is via a foot switch built into the same box that houses the electronics. This arrangement is part of all commercial effects pedals. Another common method relies on a dedicated bypass box, which is simply wired in series with the effects input and output leads. In the latter approach, the bypass function physically switches the effects box out of the signal path. This is termed “hard” bypassing, as opposed to “soft” bypassing, where some part of the effects electronics is still in-circuit (usually an input buffer and/or line driver). “Hard” bypassing is a popular approach because it ensures that the effect has no impact whatsoever on the signal, especially in relation to loading or otherwise distorting the signal source. A good example of a do-it-yourself bypass box can be found on the web at www.geofex.com/Article_Folders/ Millenium/millen.htm Alternatively, the WidgyBox has provision for an internal DPDT “hard” bypass switch. It’s simply a matter of removing the two wire links adjacent to the input and output sockets and wiring up switch S2 as shown on the circuit diagram (Fig.1). We envisage an internal switch being used in conjunction with a more robust (“stomp proof”) metal case! IC1b is configured as a non-inverting stage and operates with a gain of 12.8. It has two important roles, the first being to drive a pair of back-toback diodes (D1 & D2) whose job it is to perform the soft clipping function. Clip job The way that this works is quite straightforward. Once the peak signal level exceeds the forward voltage (0.2–0.4V) of the diodes, they start to conduct, thus clipping the highs and Fig.2: moderate soft clipping. The top waveform shows the signal into op amp IC1b, while the bottom waveform shows the signal across the clipping diodes (D1 & D2). Note the smooth waveform peaks. Compression is already quite noticeable, nearing a 2:1 ratio. siliconchip.com.au lows off the waveform. In addition, the non-linear conduction characteristic of the diodes give the peaks a smooth, rounded appearance. Regardless of increasing drive level, the diodes continue to clip the signal to about the same voltage, resulting in even more waveform compression (and distortion). At very high drive levels, IC1b’s second role comes into play – it starts to hard clip the signal. What happens is that the amplified signal level exceeds Fig.3: this is the maximum soft clipping signal, again taken across diodes D1 & D2. Note that the rising and trailing edges are almost vertical now but we still have rounded peaks. The compression is now quite high and this also imparts quite a degree of sustain. May 2003  25 Fig.4: maximum hard (and soft) clipping. The top waveform shows the hard-clipped op amp output. At the bottom, we can see what it looks like across the diodes. The amplitude isn’t much different to Fig.3 but the peaks have been “flattened”. the op amp’s maximum available output swing – so it is abruptly clipped. This is normal behaviour for any over-driven op amp and it’s exactly what we need for our hard clipping function! Fig.5: fiddling with the tone controls has a bigger effect than you might expect, because it’s boosting or cutting the harmonics as well. Here’s what the output of the box looks like (bottom waveform) when we wind up the bass boost. As a matter of interest, the TL072 clips non-symmetrically. This suggests that not only do we get the higher-order harmonics mentioned earlier but also a larger proportion of even rather than odd multiples. Note that we’ve specified Schottky diodes for D1 & D2 as they have a lower forward voltage than the common 1N4148/1N914 varieties. This gives a larger adjustment range between soft and hard clipping, allowing more waveform compression and increasing the “sustain” effect. Tone controls This close-up view shows the final version of the PC board. Take care to ensure that all polarised parts are installed the right way around. 