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Digital Effects Unit for Musicians This neat Digital Effects Unit can produce a wide range of sound effects to enhance a musical instrument. It can be driven from a guitar or line source (or from both) & uses the latest in digital delay technology. By JOHN CLARKE Adding effects to musical instruments is very popular amongst musicians since they can create their own unique sounds. These effects can vary from the more natural, such as adding auditorium ambience, to the grossly exaggerated where the origi­nal sound becomes unrecognisable. The SILICON CHIP Digital Effects 26  Silicon Chip Unit is based on a micro­processor and provides adjustable echo, delay, reverberation and vibrato effects. It can be used with guitars, electric keyboards and organs, mixing consoles and other sources capable of provid­ing a 50mV to 2V RMS output signal. By connecting this unit into the signal path before the amplifier, you can quickly tailor the sound to your requirements – from adding some interesting reverberation effects to guitar work to pulsating vibrato effects for an electronic keyboard. Alternatively, you can just add in some echo to make a room sound more “alive”, or you can use a combination of effects for some really way-out sounds. These combined effects can be instantly switched in or out of circuit using a single switch (Effects In/Out) on the rear panel. Main features As can be seen from the photos, the unit is housed in a compact case with a sloping front panel. This panel carries a 2-digit LED display that shows either the delay period in millisec­onds or the vibrato rate in Hz. The delay period can be varied from 1-64ms, while the vibrato rate can be varied from 1-20Hz. Immediately to the right of the display are four pushbutton switches, the first of which toggles the display mode between delay and vibrato. Two LED indicators, one above the switch and the other below it, are used to show the current display mode. The next two switches are labelled DOWN and UP and these set both the delay and the vibrato rate, depending on the display mode selected. Pressing a button in delay mode, for example, progressively alters the delay period in discrete 1ms steps. Conversely, if vibrato mode is selected, the display alters in 0.5Hz steps up to 10Hz and then in 1Hz steps up to 20Hz. The fourth pushbutton switch is simply used to toggle the vibrato on or off. A LED indicator above this switch lights when the vibrato is on, while another LED situated immediately above the 7-segment displays flashes at the selected rate (1-20Hz). The remaining controls on the front panel are the Echo switch and the Reverberation and Vibrato Depth potentiometers. These controls have no effect on the LED displays, however. They simply switch the echo in or out and vary the amounts of rever­beration and vibrato. Several controls are also located on the rear panel and these include a Power on/off switch, an Attenuation control and the previously mentioned Effects In/Out switch. Also on the rear panel are a power socket, two signal input sockets (guitar and line) and an output socket. If necessary, both inputs can be used together – the two input signals are simply mixed together before any effects are added. An important point here is that the output signal level remains unchanged when the Effects In/Out switch is operated. This prevents the volume from changing each time the effects are switched in or out. Obtaining sound effects All of the effects are based around the delay function. In practice, this simply involves storing the incoming signal and then replaying it some time later. This delay is adjustable from 1-64ms. Mixing the delayed signal with the original signal gives the echo effect and, when a long delay is select- Main Features ♦ ♦ ♦ ♦ ♦ ♦ ♦ ♦ ♦ ♦ Echo, delay, reverberation & vibrato effects Microprocessor controlled delay period & vibrato rate with 2-digit display Digital delay processing Low noise and distortion Delay adjustable from 1-64ms in 1ms steps Vibrato rate adjustable from 1-10Hz in 0.5Hz steps & from 10-20Hz in 1Hz steps User presets to select settings on power-up Input attenuator to prevent signal overload Click-free switching between