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If you’re a Muso, it’s pounds to peanuts that you have a collection of leads which need testing on a regular basis. This is a bit of chore if you use a multimeter but it’s a snap with this handy Musicians’ Lead Tester. By JOHN CLARKE EL Cheapo Musicians' Lead Tester L ET’S FACE IT, the various audio leads owned by musos do not have an easy life. Microphone and guitar leads are often damaged in transit from one gig to another or while setting up. They can become intermittent and cause nasty crackling noises in the sound or heavily attenuate the signal level due to shorts or breaks in the cable itself or in the connectors. Yes, you can check leads using a multimeter; probing each terminal to measure continuity from one end to another and to check for possible shorts. But you only have to do it a few times to know that the results are usually not very reliable. This is because you are never sure whether the multimeter probe has made good contact or if it has shorted to an adjacent pin or to the shell of the plug. There has to be an easier way. Of course, commercial cable testers are available. They usually comprise an XLR (eXtension Line Return) panel plug and panel socket plus 6.35mm jack sockets in a small box. You just plug the lead into the relevant sockets, then an array of LEDs and pushbuttons allow the cable to be tested. They check for shorts and correct intercon42  Silicon Chip nections and give you the result. Some even tell where the break or short is in the lead. Such testers are not cheap. But our Musicians’ Lead Tester is cheap. It should only take a couple of hours to build and will save many hours of frustration with leads. It indicates shorts, open circuits and how the lead is connected. You can even wobble the lead under test to verify an intermittent connection. And guess what? It is so cheap that it does not have a PC board and only a few components. Sadly, it won’t tell you where the fault is in a bad cable but what do you want? A cheap tester or an expensive one? This one’s cheap. The tester comprises an XLR panel socket connected in parallel with a stereo 6.35mm panel jack socket and an XLR panel socket plug connected in parallel with a stereo 6.35mm panel jack socket. This will allow testing of cables with XLR plug to XLR socket, XLR plug to jack plug, XLR socket to jack plug and jack to jack termina- Parts List 1 plastic Jiffy box 158 x 95 x 53mm, or sealed ABS box 115 x 90 x 55mm 1 front panel label 1 metal shell XLR panel socket 1 metal shell XLR panel plug 2 stereo 6.35mm panel mount jack sockets 1 single pole 12-position or 3-pole 4-position rotary switch (S1) 1 knob to suit S1 1 momentary pushbutton switch (S2) 4 5mm high-brightness red LEDs (LED1-LED4) 4 5mm LED bezels 4 1N4004 1A diodes (D1-D4) 4 1kΩ 0.25W resistors 1 9V battery 1 9V battery clip 4 M3 x 10mm screws and nuts 1 200mm length of green hookup wire 1 200mm length of red hookup wire 1 200mm length of black hookup wire 1 200mm length of blue hookup wire 1 100mm length of 0.8mm tinned copper wire incorrect LED will light com­pared to the switch selection. Jack plug test Fig.1: the circuit uses an XLR plug and socket pair, two jack sockets, two switches and a number of LED indicators. tions. Mono or stereo jack plugs are catered for. The circuit comprises four LEDs which are used to test the four possible connections in an XLR lead. These are the shell (case) connection, the pin 1 earth (shield) and pins 2 & 3 signal leads. The ground for the 6.35mm jack socket connects to pin 1, the tip to pin 3 and the ring to pin 2. A rotary switch (S1) selects each of the four connections in turn and the LEDs light according to the connection made. The circuit is powered via a single 9V battery and there is a 1kΩ current limiting resistor for each LED. A battery test switch (S2) lights the four LEDs. XLR to XLR test In a good XLR-XLR lead, there will be continuity between pin 1 on the socket and pin 1 on the plug and so LED2 will light up when switch S1 is in position 2. Similarly, a good pin 2 connection on the XLR lead will allow LED4 to light when S1 is in position 3. LED3 will light when S1 is in position 4. LED1 may or may not light depending on whether the shell is connected. If the shell is connected to the pin 1 shield, then both LED1 and LED2 will light in positions 1 and 2 of switch S1. Note that it is wise for microphone leads to have the plug and socket shells connected to pin 1 to ensure effective