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The Roadies’ Fr A tester for XLR and jack plug cables As the name suggests, this tester is designed for anyone who regularly has to check cables fitted with XLR plugs and/or 6.35mm jack plugs. Coloured LEDs on the tester clearly indicate good cables and bad, making cable checking a simple task. Design by PAUL HOAD I ONCE THOUGHT THAT the first choice in test gear when fault finding must surely be a multimeter. To test something as basic as a microphone cable, one would only need to use the multimeter’s inbuilt continuity tester and listen for tone or no tone. Purpose built testers were for “laypersons” who could not properly use a multimeter or interpret the test results! Then one evening my brother asked me to “come over for dinner and, while you’re here, sort out some crook microphone leads”. George is the administrator of the New Theatre in Sydney and so he has a few cables to check, as you might expect. “No problem!”, I thought. “As long as I remember to bring my multi­ meter, most of the time will be spent eating and drinking a few glasses of red”. The first thing I wished I’d brought were some small alli­gator leads to clip onto the pins of the male XLR plugs. Even so, there would still be a risk of shorting adjacent pins. My other problem was getting reliable contact between the meter probes and the female XLR pins. They are physically larger than the ends of standard meter probes and when inserted, result in a sloppy fit. Using an extra set of hands (George’s) was essential to ensure that one meter probe was held in contact with the male XLR pins. LEFT: the Roadies’ Friend tests all aspects of male XLR to female XLR cables and also male and female XLR to 6.5mm jack cables. Various LEDs show the condition of the cable, whether it is functional or where faults lie. Note the flush-mounted pushbuttons which insure that no damage will occur if it is stepped on! 60  Silicon Chip riend Sounds easy, doesn’t it? Well, not really. You see the pin numbering for male and female XLR sockets is mirror-imaged. So you have to keep an eye on both ends and get your numbers right. Oh yes! I almost forgot: pin 1 is the shield and is usually always wired correctly but this is not always the case with pins 2 and 3. These connect the balanced pair of wires which carry the music. So long as they are correctly soldered, the lead will work fine, even if 2 and 3 are swapped. Of course this results in functional leads that have unknown phase characteristics. Therefore, when testing a cable, you have to test each conductor against every other in order to locate out-ofphase leads. Short circuits also need to be checked in this fashion. I can’t remember if I found the crook leads and if I did, chances are the faults would not have been intermittent since any attempt to flex the cable near the connectors would have caused the meter probes to fall out or short other pins. What a hassle! This all became the inspiration behind the Roadies’ Friend Lead Check­ er and a significant mind shift on my part about the virtues of multimeters versus purpose-built testers. The Roadies’ Friend tests all aspects of male XLR to female XLR cables and also male and female XLR to 6.5mm jack cables. Various LEDs show the condition of the cable, whether it is functional or where faults lie. Since the Roadies’ Friend can be expected to be used in rough and tumble situations, it had to be designed to be rugged and difficult to damage. It had to be possible to walk on it without causing any damage, apart from incidental scratches! That meant that it had to have a strong case, no protruding switches and no on/off switch. There’s no point in having Fig.1: this is the basic concept of the cable tester, with one LED associated with each pin of the two XLR sockets and two others to show shorts to the XLR shells. an instrument like this if it can be accidentally turned on and then stay on to flatten its battery. The photos show one of the later prototypes and as you can see, it’s a pretty basic instrument with no bells and whistles. The front panel LEDs and arrows make it virtually self-explanatory. If you plug in a good XLR male to XLR female cable, for example, and then press the SCAN button, six LEDs will come on, one for each pin in each connector. If there is a short from a pin to the XLR shell, another LED, associated with the particu­lar socket, will come on. Alternatively, you can use the STEP button to individually test each conductor in the cable. Circuit description No fancy microprocessor controlled circuitry is used in this project and nor are there any special