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Audio Signal Injector & Tracer . . . with optional tiny add-on RF probe This Audio Signal Injector/Tracer is ideal for troubleshooting AM radio and audio circuits. It comprises a 1kHz oscillator (the Injector) and an in-built preamp and amplifier with a headphone jack (the Tracer) so you can trace signals right through an amplifier or radio circuit to locate faults. By JOHN CLARKE This photo shows the complete Audio Signal Injector/Tracer together with its optional RF Demodulator Probe at right. A T SOME STAGE, everyone involved in electronics will need to find a fault in an audio circuit. It might be a circuit you have just built, a repair job for a friend or a job to be done in your workplace. And while you can often check voltages if you have a circuit diagram, sooner or later Main Features •  Hand held & battery powered •  1kHz injector output •  Adjustable injector level •  Tracer input attenuator •  Tracer volume control •  Audio output to headphones or small speaker •  Low battery indication •  Optional RF probe for AM modulation detection 60  Silicon Chip you will probably need to trace the progress of an actual signal though the various stages. For example, you might feed a signal into the input and then find that it disappears as it feeds through a capacitor. The obvious conclusion would be that the capacitor is faulty (open) or it has not been properly soldered into circuit. To do this sort of fault-finding, you need a suitable signal (one you can hear) and a small amplifier so you can listen to the signal at various stages in the amplifier or AM radio being tested. So our Audio Signal Injector/Tracer has a 1kHz oscillator as the Injector and a small amplifier as the Tracer. AM radio adds an extra complication because you need to listen to a modulated radio signal as it goes from stage to stage in the circuit. For that you need an optional RF demodulator probe and we show how to build one in the article on page 68 of this issue – it’s tiny! Mind you, if you are repairing an amplifier, you may not need the Injector’s audio signal, provided you have a CD player or even a smart phone which has music tracks. One the other hand, a music signal is not always ideal if you are using an oscilloscope and want to see if the signal becomes distorted at a particular stage in the circuit. In that case, you might find the Injector more convenient as you trace a signal of known shape through the circuit. As mentioned, our Injector is a 1kHz oscillator and you can see the shape in the accompanying scope grab designated Scope 1. It looks a bit like a sinewave but is actually a somewhat “rounded” square wave. It has a maximum amplitude of about siliconchip.com.au The Audio Signal Injector/Tracer is ideal for tracking down faults in audio amplifiers and preamplifier stages. And by adding the optional RF Demodulator Probe, it can be used to trace signals through the RF stages of AM radios as well. You can listen in to the traced signal via either headphones or an external speaker but the latter should be used if checking high-voltage circuits. Specifications 2V RMS but it can be adjusted down to just few millivolts. This means that it will cover virtually all signal tracing situations, from sensitive audio preamplifiers and the audio sections of AM/FM radios, right up to high-powered guitar and public address amplifiers. Then we come to the Signal Tracer. It needs a small amplifier to listen to small signals in sensitive circuits but it also needs an input attenuator so that it is not overloaded by the much larger signals, perhaps 50V or more, that you might find in a high-powered amplifier. You also need a volume control so that your ears are not blasted as you step through a circuit. Finally, both the Injector and Signal Tracer need to be protected from any high voltages that may be present in a solid-state or valve circuit. If you feed the Injector into a circuit operating at 300V DC, for example, you siliconchip.com.au Power: 9V at 2.3mA Tracer input impedance: ~6.45MΩ to 10MΩ, depending on attenuator setting Tracer signal gain: adjustable from 2x to 20x Tracer attenuator: 1:1, 1:10, 1:100 & 1:1000 Tracer signal frequency response: 70Hz to 3kHz Injector signal: 1kHz rounded square-wave Injector signal