effects in & effects out No change in signal level between effects in & effects out Specifications Frequency response ��������������������-3dB at 25Hz and 15kHz Signal to noise ratio ����������������������78dB wrt 1V unweighted; 82dB wrt 1V A-weighted Overload capability �����������������������800mV RMS for guitar input (using attenuator); 4.5V RMS for line input (using attenuator) Sensitivity �������������������������������������guitar input 50mV; line input 300mV Distortion ��������������������������������������1% at 1kHz and 300mV output Vibrato sinewave distortion ����������7% at 20Hz (mainly switching noise) Vibrato sinewave level ������������������within 0.6dB from 1Hz to 20Hz Input impedance ���������������������������10kΩ for line input; 47kΩ for guitar input ed, it simulates the effect of a very large hall or outdoor venue. For delay settings of say less than 20ms, we obtain a phase shifting effect. This occurs because the delay time is now simi­lar in magnitude to the period of the input signal. What happens is that the original and delayed signals are added together when they are in phase and are subtracted when they are out of phase. The resulting sound effect is similar to the Doppler shift effect that occurs with a rotating loudspeaker. By contrast, reverberation occurs when the delayed signal is fed back to the input to produce multiple echoes. With a short delay time, the results can be similar to the phase shifting effect described above but the extra multiple echoes make the effect more powerful. The vibrato effect is obtained by continuously varying the delay above and below a preset period at a rate somewhere between 1Hz and 20Hz. This effect can warble a voice or make a normal guitar sound like an Hawaiian guitar. A small amount of vibrato can also improve the phase shifting effect. Digital delay chip One advantage that this Digital Effects Unit has over many other designs is its very low noise and distortion. This has been made possible by basing the design on the M65830P digital delay IC from Mitsubishi – a device designed mainly for use in surround sound decoders. It is a very versatile device and can provide 64 separate delay periods, as selected by a serial code applied to its data pin. The device works by first converting the incoming analog signal to a digital format which is then clocked into a memory. This digital signal is then clocked out at the end of the delay period and converted back to an analog form. In addition, the M65830P also contains several op amps so that input and output filters can be added to the circuit without the need for addi­tional ICs. About the only drawback to using February 1995  27 LINE INPUT 15kHz LP FILTER IC3 MIXER IC1c AMP IC1b GUITAR INPUT ECHO S2 MIXER IC1a ATTENUATION VR1 IN DELAY IC3 15kHz LP FILTER IC3 OUT MIXER IC1d RELAY OUTPUT OSCILLATOR CONTROL INPUT x5.7 REVERBERATION VR2 18Hz LP FILTER IC2 VOLTAGE CONTROLLED OSCILLATOR IC4 VIBRATO LEVEL VR3 PC1-PC3 PC4 DIP SWITCH POWER-ON RESET PD3-5,PD7 PC0 MICROPROCESSOR IC5 (PD0-PD2)(PA0-PA7)(PB0-PB7)(PC7-PC5) LED1 VIBRATO MODULATION LED3 DELAY(ms) S4 DISP1 DISP2 the M65830P is that it is not easy to drive using standard counters and gates. This is because the control signal must be in serial form and must in­clude various identification, sleep and mute codes. This means that some form of programmed device (an EPROM or a microproces­sor) should be used in order to simplify the circuitry involved. We decided to use a microprocessor to do the job because this could also be used to perform a range of other tasks without increasing circuit complexity. In fact, the displays, switches and LEDs are all driven directly driven from the microprocessor lines. The microprocessor is also used to generate the vibrato waveform. Block diagram Fig.1 shows the block diagram of the unit. At the heart of the circuit is IC3, the M65830P digital delay chip mentioned above. To simplify matters, 28  Silicon Chip LED2 S6 DOWN S5 UP S7 VIBRATO ONOFF LED4 VIBRATO RATE(Hz) it is shown with just four connec­tions to the outside world: the signal input and output lines, the oscillator input and the control input (which actually con­sists of three lines). The control lines come from outputs PC1-PC3 on