purpose or hard-to-get ICs. There are just three garden-variety CMOS ICs and not a lot else. The cleverness of the design lies not so much in the cir­cuit but in the front panel design and the use of LEDs to indicate the various cable conditions. The basic operating principle is shown in Fig.1. A 3-position switch (IC2) is used to pass current via LED6, LED7 or LED8 through a pin at one end of the cable. These three LEDs are the ‘send’ LEDs. At the other end of the cable, we monitor the currents through all pins, including the connector housing. Five ‘receive’ LEDs (LEDs 1-5) are wired to do this monitoring. If pin 1 is to be checked then LED6 will receive a low from IC2 and current will flow through R8, LED1 and R1 from the positive supply. Both ‘pin 1’ green LEDs will be on and all others will be off. The current drain is about 10mA and will cause a voltage drop of about 4V across ZD1. As ZD1 is a 5.1V type, it will not conduct. Open circuits & transposed wires If the cable is open-circuit, no current will flow through R8, so LED1 will be off (open pin 1). ZD1 conducts across the open circuit and lights LED6. If ZD1 was not included, LED6 Where To Buy The Roadies’ Friend The copyright for this project is owned by the designer, Paul Hoad. The Roadies’ Friend is priced at $115 for the fully assembled and tested version and $65 for the full kit. Payment may be made by cheque or postal money order to Hoad Electronics, Box 19, 314A Pennant Hills Rd, Carlingford NSW 2118. Phone/fax (02) 9871 3686. June 1998  61 Fig.2 (left): the complete circuit of the Roadies’ Friend. Counter IC1 is cycled through three possible outputs by oscillator IC3c to drive currents through the cable under test. Good and bad cables are then indicated by the eight LEDs. would also be off and we would not know which pin was being checked. Because the corresponding ‘send’ and ‘receive’ LEDs have the same colour, it is easy to detect transposed wires. These faults result in different coloured ‘send’ and ‘receive’ LEDs being lit. Short circuits If pins 1 & 2 of the XLR plug were to touch, then the cur­rent would increase through LED6 due to R8 and R9 being in paral­lel and this current would be equally shared between LED1 & LED2. These two ‘receive’ LEDs would both be ‘on’, indicating the short circuit. R8 and R9 increase the dynamic resistance of LED1 and LED2. If they were not included, then the LED with the lowest turn-on voltage would light and the other LED may be off. A worst case short-circuit in a cable (all wires shorted together) would see all the receive LEDs sharing the current from a single ‘send’ LED. This equates to only 3.5mA per receive LED and 17mA for the send LED. Surprisingly, this does not cause the big difference in brightness between LEDs that you might expect. The 3mm receive LEDs are physically smaller and appear subjec­tively brighter at lower currents than do the larger 5mm types used for the send LEDs. There is also little apparent difference in brightness in the larger LEDs operating at 10mA or 17mA. The relative values of resistors R8-R12 and R1 ensures that LED intensity is largely independent of the number of short circuits in a cable. For example, if the values of R8R12 are very large compared to R1, then the total current will change significantly for each additional short circuit, hence the send LED would vary in brightness compared to the receive LEDs. The reverse is true when R1 is large compared to R8, etc. The send LED would then have a constant brightness while the receive LEDs would dim in 62  Silicon Chip bright­ness with each additional short circuit. The values chosen repre­sent a good compromise. Plugs and sockets Let’s now have a look at the full circuit of Fig.2. There are six panel-mounted sockets in the tester. Each XLR socket is paralleled with its associated stereo 6.35mm jack socket: sleeve to pin 1, ring to pin 3 and tip to pin 2. The tip and sleeve of the mono receive socket is also paralleled. The ‘send’ mono socket (SK3) is a little different though, as will be explained in a moment. The circuit can be conveniently split into two parts. The part we’ll describe first does the actual testing of each wire in the cable. Whenever the output of paralleled inverters IC2e & IC2f goes low, it sends a ‘test’ signal via diode D8 to SK3 to see if a mono plug has been inserted. If this socket is empty and there­fore the integral switch contact is closed, Q2 is biased on via diode D8 and resistor R7. Q2 turns on LED8 and current can now flow to either the ring of the stereo jack socket or pin 3 of the XLR socket. Diode D7 isolates this ‘test’ signal from pin 2 on the XLR