level: adjustable from 0-2V RMS (5.6V peak-peak) with a 9V supply Headphone output: 6.6V peak-peak maximum into 16Ω with a 9V supply Test circuit DC voltage: ±300V DC maximum recommended don’t want it to be blown to shreds and by the same token, if you touch the Tracer probe onto a similar highvoltage point, you don’t want it to be “cooked”. Our circuit takes care of those possibilities. Our Injector/Tracer is housed in a compact plastic case with an internal battery compartment. It has a pair of jack sockets for the output of the Injector and a BNC socket for the input to the Tracer. Next to that socket is a 4-position slide switch for the Attenuator which has settings of 1:1, 1:10, 1:100 and 1:1000. Input impedance The input impedance of the Tracer is rather high, varying between about 10MΩ and 6.45MΩ, depending on the setting of the input attenuator. This means that the impedance of the June 2015  61 Scope 1: the 1kHz waveform generated by the oscillator looks a bit like a sinewave but is actually a “rounded” square wave. It has a maximum amplitude of about 2V RMS but can be adjusted down to just a few millivolts. Tracer will not load down or affect the operation of the circuit being tested. The high-impedance input also means that the Tracer probe can be used to directly test ceramic (crystal) phono cartridges or piezoelectric pick-ups on musical instruments such as a violins. To connect signal to the Tracer you can use a 1:1 oscilloscope probe or any shielded cable with a BNC plug at one end and a suitable connector at the other, such as an RCA plug or a pair of alligator clips. More about this later in the article. The on/off switch, a power LED and the two knobs for the Injector level   Warning! When using the Audio Signal Injector/Tracer with high-voltage circuitry (eg, in a valve radio), take care not to touch any part of the circuit with your hand. Always treat the circuit as though it has mains voltage present. As stated in the article, use a small extension speaker rather than headphones when using the unit with high-voltage circuitry. Small non-powered extension speakers are available for use with iPods and similar MP3/MP4 players. The use of a small speaker will remove the possibility of deafening clicks or even a high-voltage shock should there be a fault within the Audio Signal Injector/Tracer or if the earth lead becomes disconnected. 62  Silicon Chip Scope 2: this scope grab shows the Schmitt trigger operation of IC1a. The yellow trace shows the charging and discharg­ ing of the 6.8nF capacitor from 3V to 6V etc, while the green trace shows the resultant square-wave output at pin 1. and Tracer volume controls are at one end of the case while the 3.5mm headphone jack is on the side, adjacent to the 4-position Attenuator switch. Circuit description Let’s now take a look at the circuit of the Audio Signal Injector/Tracer – see Fig.1. As shown, it’s based on an LMC­ 6482AIN CMOS dual rail-to-rail op amp and a handful of other components. One op amp is used for the Signal Injector while the other is used for the Tracer. The output frequency of 1kHz is set by the 100kΩ resistor and 6.8nF capacitor connected to pin 2, the non-inverting input. The three resistors connected to the pin 3 inverting input set the threshold voltage (at pin 3) at 1/3Vcc or 2/3Vcc, depending on whether the output of IC1a is high or low. So with Vcc = 9V, the input (threshold) voltage at pin 3 will be either +3V or +6V. When power is applied to the circuit, the 6.8nF capacitor at pin 2 will be discharged (ie, 0V), so pin 2 will be lower than pin 3. Therefore the output at pin 1 will be high (+9V) and this charges the 6.8nF capacitor via the 100kΩ resistor between pins 1 & 2. When the capacitor voltage rises just above 6V, pin 2 becomes higher than pin 3 and so the op amp’s pin 1 output switches low, to 0V (remember, this is a “rail-to-rail” op amp). So now pin 3 is at 3V and the capacitor discharges via its 100kΩ resistor until pin 2 is just below pin 3, whereupon the pin 1 output goes high again to recharge the capacitor. This continuing cycle generates a 1kHz square wave which is filtered using a 6.8kΩ resistor and 22nF capacitor to give a “rounded” waveform, as shown in Scope 1. The Schmitt trigger operation of IC1a is demonstrated in Scope 2, which shows the charging