the micro­processor IC5, with the data on PC1 setting the delay number (1-64). Another output from the microprocessor, PC4, is fed to low pass filter stage IC2. Its output in turn is fed via VR3 to voltage controlled oscillator (VCO) stage IC4. By this means, the VCO varies its output frequency accord­ing to the data from PC4 and the setting of VR3. This frequency sets the basic minimum delay period (ie, to 1ms when there is no vibrato). Let’s look now at the signal inputs. The guitar signal is first amplified by 5.7 in IC1b to boost it to line level and then mixed with the line input signal using IC1a. From there, the signal passes via attenuation control VR1 to IC1c Fig.1: block diagram of the Digital Effects Unit. The incoming signals are amplified, mixed & filtered before being fed to a digital delay line based on IC3. Microprocessor IC5 controls the delay line (via PC1PC3) & also provides a control signal (via PC4 & IC2) for IC4 which, in turn, provides the clock signal. where it is mixed with the delayed signal fed back via the reverberation control (VR2). Note that the line level is defined as 285mV. However, signals up to 4.5V RMS can be accommodated by using VR1 to attenuate signals above 1.1V RMS. This attenuation is necessary to prevent signal clipping within IC3. Following IC1c, the mixed signal is applied to the digital delay stage via a 15kHz low-pass filter. This filter keeps un­wanted high frequencies out of the delay line to avoid spurious effects. Note that the op amp used for this lowpass filter stage is actually contained inside IC3. A similar 15kHz low-pass filter stage is also used at the output of the delay line. The filtered output from the delay line is fed to mixer stage IC1d and finally to the output via a set of relay contacts. The relay itself is controlled by the PC0 output of IC5; this keeps the relay contacts open for a few seconds after power is applied to eliminate switch-on “plops”. Echo is added to the output by closing switch S2, so that the signal from IC1c is mixed with the delayed signal inside IC1d. VR2 sets the level of the delayed signal that’s fed back to IC1c and thus controls the reverberation, while VR3 sets the vibrato level (or depth). Vibrato is produced by applying a varying frequency (from the VCO) to the oscillator input of IC3. To provide a natural vibrato sound, this variation should be sinusoidal in nature. This is achieved by first producing a pulse width modulated signal at the PC4 output of IC5 and then filtering it to produce a smooth sine wave. This is then fed through low-pass filter stage IC2 and used to modulate the VCO. If the vibrato effect is switched out, however, IC2’s output sits at a constant level and so the VCO’s output frequency remains fixed. As well as controlling the effects circuitry, the micropro­ cessor also drives the LED displays and accepts inputs from the four pushbutton switches (S4-S7). Finally, the PD3, PD4, PD5 & PD7 lines of IC5 connect to a 4-way DIP switch. These switches set the initial configuration of the effects unit at power up and can be configured to suit your requirements – see Table 1. For example, the DIP switches can be set so that the vibra­to function is either on or off at power up and there are options for setting the initial delay and vibrato rate. Circuit details Refer now to Fig.2 for the full circuit details. IC1b, the guitar preamplifier, functions as an inverting stage with a gain of 4.68, as set by the ratio of the 220kΩ and 47kΩ feedback resistors. Its output is fed to mixer stage IC1a which functions with a gain of -1.2 for guitar signals (thus giving an overall gain of 5.7) and -1 for line level signals. Note that the guitar socket shorts the input to ground when no plug is inserted. This is done to reduce hum pickup when this input is unused. The output from IC1a is coupled to VR1 via a 10µF ca­pacitor and then fed to pin 23 of IC3 via IC1c, as described pre­viously. The RC network connected to pins 22 and 23 forms part of PARTS LIST 1 PC board, code 01301951, 141 x 131mm 1 PC board, code 01301952, 41 x 146mm 1 desk console case, 170 x 143 x 31 x 55 (Jaycar Cat. HB6092 or equivalent) 1 front panel label, 168 x 143mm 1 12VAC 300mA plugpack 3 6.5mm mono panel mount