26  Silicon Chip The distorted signal is routed to a Baxandall type tone control network, based around op amp IC2 and potentiometers VR2, VR3 & VR4. These pots and their associated resistors and capacitors form the feedback network between the op amp’s inverting input and its output. Each of the bass, mid and treble networks can be considered separately since they are connected in parallel between the signal input following IC1b and the output of IC2 at pin 6. Furthermore, the wiper of each pot is effectively connected to the inverting input (pin 2) which is a virtual ground. Operation of the bass control is as follows: with VR2 centred, the value of resistance connected between the output from IC1b and pin 2 of IC2 is the same as that between pins 2 & 6 and this sets the gain to -1. The 15nF capacitor has no effect since it is equally balanced across the potentiometer. If we move the wiper of VR2 to the full boost position (ie, rotate the pot shaft fully clockwise), we get 19kΩ (18kΩ + 1kΩ) between the input and pin 2 of IC2 and 119kΩ between pins 2 & 6. In addition, the 15nF capacitor siliconchip.com.au Table 2: Capacitor Codes Value 220nF 100nF 15nF 12nF 2.7nF 1.5nF 150pF 39pF 10pF µF Code EIA Code IEC Code 0.22µF 220n 224 0.1µF 100n 104 .015µF  15n 153 .012µF  12n 123 .0027µF  2n7 272 .0015µF  1n5 152 150pF 150p 150 39pF  39p  39 10pF  10p  10 is across the 100kΩ resistance in the feedback loop. Without the capacitor the gain would be -119kΩ/19kΩ or -6.3 at all frequencies. But with the capacitor, the gain is high only at around 50Hz and as the frequency rises it comes back to -1 (ie, overall unity gain). Thus we have bass boost. Conversely, when VR2 is wound fully anticlockwise, the position is reversed and we get a gain of 19kΩ/119kΩ or -0.16 (-16dB). The capacitor is now on the input side and provides less gain at frequencies below 100Hz but with gain increasing to -1 at frequencies above 100Hz. Thus we have bass cut. Various settings of VR2 between these two extremes will provide for less boost and cut. The midrange section works in a similar manner except that there is now a 12nF capacitor between VR3’s wiper and pin 2. This, along with the 2.7nF capacitor across VR3, gives a band­pass filter, so we either boost or Fig.6: here’s how to install the parts on the PC board. Install the smaller parts first before moving on to the output sockets, the battery holder and (finally) the pots (see text). cut the midrange frequencies. The treble control operates with no capacitor across VR4 but has a 1.5nF capacitor between its wiper and pin 2 to produce a high-frequency boost or cut at 10kHz. A 39pF capacitor between pins 2 & 6 of IC2 provides a high-frequency rolloff to prevent oscillation which could otherwise occur when the treble control is set for maximum boost. Similar­ly, the 1kΩ resistor in series with pin 2 is there to attenuate Table 1: Resistor Colour Codes o No. o  1 o  1 o  2 o  2 o  3 o  2 o  2 o  3 o  1 o  2 o  2 o  1 o  1 siliconchip.com.au Value 1MΩ 220kΩ 100kΩ 47kΩ 39kΩ 18kΩ 12kΩ 10kΩ 3.3kΩ 2.2kΩ 1kΩ 150Ω 100Ω 4-Band Code (1%) brown black green brown red red yellow brown brown black yellow brown yellow violet orange brown orange white orange brown brown grey orange brown brown red orange brown brown black orange brown orange orange red brown red red red brown brown black red brown brown green brown brown brown black brown brown 5-Band Code (1%) brown black black yellow brown red red black orange brown brown black black orange brown yellow violet black red brown orange white black red brown brown grey black red brown brown red black red brown brown black black red brown orange orange black brown brown red red black brown brown brown black black brown brown brown green black black brown brown black black black brown May 2003  27 Parts List 1 PC board coded 01105031, 117mm x 100.5mm 1 110 x 140 x 35mm (L x W x H) plastic instrument case (Jaycar cat HB-5970) 1 DPDT PC mount toggle switch (S1) (Jaycar ST-0365) 2 6.5mm PC-mount stereo switched sockets (CON1 - CON2) (Jaycar PS-0190) 1 2.1mm PC mount DC socket (CON3) 1 9V PC mount battery holder 3 100kΩ 16mm PC mount linear pots (VR2 - VR4) 2 10kΩ 16mm PC-mount log pots (VR1, VR5) 5 knobs to suit pots 20mm length of small heatshrink tubing 70mm length of light duty hook-up wire 320mm length of 0.71mm tinned copper wire 2 1N4004 1A silicon diode (D3,D4) Semiconductors 2 TL072CP dual op amps (IC1,IC3) 1 TL071CP op amp IC (IC2) 1 3mm high-brightness red LED (LED1) 2 BAT43 schottky diodes (D1,D2) (Jaycar ZR-1141) Resistors (0.25W, 1%) 1 1MΩ 3 10kΩ 1 220kΩ 1 3.3kΩ 2 100kΩ 2 2.2kΩ 2 47kΩ 2 1kΩ 3 39kΩ 1 150Ω 2 18kΩ 1 100Ω 2 12kΩ RF signals; it stops radio breakthrough. Being able to boost or cut the distorted signal in three distinct bands gives you a lot of control over your Capacitors 1 100µF 25V PC electrolytic 2 10µF 16V PC electrolytic 2 47µF 16V non-polarised PC electrolytic (Jaycar RY-6820) 1 22µF 16V non-polarised PC electrolytic (Jaycar RY-6816) 2 2.2µF 16V non-polarised PC