and discharging of the 6.8nF capacitor from 3V to 6V etc in the yellow trace. The lower green trace shows the resultant square-wave output at pin 1. Note that the amplitude of the square-wave is shown as 9.8V – we used a fresh 9V battery. Potentiometer VR1 connects across the 22nF capacitor to provide the Injector level control. This is AC-coupled to the output terminal via a 100nF 630V capacitor. We specified a high voltage rating for this capacitor so that the Injector output can be connected to a high voltage on the circuit under test without damage. For the same reason, diodes D2 & D3 clamp any high voltage from an external circuit (eg, a valve radio being tested) at the wiper of VR1 to 0.7V above or below the 9V and 0V supply rails. The 10MΩ resistor across the 100nF capacitor is there to discharge the capacitor when it is disconnected from the circuit under test. The 1kΩ resistor in series with the Injector output limits peak current to the clamping diodes. Tracer circuit The input signal from the BNC siliconchip.com.au POWER D1 1N5819 +9V A 100k 100k K IC1: LMC6482AIN 3 1 IC1a 2 100k 100k D2 A 6.8k INJECT LEVEL VR1 10k LIN 22nF 100nF K K INJECT OUT 1k 100nF 16V A 2.2k GND 10M D3 BANANA SKT A BC 327, BC33 7 +9V B K BNC TRACER INPUT D4 100k 9.1M 910k 91k 1:1 ATTENUATOR S2 1nF 100k A 10M 1:10 E 8 5 7 IC1b 6 4 E 1:100 1:1000 C 10M 2.7k A 100k 10 µF 16V 100 µF 16V CON1 Q1 BC327 3.5mm JACK SOCKET 100k VR2 50k LOG D1: 1N5819 1 µF 16V A VOLUME LED1 2.7k K A SC C 220pF D5 10k 20 1 5 Q2 BC337 620Ω B K E C B 1kV 9V BATTERY ZD1 5.6V 100 µF BANANA SKT 630V K 6.8nF A K S1 λ LED1 AUDIO SIGNAL INJECTOR & TRACER K ZD1 A K D2–D5: 1N4004 A K Fig.1: the circuit is based on dual op amp IC1. IC1a operates as a Schmitt trigger oscillator and this generates the injector signal, with VR1 setting the output level. The traced signal is fed in via a switched attenuator and then fed to op amp IC1b. Its output signal is then buffered by Q1 & Q2 and fed to CON 1, while VR2 sets the op amp gain. socket is fed to 4-way slider switch, S2 and the attenuator resistors. The resistors provide for division ratios of 1:1, 1:10, 1:100 and 1:1000. Following S2, the signal is coupled via a 1nF 1kV ceramic capacitor to the pin 5 non-inverting input of IC1b. This is tied via two series-connected 10MΩ resistors to a voltage divider (two 100kΩ resistors) which provides a reference at 4.5V ie, half the 9V supply. Diodes D4 & D5 clamp any high voltage input signals to 0.6V above or below the 9V supply rails. IC1b is connected as a non-inverting amplifier and its pin 7 output drives a complementary emitter follower stage using transistors Q1 & Q2. These provide a buffered output to the headphone socket via a 100µF coupling capacitor. Note that the emitter follower output stage is operated with no quiescent siliconchip.com.au current but is within the negative feedback loop of the op amp to minimise crossover distortion. The 50kΩ volume control (VR2) is also in the op amp’s feedback loop, connected in series with a 2.7kΩ resistor. In conjunction with the 1µF capacitor and series 2.7kΩ resistor from pin 6 to 0V, this allows the AC gain to be varied from between two and 20. The DC gain is unity, by virtue of the 1µF capacitor. Note that while the amplifier is mainly intended to drive headphones, it can also be used to drive a small speaker and we recommend this if you are doing signal tracing in a highvoltage circuit which might cause deafening clicks when you touch the probe on high voltage points. ates from a 9V battery, fed in via toggle switch S1. Diode D1 gives protection if the battery is inadvertently connected the wrong way around. A high-intensity red LED is used for power indication. It is bright when the supply is at 9V but drops to a dim glow when the battery is flat, by virtue of ZD1, a 5.6V zener diode in series with the LED. When the battery is fresh, ie, putting out 9V or maybe as much as 10V, we will have 1.8V across the red LED, 5.6V across ZD1 and 1.6V or more across the 1kΩ resistor so that 1.6mA or more flows through LED1. As the voltage falls, the voltage across the 1kΩ resistor also falls. At a battery voltage of 7.4V or less, there is very little voltage across the 1kΩ resistor and so LED1 will be dim. Power supply RF demodulator probe As already noted, the circuit