socket and the tip of the stereo socket. Plugging into the mono socket (SK3) opens the integral switch contacts and stops the bias to Q2 which turns off LED8 and the abovementioned pins. As a result, only two send LEDs (LED6 & LED7) are available when a mono plug is inserted. This avoids confusion when checking unbalanced leads, such as guitar, which only use two conductors. Also, the user can plug RCA, BNC or other unbalanced adaptors at this point. A stereo/mono panel switch is also avoided! The digital bit So much for the testing side of things. The rest of the circuit controls the low signals which are passed via the three ‘send’ LEDs. The heart of this section of the circuit is the 4017 counter IC1. It receives clock pulses from IC3c and it counts so that pins 3, 2 & 4 go high, in sequence. Each of these three outputs is inverted and buffered by IC2 to become the low signals fed via the three send LEDs. The inverters are paralleled to increase output current. You need to create a wiring harness in the box, as shown in this photograph. Start by in­stalling the two XLR sockets and then run the wires as shown. Install the jack sockets, wire them up and then remove them to dangle like this so that the PC board can be fitted. NAND gates IC3a & IC3b are connected together to work as an RS flipflop which is controlled by switches S1 & S2. If S1 is pressed, pin 11 of IC3 will be low and diode D1 will not conduct. Under these conditions, IC3c will oscillate at a frequency deter­mined by resistors R2 & R3 and capacitor C1, at pins 5 & 6. All three ‘send’ LEDs will be strobed and appear to be on continually. This is the ‘scan’ mode which tests the cable au­tomatically. If S2 is pressed, pin 13 of IC3b will be pulled low via diode D3. This will cause pin 11 to go high and D1 will June 1998  63 Fig.3: make sure you follow the steps in the text when assembling the cable tester. The PC board must be temporarily installed in the box when the LEDs are soldered in place. conduct, disabling oscillator IC3c. Pressing S2 also causes diode D6 to conduct and pull pins 5 & 6 low. IC3c now functions as a switch debouncer for S2. Each press of S2 results in a clean, debounced pulse which clocks IC1. This is the ‘step’ mode. Therefore, the user uses switch S2 to ‘step’ through each conductor in a cable. Whenever switches S1 or S2 are pressed, diodes D2 or D5 will conduct to charge the 10µF capacitor C2. This biases on FET Q1 which supplies voltage to the rest of the tester circuit. If no more buttons are pressed, C2 discharges via R6 and so Q1 turns off. This provides the automatic switchoff feature for the circuit. The turn-off delay, after the last button is pressed, is about 20 seconds, long enough to assess the condition of any cable. Diodes D3 and D4 prevent current flowing via the internal diodes of the NAND gates to ground (0V). Without these diodes, Q1 will not turn off as it should. Assembly procedure The Roadies’ Friend is housed in a UB3 plastic zippy box from Dick Smith Electronics. Made of ABS plastic and with heavy internal ribs, this is a very sturdy enclosure. Similar looking enclosures from other sources were not so good. As a guide, if the box can be twisted or easily flexed then it won’t do. So why is nothing mounted on the lid? Plugs get stuck for all sorts of reasons; eg, different tolerances, not pushing the release mechanism properly and so on. Repeated ‘struggles’ to remove recalcitrant plugs (good word that, recalcitrant) would quickly weak­en the threads of the self-tapping screws and buckle or crack the plastic lid. Therefore everything is mounted in the base of the box for greater strength. Also, the XLR sockets are mounted inside the box to provide further mechanical support when pulling stuck plugs. 12mm PC standoffs ensure that the switches are flush with the face of the tester. They can’t accidentally be turned on. Assembly steps The componentry for this project is really squeezed into the UB3 plastic utility box so it is necessary to do the assembly in a particular sequence. Follow the steps below: (1) Solder all the components, except the LEDs, to the PC board. The component overlay and wiring diagram is shown in Fig.3. Make sure you do not confuse the zener diodes with the ordinary diodes and watch the polarity of all diodes and electro­lytic capacitors. (2) The Mosfet (Q1) needs to be Resistor Colour Codes ❏ No. ❏  1 ❏  2 ❏  3 ❏  1 ❏  5 64  Silicon Chip Value 2.7MΩ 390kΩ 15kΩ 220Ω 180Ω 4-Band Code (1%) red violet green brown orange white yellow brown brown green orange brown red red brown brown brown grey brown brown 5-Band Code (1%) red violet black yellow