electrolytic (Jaycar RY-6804) 5 220nF (0.22µF) 50V MKT polyester 2 15nF (.015µF) 50V MKT polyester 1 12nF (.012µF) 50V MKT polyester 1 2.7nF (.0027µF) 50V MKT polyester 1 1.5nF (.0015µF) 50V MKT polyester 3 150pF 50V ceramic disc 1 39pF 50V ceramic disc 1 10pF 50V ceramic disc sound – more, in fact, than is possible with many commercial units, which commonly provide only one or two bands of adjustment. Switch S1 has been included to allow you to quickly bypass the tone circuitry altogether should you wish to control it elsewhere in your setup. Level control & output IC2’s output is AC-coupled via a 2.2µF capacitor to VR5. This pot allows you to set the output level to match the input, thus preventing any noticeable jump in volume when the WidgyBox is switched in and out (see the panel entitled “Effects Bypassing: The Different Methods”). From there, the signal is AC-coupled via a 220nF capacitor to op amp IC3a. This op amp is configured as a voltage follower – it simply buffers the incoming signal and passes it through unchanged. A 150Ω resistor decouples IC3a’s output from any cable capacitance, thereby ensuring stability under all conditions. This is followed with a 47µF capacitor to remove the DC offset. Finally, a 10kΩ resistor terminates the output to ground, ensuring that there are no nasty clicks when the box is hot-switched into the signal path. Power supply In keeping with other popular effects pedals, power for the unit is provided by a 9V alkaline battery. The current drain is only about 12-15mA, so you’ll get more than a days’ continuous use and many days of intermittent use before a swap is required. Alternatively, power can be provided by a 9V DC plugpack. Be aware, though, that most unregulated plug­ packs put out much more voltage than their rating at these low current levels. Fig.7: these full-size artworks can be used as drilling templates for the front and rear panels. Drill small pilot holes to begin with, then carefully enlarge each hole to size using a tapered reamer. 28  Silicon Chip siliconchip.com.au Although this won’t damage your box, the higher voltage will alter the characteristics of the distortion effects at high drive settings. If you have a plugpack with selectable output voltages, you may find that the 7.5V setting provides about 9.5V under light load, which is ideal. Note that the negative terminal of the battery connects to earth via the switch contacts of the DC input socket (CON3) and the middle and common contacts of the guitar input socket (CON1). This means that you’ll need to plug in your guitar to power up the box. It also means that when a plugpack jack is inserted, the battery is disconnected. This feature is very important, otherwise the plugpack would attempt to charge the battery and that could have loud and startling consequences! Finally, the half supply voltage rail (ie, +V/2) needed by all of the bias networks is generated by op amp IC3b and its associated circuitry. Two 47kΩ resistors divide the +V rail in half, after which it is filtered by a 10µF capacitor and then buffered by op amp IC3b. A 100Ω resistor in series with IC3b’s output decouples the large 10µF filter capacitor. Construction With the exception of the power LED, all components mount on a single PC board, coded 01105031. Using the overlay diagram in Fig.6 as a guide, begin by installing the eight wire links using 0.7mm tinned copper wire or similar. Note that the two links adjacent to the input and output sockets (CON1 & CON2) can be left out if you intend fitting a foot switch to the box but more on that later. Install the low-profile components first, beginning with the resistors and diodes (D1-D3). Follow with the three op amp ICs (IC1-IC3). Make sure that you have the pin 1 (notched) end of each IC oriented as per the overlay diagram. In addition, note that IC2 is a TL071 (single) op amp, whereas the others are TL072 (dual) versions. Don’t mix them up! The two jack sockets (CON1 & CON2) and the DC socket (CON3) can go in next. When inserting the jack sockets, push them all the way down until the shoulders of all pins make contact with the PC board surface. Follow with the battery holder, which siliconchip.com.au The assembled PC board fits neatly into a low-profile plastic instrument case. Note that the PC board shown here is a prototype version and differs slightly from the final version shown in Fig.6. should be secured to the PC board with No. 4 x 6mm self-tapping screws prior to