oper- As previously noted, if you want June 2015  63 VR1 100 µF Q2 + This photo shows how switch S2 is mounted. It’s soldered to a pin header so that its top metal face is 12.5mm above the PCB. BC337 620Ω 2.7k 4004 4004 D5 1kV 220pF 2.7k 1 µF D4 LMC6482 100k 6.8k 100k 2.2k D3 22nF Q1 PHONES Inject fitted. These are installed at the five external wiring points, at TP GND (near LED1) and at the bottom right of S2. IC1 can then be soldered in place. Do not use a socket for this IC, as this would exacerbate noise pick-up. CON1 100k 10M 10M 100k 1k 4004 S 100nF 630V – (-) TO BATTERY CLIP to troubleshoot an AM radio with the Tracer, you need to have an additional demodulator probe for the amplitude-modulated (AM) RF signals that should be present in the circuit being tested. As stated, a suitable RF demodulator probe is described on page 68 of this issue. Construction The Audio Signal Injector/Tracer is built on a double-sided PCB coded 04106151 (85 x 63mm). This is housed in a plastic remote control case measuring 135 x 70 x 24mm. A panel label measuring 114 x 50mm is attached to the front of the case. To make the assembly easy, the PCB 100k TO SHIELD PCB 10k S2 91k + 910k 9.1M 10M + GND Tracer GROUND SOCKET 1nF R TRACER INPUT BNC SOCKET 10 µF T CUT LUGS SHORT (SEE TEXT) AUDIO SIGNAL INJECTOR & TRACER D2 INJECTOR OUTPUT SOCKET 4004 6.8nF + 100 µF 100k 100k 5 V6 ZD1 100k TP GND D1 50k LOG 10k LIN 100nF BC327 LED1 S1 VR2 15160140 K IC1 A 5819 Fig.2: follow this parts layout diagram to build the PCB assembly. Be sure to install the 100nF 630V and 1nF 1kV capacitors in the positions indicated and note that S2 is mounted on a pin header (see photo). /1 /10 COM Installing switch S2 /100 /1000 Switch S2 does not mount directly onto the PCB but is instead raised off the PCB using a 6-way DIL pin header. Before installing this DIL header, remove a pin from each side so that there are three pins, then a gap, then two pins (ie, on each side of the header to correspond with the switch pins). That done, position the header on the PCB with the longer pins facing upwards, then push each pin down so that it extends only 5mm above the top of the PCB. The pins on the underside EARTH PC STAKE FOR S2's METAL COVER is designed to mount onto the integral mounting bushes within the case. The top of the PCB is also shaped to fit around the case mounting pillars at that end – see Fig.2 and photo. Fig.2 shows the parts layout on the PCB. Begin by the installing the resistors. Table 1 shows the resistor colour codes but it’s also a good idea to check each one with a digital multimeter before soldering it to the PCB. The diodes can go in next. Note that there are two different types – D1 is a 1N5819, while D2-D5 are 1N4004s. Be sure to mount them with the correct polarity, then install zener diode ZD1, again taking care with its polarity. The seven PC stakes can now be   Table 2: Capacitor Codes Value µF Value IEC Code EIA Code 100nF 0.1µF   100n   104 22nF 0.022µF   22n  223 6.8nF 0.0068µF   6n8  682 1nF 0.001µF    1n  102 220pF   NA  220p  221 Table 1: Resistor Colour Codes   o o o o o o o o o o o o No.   3   1   1   8   1   1   1   2   1   1   1 64  Silicon Chip Value 10MΩ 9.1MΩ 910kΩ 100kΩ 91kΩ 10kΩ 6.8kΩ 2.7kΩ 2.2kΩ 1kΩ 620Ω 4-Band Code (1%) brown black blue brown white brown green brown white brown yellow brown brown black yellow brown white brown orange brown brown black orange brown blue grey red brown red violet red brown red red red brown brown black red brown blue red brown brown 5-Band Code (1%) brown black black green brown white brown black yellow brown white brown black orange brown brown black black orange brown white brown black red brown brown black black red brown blue grey black brown brown red violet black brown brown red red black brown brown brown black black brown brown blue red black black brown siliconchip.com.au can then be soldered to their respective pads, making sure that the header itself is flush against the PCB. Once it’s in position, switch S2 can be mounted by soldering its pins to the top of the header pins, so that its top metal face sits 12.5mm above the PCB (see photo). The best way to do this is to lightly tack-solder two diagonally-opposite pins first, then make any necessary adjustment before soldering the remaining pins. Don’t forget to resolder