brown orange white black orange brown brown green black red brown red red black black brown brown grey black black brown ABOVE: your PC board should look like this when it is complete. You will need to temporarily install it in the box when soldering the LEDs in place. LEFT: this is what the assembly looks like with the board installed. Make sure that the jack sockets don’t touch the underside of the board when they are installed. handled carefully to avoid damage from static discharges. It should be supplied packed with its leads stuck into a piece of black conductive foam and it is a good idea to leave this in place while the device is soldered into the PC board. Note that the gate (middle pin) of Q1 must be cranked out to fit in its respective PC hole. The two PC switches are installed with their flat sides facing the 4017 IC, as shown on the component overlay in Fig.3. (2) Attach the three Nylon standoffs to the PC board and insert the LEDs but don’t solder them to the board at this stage. Note that LED5 is reversed in orientation compared to the other seven. (3) Temporarily attach the PC board with its standoffs to the base of the box. Gently push the LEDs through their respective holes so that they are barely proud of the front panel. This sets the LED leads to the correct length for soldering. (4) Remove the PC board from the box and solder the battery snap con- nector to the board. Connect a battery and press the SCAN switch S1. LEDs 6, 7 & 8 should immediately light and then go out after 20 seconds or so. Pressing the STEP switch repeated­ly should then light LEDs 6, 7 and 8 in sequence. If these checks are not OK, you will have to carefully go over your work to find any mistakes. In our experi­ence, most faults are due to missed solder joints or solder splashes shorting between tracks. (5) Install the two XLR sockets from June 1998  65 Parts List 1 PC board, 97 x 73mm 1 UB3 plastic utility box, 130 x 68 x 42mm (DSE Cat H-2853) 1 front panel label 1 male XLR socket (SK1) 1 female XLR socket (SK4) 2 6.35mm stereo jack sockets (SK2,SK5) 2 6.35mm switched mono jack sockets (SK3,SK6) 2 round momentary contact PCB switches (S1,S2) 1 216 9V battery and snap connector 1 10-way IDC connector 3 12mm tapped 3mm Nylon spacers The battery compartment is made by sliding a suitable piece of Veroboard or copper laminate into the vertical slots, as shown here. Line the compartment with foam rubber to prevent the bat­tery from coming into contact with the underside of the PC board. the inside of the box. Don’t install the other sockets at this stage. (6) Make a wiring harness to connect the six connectors via a short length of ribbon cable to an IDC transition plug. Ensure that the wiring loops down to the bottom of the box between the connectors and that it loops outside the XLR connectors and not through them. (7) Temporarily install the four jack sockets and solder the appropriate wires to them. Then remove the sockets from the box, leaving them hanging from the harness. (8) Solder the IDC plug to the PC board. (9) Install the PC board into the box with 3mm screws. Check that 66  Silicon Chip the LEDs are correctly aligned and make sure that the wiring harness is not fouling the PC board and is laying neatly along the sides of the box. (10) Reinstall the jack sockets and check the clearance between each socket and the PC board. (11) Make the battery compartment with a suitable piece of Vero­board, matrix board or PC laminate fitted into the appropriate slots in the case. Then place a piece of thin foam rubber to insulate the PC board and prevent the battery from moving. One of the photos shows this clearly. When everything is complete, connect the battery again and push the SCAN switch. LEDs 6, 7 & 8 should light up as before. Now you should Semiconductors 1 4017 decade counter (IC1) 1 4069 hex inverter (IC2) 1 4093 quad NAND Schmitt trigger (IC3) 1 NDF0610 P-channel Mosfet (Q1) 1 BC558 PNP transistor (Q2) 8 1N914, 1N4148 diodes (D1-D8) 3 BZX79-5V1 5.1V 400mW zener diodes (ZD1,2,3) 3 3mm green LEDs (LED1,4,5) 1 3mm red LED (LED2) 1 3mm yellow LED (LED3) 1 5mm green LED (LED6) 1 5mm red LED (LED7) 1 5mm yellow LED (LED8) Capacitors 1 33µF 25VW PC electrolytic 1 10µF 25VW PC electrolytic 1 0.1µF MKT or greencap metallised polyester Resistors (0.25W, 1% or 5%) 1 2.7MΩ 1 220Ω 2 390kΩ 5 180Ω 3 15kΩ Miscellaneous 3mm mounting screws, nuts and washers, ribbon cable, foam rubber, Veroboard, solder. connect a variety of cables and simulate shorts, transpositions and open circuits to check that all cable faults are detected. Screw the lid onto the case and your SC Roadies’ Friend is complete.