soldering. Next, install all the capacitors. The 100µF and two 10µF electrolytic capacitors are polarised and must go in the right way around. The remaining five electrolytics are non-polarised (marked “NP” on the overlay) and can go in either way. Potentiometers VR1-VR5 and switch S1 should be installed last of all. Start with VR1 but solder its middle pin only. Lift the board to eye level and examine the position of the pot from the front and side. It should be sitting perfectly “square”. Why bother? – well, when we eventually fit the front panel, this step helps to ensure that all the pot shafts Fig.8: having heard all the stories about valve distortion, we were consumed with curiosity and had to have a look at it ourselves. A kind gentleman loaned us his valve guitar amplifier and we captured this waveform when it was overdriven. Man, that doesn’t look too soft, does it? May 2003  29 switch (S1), ensuring that it is seated firmly on the PC board surface before soldering Case preparation The rear panel carries the 6.5mm stereo switched sockets and includes an access hole for the DC power socket. Note that the PC board in this photo is the final version, as shown in Fig.6. are aligned, improving appearance and minimising stress on solder joints when the nuts are tightened. Adjust the pot position as necessary and then solder the remaining two pins. Repeat this procedure for the other four pots. Finally, install the tone bypass As supplied, the bottom half of the case contains eight mounting posts. The four outermost posts are used to support the PC board, while the four inner posts are not required and must be removed. This can be done using a chisel or an oversized drill bit. The templates shown in Fig.7 provide the quickest and easiest method of getting all the holes in the right places for the front and rear panels. Photocopy the templates, cut them out and carefully align and tape each one to a blank panel. First, gently centre-punch the holes directly through the templates, then remove them and drill 1mm pilot holes for each mark. Don’t attempt to jump directly to a large diameter drill, as you may split a panel or get the holes off-centre. Instead, drill the holes progressively larger in several steps. Some constructors won’t have fractional drill sizes all the way up to the large diameters of the pot shafts and jack sockets. In this case, a tapered reamer is ideal for enlarging the holes to their final sizes. Trial fit The front panel should not be forcibly fitted over the pot shafts. If the holes are correctly sized for the shafts but the panel is still a tight fit (or won’t fit!), then the holes are obviously out of alignment. Increase the hole sizes as necessary to get an easy fit. This is quite important; a good fit keeps all the pot shafts in alignment. With the drilling done, slide the panels into place and loosely install washers and nuts on all the pots and the two jack sockets. The assembly should now slip home in the case bottom without too much trouble. Check that you can sight the four mounting post holes through the PC board holes and that the posts actually make contact with the underside of the board. If all is well, tighten up the nuts by hand. Grounding the pots Fig.9: this is the full-size etching pattern for the PC board. 30  Silicon Chip To minimise extraneous noise, the metal shells of the pots must be connected to the ground (0V) rail. This is achieved by soldering a single length of tinned copper wire to the metal top of each pot and terminating it to the siliconchip.com.au PC board at either end. The overlay diagram (Fig.6) and the various photos show where to position this wire. In order to get the solder to adhere to the pots, remove a small spot of the cadmium plating on each pot with an ink rubber or scouring pad and clean the area with alcohol. That done, pretin the spot with a fairly hot iron and large gauge multicore solder before attempting to attach the earth wire. Installing the LED Sound Fun: Experimenting With The Circuit Like to experiment a little? Then check out these ideas! As explained in the text, the high-pass filter formed by the 15nF capacitor and 100kΩ resistor at the input of IC1b provides for some pre-distortion equalisation. The 3dB point of this filter is around 100Hz. A higher or lower point may better suit your system. We suggest an upper limit of about 300Hz (no lower limit). Here are some example values: for a 194Hz 3dB point, use 