sockets with SP switch 1 SPDT toggle switch (S1) 1 SPST rocker switch (S2) 1 DPDT toggle switch (S3) 4 grey momentary click action PC board switches (S4-S7) 1 4-way DIL switch (DIP1) 1 10kΩ 16mm log pot (VR1) 2 10kΩ 16mm linear pots (VR2-3) 1 10kΩ horizontal trimpot (VR4) 1 DC panel mount power socket plus 2.5mm screws & nuts 3 15mm knobs 1 micro U heatsink (18 x 19 x 10mm) plus 3mm screw & nut 1 5V reed relay (RLY1) (Jaycar Cat. SY-4036 or equivalent) 1 40-pin IC socket 4 9mm tapped standoffs 4 3 x 6mm screws 4 3 x 6mm countersunk screws 1 solder lug 1 22 x 34mm piece of 3mm-thick red Perspex 1 4MHz parallel resonant crystal (X1) 2 HDSP-5301 red common anode 7-segment displays (DISP1, DISP2) 4 3mm red LEDs (LED1-LED4) 22 PC stakes 5 cable ties Semiconductors 1 TL074 quad op amp (IC1) 1 LM358 dual op amp (IC2) 1 M65830P single chip digital delay (IC3) 1 ICM7555, LMC555CN or TLC555 CMOS timer (IC4) 1 programmed MC68HC705C8 microprocessor (IC5) 1 7805 5V 1A 3-terminal regulator (REG1) 6 1N4004 1A diodes (D1-D6) 1 BC338 NPN transistor (Q1) 1 BB212 varicap diode (VC1) Wire & cable 1 250mm-length of 6-way rainbow cable (2.54mm separation) 1 60mm-length of twin shielded audio cable 4 300mm-lengths of hook-up wire (red, blue, yellow & green) 1 200mm-length 3-way rainbow cable 1 300mm-length 0.8mm tinned copper wire Miscellaneous Solder, heatsink compound the 15kHz low pass filter shown on the block diagram (Fig.1). This filter prevents frequencies that are greater Capacitors 1 1000µF 25VW PC electrolytic 3 100µF 16VW PC electrolytic 1 47µF 16VW PC electrolytic 1 22µF 25VW PC electrolytic 9 10µF 16VW PC electrolytic 2 1µF 16VW PC electrolytic 2 1µF MKT polyester 1 0.27µF MKT polyester 7 0.1µF MKT polyester 1 0.039µF MKT polyester 2 0.022µF MKT polyester 1 0.01µF ceramic 2 560pF ceramic 2 150pF ceramic 2 33pF ceramic Resistors (0.25W, 1%) 1 4.7MΩ 1 8.2kΩ 2 220kΩ 1 4.7kΩ 3 100kΩ 1 3.3kΩ 2 47kΩ 1 1.8kΩ 4 39kΩ 1 1kΩ 1 27kΩ 1 330Ω 5 22kΩ 1 270Ω 2 18kΩ 1 100Ω 20 10kΩ 1 82Ω Where to get the microprocessor The programmed MC68HC705­C8 microprocessor (IC5) is avail­ able from retailers as part of a complete kit, or can be purchas­ed separately from SILICON CHIP for $45 plus $6 p&p. than half the sampling frequency of the digital delay from causing spurious conversion products which would February 1995  29 10k 10k LINE 285mV 10 10k +16V 13 IC1a TLO74 12 8.2k GUITAR 47k 10 1 10k ATTENUATION VR1 10k LOG 220k 10 14 10 4 2 1 IC1c 3 0.27 15kHz ANTI ALIAS FILTER 10k 9 IC1b 10 +16V 8 18k 23 150pF 39k 22 39k 560pF 100k OUT 82  20 10 11 IN .022 21 100k 100 17 +5V 0.1 MODULATION FILTER 22k 3 22k 22k 22k 1 .039 22k 4.7k 6 IC2a 5 LM358 1 18Hz LP FILTER 1.8k 1 IC2b TP1 10k 2 0.1 0.1 18 270  5 7 1 VC1 VOLTAGE BB212 CONTROLLED OSCILLATOR +5V FREQUENCY SET VR4 10k 3 4 100 16VW 6 A1 A2 10k 2 8 0.1 100k K 8 IC4 7555 7 VIBRATO LEVEL VR3 10k LIN 27k +16V 3.3k 4 0.1 .01 +5V 10k S6 10k 10k 29 PD0 DOWN S5 30 UP S7 PD1 31 PD2 VIBRATO 24 PC4 +5V DIPSW1 POWER S1 DELAY PRESET D1-D4 4x1N4004 V+ VIBRATO PRESET +16V D5 1N4004 12VAC 300mA IN 22 25VW 1000 25VW REG1 7805 GND OUT 10k 10k 10k 10k 4 32 PD3 33 PD4 34 PD5 36 PD7 3 2 1 20 +5V 10 16VW B I GO DIGITAL EFFECTS UNIT 30  Silicon Chip A2 K A1 E VIEWED FROM BELOW C A K 37 +5V 0.1 100 1 24 10k 16 15kHz LP FILTER .022 15 ON 39k IC3 M65830P 14 IN 10k RLY1 560pF 39k 150pF OUT 1 10k 13 OUT 7 9 3 10 11 12 10 7 IC1d 5 100  OUTPUT 47k 220k REVERB VR2 10k LIN 19 OUT TP2 6 IN EFFECTS S3a 10 XIN REQ SCK DATA 2 4 5 6 ECHO S2 18k 47 IN S3b +5V VIBRATO MODULATION LED1 0.1  VIBRATO ON LED2 10k 330 330  25 26 27 PC3 PC2 PC1  V+ 22 23 40 PC5 2 PC6 IRQ 3 D6 1N4004 VPP 1 RESET RLY1 10 PC0 1k 10k 28 B C Q1 BC338 E IC5 MC68HC705C8 +5V 10k DELAY LED3  330  PC7 38 4.7M X1 4MHz 33pF 33pF 39 PA PA PA PA PA PA PA PA 0 7 6 5 4 3 2 1 4 5 6 7 8 9 10 11 PB PB PB PB PB PB PB PB7 0 1 2 3 4 5 6 12 13 14 15 16 17 18 7x 330 8x 330  10 9 7 6 1 2 4 G F A B E D C 5 A DP F B G E C D DISP1 DP HDSP5301 3,8 MSD 21 19 330  DISPLAY S4 VIBRATO RATE (Hz) DELAY (ms) VIBRATO RATE LED4  1 2 4 10 9 7 6 E D C G F A B A F G E B C D DISP2 DP HDSP5301 3,8 LSD +5V Fig.2: IC3 (the digital delay line) forms the heart of the circuit, while microprocessor IC5 provides the control signals & drives the various LED displays. IC5 also