the first two pins, to ensure reliable connections. Once it’s in position, the adjacent earth PC stake is soldered to the earth tag on S2’s metal cover. Completing the PCB Now for the capacitors. Install the 100nF 630V polyester and 1nF 1kV ceramic capacitors in the positions shown, then install the remaining MKT polyester types. The electrolytics can then go in, taking care to fit each one with the polarity as indicated on Fig.2. Note that the tops of the electrolytics must be no more than 12.5mm above the PCB, otherwise you will not be able to fit the case lid later on. Follow with potentiometers VR1 & VR2, toggle switch S1 and the 3.5mm socket. VR1 is a 10kΩ linear potentiometer while VR2 is a 50kΩ log potentiometer, so don’t get them mixed up. LED1 can then be installed – it mounts horizontally with its leads bent down through 90° exactly 7mm from its lens, so that they go through the PCB pads. Push it down so that its horizontal lead sections sit exactly 6mm above the PCB (use a 6mm-wide cardboard spacer) and check that it is correctly orientated before soldering it to the PCB. That completes the PCB assembly. It can now be checked and placed to one side while the case is drilled. Preparing the case Figs.3 & 4 show drilling templates for the front panel and for the top of the case. They can either be photocopied from the magazine or downloaded as PDF files from the SILICON CHIP website and printed out. It’s just a matter of cutting the templates out, temporarily attaching them to the case panels and then drilling the various holes. The top of the case requires holes for potentiometers VR1 & VR2, switch S1 and LED1, while the front panel is drilled to accept the two siliconchip.com.au banana sockets, the BNC socket and slide switch S2. The rectangular cut-out for S2 is best made by drilling a row of holes inside the cut-out area, joining these and then filing the job to shape. The two banana socket holes can simply be drilled and reamed to size but the BNC socket hole needs to be shaped as shown on the template. It can be made by first drilling a small hole in the centre, then finalising its shape using small files, with the flat side positioned as shown. A hole must also be cut in one side of the case to accept the 3.5mm jack socket. To do this, temporarily position the PCB in the case, mark out the socket position, then remove the board and make a semi-circular notch in the base using a small round file. Once that’s been done, temporarily assemble the case and complete the hole by filing a matching semi-circular cut-out in the lid. Finally, you have to remove an internal pillar inside the case lid so that it doesn’t foul the nut for the earth banana socket. This can be done using side cutters. Note also that, as provided, the banana socket terminals are too long for the case and have to be shortened by 5mm. A fine-tooth hacksaw blade is the best tool for this job – do not bend the terminals, as they will break. File off any sharp edges after cutting them to length. Having drilled all the holes, the front panel label can be attached. This can be downloaded from the SILICON CHIP website, printed out (preferably onto photo paper) and affixed to the lid using either glue or neutral-cure silicone. Alternatively, for a more rugged label, print it out as a mirror image onto clear overhead projector film (be sure to use film that suits your printer), so that the printed side will be on the back of the film when the label is affixed. The film will have to be attached using a light-coloured silicone applied evenly over the surface, as the lid is black. Another option is to print the panel onto either an A4-size “Dataflex” sticky label (for ink-jet printers) or a “Datapol” sticky label (for laser printers) and directly attach this to the case lid. These labels are available from http://www.blanklabels.com.au and sample sheets are available on request to test in your printer. INJECTOR OUT + TRACER VOLUME INJECT LEVEL POWER WARNING! Do not use headphones or earbuds when testing high voltage HEADPHONES circuits. Use extension speaker instead. 