8.2nF instead of 15nF; for 284Hz, use 5.6nF. It is also possible to experiment with the distortion-making section of the circuit. For example, replacing one of the Schottky diodes with a common 1N4148 will create non-symmetrical clipping for quite a different sound. You could also substitute germanium diodes, which have softer turn-on characteristics. Have fun! To mount the LED, first strip and tin the ends of two 30mm lengths of light-duty hook-up wire. That done, shorten the LED leads to about 8mm, solder one end of each wire to a LED lead and insulate the connections with heatshrink tubing. Finally, slip the LED into position in the front panel and solder the two leads to the PC board as shown in Fig.6. Be sure to install the LED with the correct polarity, though. The flat edge on the LED body goes towards the edge of the case (see Figs.1 & 6). If necessary, the LED can be fixed in position with a spot of glue or silicone sealant. and measure between pins 4 & 8 of both IC1 and IC3. That done, repeat this measurement between pins 4 & 7 of IC2. In all cases, the reading should be about 9.2V. Now touch the negative probe of your meter to the negative battery terminal and the positive probe to pin 2 of IC2. Your reading should be very close to half the voltage measured above (about 4.6V). Testing Final touches A few quick voltage measurements around the circuit will help to confirm that your project is ready for use. You’ll need a fresh 9V battery, a mono jack plug and a multimeter. Fit the battery and insert the plug in the input socket (CON1). The plug can be on one end of your guitar lead but don’t connect anything to the other end just yet! As soon as the plug is inserted, the power LED should light. If it doesn’t, then remove the plug immediately and check the orientation of the LED. Also, check that there is continuity through the DC socket (CON3) switch contacts, which can be identified by tracing the negative connection from the battery. If the above checks don’t identify the problem, then suspect a short or low resistance between the +V rail (battery positive) and ground (battery negative). You may have inadvertently reversed one of the ICs or perhaps there is a solder bridge between tracks somewhere. Follow the +V trace around the board to track it down. OK, let’s assume your LED lights up. Next, we’ll check that power arrives at each op amp IC supply pin. Set your multimeter to read DC volts The next step is to secure the PC board to the case posts with four No. 4 x 6mm self-tapping screws. Before tightening the screws, it’s a good idea to temporarily loosen off the pot and jack socket nuts, so that the assembly settles “comfortably” into position. The final job is to shorten the pot siliconchip.com.au shafts to match the knobs. Before doing this, screw the top half of the case into position and tighten up all of the pot nuts. The procedure now is to grip the tip of each pot shaft (in turn) in a vice, starting with VR1. You can then carefully cut off the unneeded section of the shaft using a hacksaw. For our prototype, only 14mm of shaft length (measured from the surface of the panel) was required for the push-on type knobs. Be sure to support the weight of the assembly during the cutting. That’s it – your WidgyBox is ready to rock! Crfedits Many thanks to Tim and Ash who were kind enough to drop in and put the prototype through its paces. SC Help – It Doesn’t Work! Before doing anything else, double-check all component values against the overlay diagram. If that doesn’t turn up anything, then some detective work is in order. If you have no output at all, then a few additional DC voltage measurements may help to narrow the problem down to a particular op amp and/or it’s immediate circuitry. Apply power and wait at least 10 seconds for the bias networks to fully charge. Don’t apply a signal to the input or connect anything to the output socket during these checks. Connect your multimeter’s negative probe to battery negative and touch the positive probe to each op amp output in turn (IC1a pin 1, IC1b pin 7, IC2 pin 6 and IC3a pin 7). Although the readings will vary slightly, they should all be close to one-half battery voltage. A large variation in any reading indicates a problem in the immediate vicinity. Alternatively, if you have output signal but varying the drive pot doesn’t change the distortion level, then suspect a problem with the feedback circuitry around IC1a or IC1b. May 2003  31