controls relay RLY1 to briefly mute the output at switch-on. February 1995  31 RLY1 100k 47k 47k 10uF 10uF 10uF 10k 560pF 0.27 1k 10k 1 10k 1uF 1000uF 560pF 10k 18k 39k REG1 18k Q1 10k 1uF 39k 82  10k 10k TP GND 10uF 39k DIP1 100k 100uF 10k 10uF 10uF 10k D1 D5 10uF IC1 TLO74 10k 10k 10k 10k 100  D2 8.2k 10uF D3 D4 220k 220k D6 39k 22uF 1 2 3 4 150pF 22k IC3 M65830P 22k 10uF 22k 100uF IC5 MC68HC705C8 150pF 0.1 0.1 X1 47uF 22k 100uF 0.1 .022 4.7M 0.1 33PF .022 33pF TP2 1 1 .039 20 22 21 7 8 9 101112 K A 131415161718 S5 20 19 22 21 S7 24 23 Fig.3: mount the parts on the two PC boards as shown here, taking care to ensure that all polarised parts are correctly oriented. Note particularly the orientation of the ICs, the 7-segment LED displays & the four pushbutton switches (S4-S7). An IC socket is recommended for IC5. subsequently be passed to the output. After processing inside IC3, the delayed signal appears at pin 15 and is then fed back into an internal op amp via pin 14. This op amp, together with the associated RC network, forms the 15kHz low-pass output filter depicted on Fig.1. The output signal then reappears at pin 13 of IC3 and is AC-coupled to pin 6 of IC1d via a 1µF capacitor and switch S3a. A 10k 330  K 330  S6 LED4 DISP2 32  Silicon Chip TP1 10k 330  10k 10k 1 1 2 3 4 5 6 10k 23 S4 DISP1 10k LED2 A 330  K K 1 24 4.7k 100k LED3 LED1 A 19 131415161718 VR4 1 0.1 1 2 3 4 5 6 IC2 LM358 27k 0.1 1 7 8 9 101112 1uF 1.8k IC4 7555 0.1 330  1uF 22k VC1 330  330  330  330  330  330  330  330  330  330  330  10k 330  330  330  3.3k 270  .01 IC1d functions as the output mixer stage. It operates with a gain of -1, both for signals from IC3 and for signals fed in from IC1c when the Echo switch (S2) is closed. VR2 sets the reverberation as described previously. It feeds a sample of the delayed output signal to the input of IC1c via S3b, a 10µF capacitor and a 10kΩ resistor. Switches S3a & S3b are used to switch the effects in or out. In the OUT position, the signal from IC1c is fed directly into IC1d and the digital delay circuitry is effectively by­passed. At the same time, the feedback signal from VR2 is switched out to eliminate any reverberation effects that would otherwise occur if VR2 was not set to its minimum position. The 220kΩ resistor connected to pin 7 of IC1d maintains the DC charge on the associated 1µF input coupling capacitor when the effects are switch­ ed out. This eliminates noise when S3a is subsequently switched to the IN position. The muting relay (RLY1) is controlled by the PC0 output of IC5. This output goes high several seconds after power is applied and turns on RLY1 via transistor Q1. Diode D6 protects Q1 by quenching the back-EMF spikes generated when RLY1 switches off. A 4MHz crystal connected between pins 38 & 39 of IC5 sets the clock frequency for IC5. This frequency is internally divided by two, so that the microprocessor actually runs at 2MHz. The 10µF capacitor on pin 1 briefly pulls this input low at switch-on to provide a reset pulse. Power supply The display PC board is mounted on the lid of the case via four 9mm tapped spacers. After mounting, adjust the height of each indicator LED so that it just protrudes through the panel. A small piece of red Perspex provides a window for the two 7-segment LED displays. Following IC1d, the processed signal is coupled to the output socket via a 10µF capacitor, a set of relay contacts and a 100Ω resistor. The latter is there to prevent IC1d from going into oscillation when a long output lead is connected. Vibrato circuitry The vibrato function is toggled on or off using pushbutton switch S7 to pull the PD2 line of the microprocessor low. The resulting PWM signal from PC4 (pin 24) is then AC-coupled to low-pass filter stage IC2a to derive a control signal for the VCO. IC2a’s non-inverting input (and thus its output) is biased to about +5V by IC2b. This latter stage functions with a gain of two and amplifies the voltage fed to its pin 3 input from the wiper of trimpot VR4. As a result, IC2a’s output sits at a con­stant +5V when the vibrato is off and varies in sinusoidal fash­ion about the +5V level when the vibrato is on. This control signal is fed to the cathode (K) of varicap diode VC1 via VR3 (the vibrato level