1 10 100 1000 + + GROUND TRACER IN TRACER ATTENUATOR SILICON CHIP Audio Signal Injector & Tracer Fig.3: this front-panel artwork can be copied or down­loaded from the SILICON CHIP website and used as a drilling template. End Panel Drilling Guide 5mm 3mm 7mm 7mm Fig.4: the end panel drilling template. Drill pilot holes first to ensure they are accurately positioned, then carefully enlarge them to size. Once the label is in position, cut out the holes using a sharp hobby knife. Making a shield PCB Since the tracer has such a high input impedance, it has the potential to pick up hum from transformers but it will also pick up the injector signal as well, due to direct radiation of the injector signal into the input attenuator and other components in the op amp’s input circuitry. We can reduce this by a significant amount by installing a small shield board, made from copper laminate, underneath the PCB, with its copper side earthed to the PCB’s GND stake. The dimensions of this shield board are shown in Fig.5. It fits between the June 2015  65 Parts List 1 remote control case, 135 x 70 x 24mm (Jaycar HB-5610) 1 double-sided PCB, code 04106151, 85 x 63mm 1 single-sided shield PCB, code 04106153, 62 x 63mm 1 panel label, 114 x 50mm 1 9mm square PCB-mount 10kΩ linear potentiometer (VR1) 1 9mm square PCB-mount 50kΩ log potentiometer (VR2) 1 SPDT PCB-mount toggle switch (Altronics S1421) (S1) 1 DP4T PCB-mount slider switch (TE Connectivity STS2400PC04) (element14 Cat. 1291137) (S2) 1 PCB-mount 3.5mm stereo jack socket (CON1) 2 knobs to suit VR1 & VR2 1 panel-mount BNC socket 1 blue insulated banana socket (Jaycar PS-0423) 1 green insulated banana socket (Jaycar PS-0422) 1 9V alkaline battery 1 9V battery snap connector 4 No.4 x 6mm self-tapping screws 7 PC stakes 1 DIL 6-way pin header 7 7 7 7 ALL DIMENSIONS IN MM 62 BLANK PCB COPPER ON UNDERSIDE 20 Semiconductors 1 LMC6482AIN dual CMOS op amp (IC1) 1 3mm high-intensity red LED (LED1) 1 BC327 PNP transistor (Q1) 1 BC337 NPN transistor (Q2) 1 5.6V 1W zener diode (ZD1) 1 1N5819 Schottky diode (D1) 4 1N4004 diodes (D2-D5) Capacitors 2 100µF 16V PC electrolytic 1 10µF 16V PC electrolytic 1 1µF 16V PC electrolytic 1 100nF 630V polyester 1 100nF 63V or 100V MKT polyester 1 22nF 63V or 100V MKT polyester 1 6.8nF 63V or 100V MKT polyester 1 1nF 1kV ceramic 1 220pF disc ceramic Resistors (0.25W, 1%) 3 10MΩ 1 6.8kΩ this, solder a short piece of wire to the copper side and then connect its other end to the earth pin (GND) for the BNC connection, on the PCB. The shield PCB is then secured inside the case using silicone adhesive. Final assembly 63 28 7 4 Fig.5: this diagram shows the dimen­ sions of the blank shield PCB. four integral pillars used to mount the PCB and it has a cut-out to clear the back of the Injector jack sockets. Alternatively, if you don’t wish make your own shield board, you can buy a ready-made board from the SILICON CHIP Online Shop (code 04106153). The shield board is installed in the case with its copper side facing downwards, away from the underside of the PCB (otherwise it would short the component pigtails!). Before doing 66  Silicon Chip 1 150mm length of hookup wire 1 50mm length of single core shielded wire Now for the final assembly. First, attach the sockets to the front panel, then solder short lengths of hook-up wire to the Inject and GND terminals on the underside of the PCB. That done, pass these leads up through their respective holes in the PCB, ready to solder to the banana socket terminals. Next, attached a short shielded cable (for the BNC socket) to the GND and Tracer PC stakes on the top of the PCB. The 9V battery snap can then be fitted. Its leads are fed through from the battery compartment before being looped through stress relieving holes in the PCB and soldered to the “+” and “–” terminals. The next step is to fit the end panel to the potentiometers, switch and LED and install this into the base of the case. The PCB is then secured using four No.4 x 6mm self-tapping screws that go into integral mounting pillars. 