shielding. However, some XLR leads may not have the shell connected, to prevent hum loops when connecting from an earthed instrument to an amplifier which has XLR inputs. A faulty lead will show some difference from the above LED conditions. Shorts between pins will light up more than one LED for selections of S1 and breaks will not light up any LED. Trans­posed leads will be shown as the Jack to XLR lead tests are trickier since there are many variants in the wiring for these. For a standard XLR to stereo jack lead, LEDs 1, 2 & 4 should light for positions 2, 3 & 4 of the switch. You may find that pins 2 & 3 are trans­posed which means that the tip and ring terminals on the jack plug connect to pins 2 & 3 respectively. This still means that the lead is good. Again, shorts are indicated with more than one LED alight and breaks are indicated with no LED alight. The shell LED lights if connected to pin 1. Mono jack to mono jack leads (guitar lead) will show a short between pins 1 and pins 2, corresponding to the ring con­necting to ground. Finally, XLR to mono leads will need to be carefully inter­preted. It is common for the unused pin on the XLR end to be connected to ground (pin 1) to provide for an unbalanced connec­tion to a balanced input. This means there may be a short shown between pins 1 & 2 or between pins 1 & 3. For a good lead there will be a connection between the tip and either pin 3 or pin 2. To find this connection, firstly push the jack plug into the socket fully to make contact with the tip. If there is a connec­tion between pin 3 and the tip then LED3 will light for position 4 of the switch. If the tip is connected to pin 2, you will find that the tester shows a short between all three connections since the tip will be at ground and pin 2 will be grounded via the ring connec­tion contacting the ground on the mono jack. You will need to pull the jack plug out one notch so that the tip of the jack plug is contacting the ring contact on the socket. The tester should then show a connection between pins 2 and tip without shorts between the other terminals. If you are uncertain about the indications from the tester, you can always try a known good lead to check how the tester performs and compare this with the suspect lead. Putting it together We built our unit into a sealed ABS box measuring 115 x 90 x 55mm. Alternatively, you could use a plastic Jiffy box measuring 158 x 95 x 53mm. Use a metal shell XLR panel socket and plug JULY 2000  43 Fig.3: this is the full-size front panel artwork. Fig.2: follow this wiring diagram to build your lead tester. The photo below shows the finished unit. Our prototype only used one stereo jack socket but you can easily add a second socket if you wish. 44  Silicon Chip and stereo 6.35mm panel mount jack sockets. The rotary switch can be a 3-pole 4-position type or a single pole 12-way type with the locking tab set so that it only rotates through four positions. The LEDs can be general purpose types but preferably high brightness, so that the Tester can be used in bright light. All the components mount on the lid with the exception of the 9V battery. Use the front panel artwork as a guide to place­ment of the components and drill out the holes required for these. The cutouts for the XLR socket and plug can be made by drilling a series of holes around the required cutout perimeter and knocking this piece out. Then file to shape. Mount each XLR and jack socket pair on either side of the box lid. The LEDs are arranged above the rotary switch so that the pointer on the knob indicates which LED is selected. By the way, our prototype used only one jack socket but the circuit shows two. Cut the switch shaft to a suitable length for the knob and attach the components to the box lid. Wire it all up according to the diagram of Fig.2. The anode lead for each LED is soldered to a strip of tinned copper wire and this becomes the positive connection for the battery clip lead. The cathode ends of the LEDs connect to the shell, pin 1, pin 2 & pin 3 of the XLR socket via 1kΩ resistors as shown. Connect the anodes of diodes D1-D4 to the shell and to pin 1, pin 2 & pin 3 of the XLR socket. Now common the cathodes and connect this point to switch S2. The other terminal of S2 connects to the wiper of S1. The battery can be held in place using a strap or clip on the bottom of the case. Battery life should be extra long and you can even use a “dead” battery from a smoke alarm, since SC the low voltage will not matter for this circuit.