control) and a 100kΩ resis­tor. Note that the +5V bias level, as set by VR4, determines the output frequency of the VCO when the vibrato is off (and when VR3 is set to minimum). VC1 and 7555 timer IC4 make up the VCO. IC4 operates in astable mode and varies its output frequency according to the capacitance of VC1. This capac- itance, in turn, varies according to the control signal from VR3. When the vibrato is off, the control signal remains con­stant and thus the VCO output also remains constant at a nominal 1MHz. Conversely, when the vibrato is on, the control signal varies sinusoidally and so the VCO output varies in similar fashion. This in turn modulates the delay period of the digital delay line to produce a pulsating sound effect. Microprocessor functions Not a lot can be gleaned from the looking at the micropro­cessor circuitry since its operation depends mainly on the soft­ware. Basically, its various I/O (input/output) lines accept inputs from the various switches (S4-S7 and DIPSW1) and drive the various LED indicators and the two 7-segment displays. Depending on the settings programmed into it via these switches, IC5 also controls the digital delay line via its PC1-PC3 outputs as described previously. And, as we have just seen, it also controls the vibrato circuitry via its PC4 output. Outputs PA0-PA7 drive the two 7-segment displays via 330Ω current limiting resistors, while PC5 & PC6 drive the two vibrato LED indicators (LED 1 & LED 2). PC7 controls LED 3 & LED 4 to indicate the display mode –when PC7 is low, LED 3 is on and when PC7 is high LED 4 is on. Power for the circuit is derived from a 12VAC 300mA plug­pack supply. Its output is full-wave rectified using D1D4 and the resulting DC filtered using a 22µF capacitor to provide a nominal 16V rail. This rail supplies RLY1 and is also fed to 3-terminal regulator REG1 via isolating diode D5. A 1000µF capacitor provides further filtering at the output of D5 and the 16V rail at this point is used to power IC1 and IC2. REG1 provides a regulated +5V output and this supplies IC3, IC4, IC5 and the LEDs. When power is switched off, the relay supply falls quickly due to the modest amount of filtering employed. As a result, the relay switches off well before the voltage across the 1000µF capacitor falls by any appreciable amount. This effectively mutes the output and eliminates any nasty switch-off effects. Software Although the software programmed into IC5 is fairly complex, we can describe some of the main features of the program. The program is divided into various subroutines and interrupts and each of these performs a separate function. At power up, the RESET program begins and this sets up the initial conditions for the I/O and monitors the DIP switch set­tings. The Delay period is initially set by the DIP switches on PD3 & PD4, while the Vibrato Rate is initially set to one of two values (either 3.5Hz or 8.5Hz) by the DIP switch on PD5. The DIP switch on PD7 selects whether the Vibrato is ini­ tially on or off. The program then waits for several seconds and then brings PC0 high and updates the delay time in February 1995  33 This view shows how everything fits together inside the plastic case. Note that REG1 is mounted with its leads bent at right angles & is bolted to the PC board along with a small heatsink. The wiring is secured using plastic cable ties. IC3 in a subroutine called UPDTE. The display is then driven by a subrou­tine called SET. The program now monitors switch­ es S4-S7. If one is pressed, it acts according to the function of the switch. This program is called POLL and is the background program that runs continuously until the power is switched off or it is interrupted by an internal interrupt program called TIMER. This interrupt program generates the PWM code for PC4 and LED 1 when Vibrato is selected. When vibrato is off, PC4’s output is a 1kHz square wave. Construction Most of the circuitry is contained on two PC boards: a main board coded 01301951 and a display board coded 01301952. Fig.3 shows the parts layout on these two boards. Before installing any of the parts, check the boards care­fully for shorts and open circuit tracks by comparing The rear panel carries (from left to right) the Effects In/Out switch, the input & output sockets, the Attenuator control, the Power switch & the DC power socket. 