1 9.1MΩ 1 910kΩ 8 100kΩ 1 91kΩ 1 10kΩ 2 2.7kΩ 1 2.2kΩ 1 1kΩ 1 620Ω Test Leads Tracer In Option 1:  1 x 1:1 oscilloscope probe Option 2:  1 x BNC plug-to-RCA plug lead fitted with a PC stake and 5mm & 10mm heatshrink tubing (see text) Option 3: 1 x BNC line plug, 1 x RCA line plug, 1 x 500mm-length of single core shielded audio cable, 1 x M4 nut, 1 x PC stake and 2mm, 5mm & 10mm heatshrink tubing (see text) Injector Out Option 1:  1 x multimeter lead set with accessory alligator clips Option 2:  1 x red banana plug, 1 x black banana plug, 1 x red alligator clip, 1 x black alligator clip, 1 x 500mm length of red medium-duty hookup wire, 1 x 500mm length of black medium-duty hookup wire (made into two banana plug to alligator clip leads). Once it’s in place, complete the wiring to the banana sockets and the BNC socket, then secure the lid to the base using the supplied screws. You will need to make sure that the wires do not interfere with the banana sockets – if they are sandwiched beneath the banana sockets, they will prevent the lid from fully closing. Similarly, any wires running over the battery compartment or over the slider switch will prevent the case from closing. If necessary, move the wires out of the way using a small screwdriver as the case is being closed. Finally, fit the battery and the assembly is complete. Test leads As mentioned earlier, a 1:1 oscilloscope probe makes a suitable test lead for the Audio Signal Injector/Tracer’s BNC input. Alternatively, a cheaper test probe can be made using a BNCto-RCA lead. This can be a commercial lead but these tend to be made from stiff large-diameter cable. A do-it-yourself cable using a line RCA plug, a line BNC plug and standard shielded audio cable will be much more flexible. The connections to siliconchip.com.au The shield board is installed in the case with its copper side facing down and is secured in place using silicone adhesive. Its copper side is connected to the GND stake on the main PCB. the BNC plug can be made using the method described in the article on the RF Demodulator Probe. The tip of the RCA plug can be used as the probe but note that the outer metal earth shell must be insulated using 10mm-diameter heatshrink tubing to prevent it making contact with the circuit under test. In addition, a PC stake can be soldered to the centre pin of the RCA plug to extend it. That’s done by first drilling a 1mm hole in the end of the plug’s tip, then inserting the PC stake and soldering it. It’s a good idea to cover the RCA plug’s centre terminal with 5mm dia­meter heatshrink tubing, leaving only the PC stake “probe” exposed. This will help prevent inadvertent shorts when probing closely-packed circuits. The injector signal can be fed out using a multimeter probe. Alternatively, you can use a lead fitted with a banana plug at one end and an alligator clip lead at the other. A banana plug-toalligator clip lead can also be used for the ground lead. Testing To check that the unit is working correctly, connect the “Injector Out” signal to the “Tracer In” (BNC) socket, then plug in headphones (or earphones) and listen for the 1kHz signal. Assuming that it’s present, check that siliconchip.com.au This is the view inside the completed unit. Make sure that the wiring leads to the banana sockets aren’t squashed under them as the lid is closed (push the leads towards the outer edge of each hole using a small screwdriver). the level varies when the “Inject Level” potentiometer, the “Tracer Volume” potentiometer and the “Tracer Attenuator” switch are adjusted. As noted above, if the tracer input is disconnected from a circuit, the unit will pick up hum and the 1kHz injector signal due to the tracer circuit’s high input impedance (ie, the 1kHz signal will be heard even when there is no connection). The pick-up level will depend on the capacitance of the input cable, the attenuator setting (S2), the injector level setting (VR1) and the gain (Volume) setting (VR2). Obviously, it will be at a maximum when the attenuator is set to 1:1 and VR1 & VR2 are at maximum but this combination of settings would not be used in practice. Basically, it’s just a matter of choosing settings to suit the job at hand and to minimise extraneous noise pick-up. Under normal use and when connected to a circuit for testing, the crosstalk from the injector will be minimal and will be swamped by the signal from the circuit under test. Ground connections Finally, note that when using the Audio Signal Injector/Tracer, the Ground banana socket must be connected to the ground of the circuit under test. This can be done using a lead fitted with an alligator clip as described above or by using the earth lead on the 1:1 oscilloscope probe. Now turn to page 68 for the optional SC RF Demodulator Probe. June 2015  67