34  Silicon Chip them with the published artworks. If all is correct, begin the assembly by installing PC stakes at all external wiring points on the main board, excluding points 1-24. PC stakes should also be installed at TP1, TP2 and TP GND. Next, install the links, resistors, diodes and ICs 1-4, taking care to ensure that the semiconductors are all correctly oriented. Table 3 lists the resistor colour codes but it is also a good idea to check them using a multimeter, as some colours can be difficult to decipher. Note particularly the row of resistors below IC5 – the resistor on the extreme left has a value of 10kΩ, while the rest are all 330Ω. The MKT capacitors can now be installed (see Table 2), followed by the electrolytic types. This done, install RLY1, VC1, Q1, VR4, X1, DIP1 REG1 and a 40-pin socket for IC5. The latter is mounted with its leads bent at right angles and its metal tab fitted with a small heat­sink. Smear the metal tab of the regulator with heatsink compound before bolting the assembly (regulator plus heatsink) to the PC board using a screw and nut. Finally, complete the main board assembly by installing IC5 in its socket. Take care to ensure that it is correctly oriented. By contrast with the main board, the display board carries relatively few components and should only take about 10 minutes to assemble. Begin by installing the resistors, then mount the two 7-segment displays and the pushbutton switches (S4-S7). The displays must be oriented with their decimal points at bottom right, while the flat sides on the switch bodies go towards the bottom of the board – see Fig.3. Push the switches down onto the board as far as they will go before soldering their leads. The four indicator LEDs can now be installed. For the time being, mount them so that they sit about 10mm ▲ Fig.4 (right): figure-8 shielded cable is used for the connections between the two input sockets (Guitar & Line) & the PC board, while the remainder of the wiring shown on this diagram can be run using medium-duty hook-up wire. Points 1-24 of the two PC boards are connected together using four 60mm lengths of 6-way rainbow cable. S3 S1 GUITAR LINE OUTPUT 28 POWER SOCKET 26 VR1 25 TP GND 29 27 MAIN PCB 1 2 3 4 5 6 20 7 8 9 101112 131415161718 19 22 21 25 S2 24 23 VR2 26 VR3 29 28 27 1 2 3 4 5 6 7 8 9 101112 131415161718 20 19 22 21 24 23 SOLDER LUG DISPLAY PCB February 1995  35 Table 1: DIP Switch Settings 1 2 3 4 DIP Switch Vibrato Off on x x x Vibrato On off x x x 3.5Hz Vibrato x on x x 8.5Hz Vibrato x off x x 1ms Delay x x on on 17ms Delay x x on off 33ms Delay x x off on 49ms Delay x x off off TABLE 2: CAPACITOR CODES ❏ ❏ ❏ ❏ ❏ ❏ ❏ ❏ ❏ ❏ Value 1µF 0.27µF 0.1µF 0.039µF 0.022µF 0.01µF 560pF 150pF 33pF IEC 1u 270n 100n 39n 22n 10n 560p 150p 33p EIA 105 274 104 393 223 103 561 151 33 above the surface of the board and secure each LED by just soldering one lead. Do not trim the leads at this stage, as the LEDs are ad­justed for height later on when the display board is attached to the front panel. Take care with the orientation of the LEDs; the anode lead of each LED is the longer of the two (see Fig.2). Refer now to Fig.4 for the wiring details. The first step is to connect points 1-24 of the two PC boards together using four 60mm lengths of 6-way rainbow cable. The main board is then mounted on integral plastic pillars in the base of the case and secured using the small self-tapping screws supplied. The display board mounts on 9mm tapped spacers which are secured to the lid of the case using countersunk screws. Attach the four spacers to the lid, then fit the red Perspex window for the LED displays. This window should be a tight fit into the front panel cutout and can be secured by applying a thin line of epoxy resin around the underside edge. The display board can now be mounted in position. Adjust the height of each indica­tor LED so that it just protrudes through the front panel before soldering the remaining leads. Next, fit the lid to the case and carefully mark out and drill holes in the rear panel for the three 6.5mm sockets, the two switches, the Attenuation pot and the power input socket. The 6.5mm sockets and the switches should be mounted 15mm down from the top edge of the rear panel, while the pot should be mounted half-way down so that its lugs can be soldered directly to the PC stakes on the board immediately below it. The power socket mounts directly below the power switch. The various items of hardware can now be mounted in posi­tion and the wiring completed. Shielded cable is used between the two input sockets and the main PC board, while the remaining wiring is run using medium-duty hook-up wire. Run the wiring along one edge of the main board (see photo) and don’t forget the lead between TP GND and the solder lug on the display board. Once the wiring has been completed, it can be tidied up and secured using a number of cable ties. Testing To test the unit, first connect your multimeter between TP GND on the main board and pin 1 of IC3. This done, set the meter to a low voltage range, apply power and check that the meter reads 5V. The display should initially show two dashes (- -) and then, after a few seconds, a number (the value depends on the DIP switch settings). The Delay LED should be also be lit. Now check that pin 8 of IC4 and pin 2 of IC5 are at +5V. Similarly, check for +16V on pin 4 of IC1 and pin 8 of IC2. If these voltage measurements are OK, check that the LED readout toggles between the Delay and Vibrato Rate modes each time S4 is pressed. If it does, check that the display increments when Up is pressed and decrements when Down is pressed. Check that the display range is 1-64 in Delay mode and 1-20 in Vibrato Rate mode. Now check the operation of the TABLE 3: RESISTOR COLOUR CODES ❏ No. ❏   1 ❏   2 ❏   3 ❏   2 ❏   4 ❏   1 ❏   5 ❏   2 ❏ 20 ❏   1 ❏   1 ❏   1 ❏   1 ❏   1 ❏   1 ❏  1 ❏   1 ❏   1 36  Silicon Chip Value 4.7MΩ 220kΩ 100kΩ 47kΩ 39kΩ 27kΩ 22kΩ 18kΩ 10kΩ 8.2kΩ 4.7kΩ 3.3kΩ 1.8kΩ 1kΩ 330Ω 270Ω 100Ω 82Ω 4-Band Code (1%) yellow violet green brown red red yellow brown brown black yellow brown yellow violet orange brown orange white orange brown red violet orange brown red red orange brown brown grey orange brown brown black orange brown grey red red brown yellow violet red brown orange orange red brown brown grey red brown brown black red brown orange orange brown brown red violet brown brown brown black brown brown grey red black brown 5-Band Code (1%) yellow violet black yellow brown red red black orange brown brown black black orange brown yellow violet black red brown orange white black red brown red violet black red brown red red black red brown brown grey black red brown brown black black red brown grey red black brown brown yellow violet black brown brown orange orange black brown brown brown grey black brown brown brown black black brown brown orange orange black black brown red violet black black brown brown black black black brown grey red black gold brown Fig.5: check your PC boards against these full-size etching patterns before installing any of the parts. Vibrato On/Off switch (S7). When the vibrato is on, its associated LED should light and the Vibrato Modulation LED (LED 1) should flash at the selected rate. Check that the rate at which this LED flashes alters according to the vibrato rate. If you have access to a frequency meter or an oscilloscope, adjust VR4 for a reading of 1MHz at TP2 when the vibrato is off. If this equipment is unavailable, simply adjust VR4 for a voltage reading at TP1 (note: not TP2) of 2.5V. This should provide a clock frequency that’s reasonably close to the mark. The unit can now be given a practical test by connecting the output to an amplifier and feeding a signal into one of the inputs. Check that the Echo On, Reverberation and Vibrato Depth controls all produce the desired effects on the sound and that the Effects In/ Out switch operates correctly. Once you become familiar with the various effects, you can set the DIP switches on the PC board so that your normally selected settings appear at switch on – see Table 1. To see how this works, let’s assume that you want the unit to power up with the following set­tings: (1) vibrato off; (2) vibrato rate = 3.5Hz; and (3) delay = 33ms. In this case, the switch settings would be: DIP1 on; DIP2 on; DIP 3 off; and SC DIP 4 on. February 1995  37