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By JOHN CLARKE Check your turntable’s speed with this white LED strobe So you have dragged out the old turntable and are playing vinyl records again. Good. But how do you know that the turntable speed is correct? The old way to do it was to use a circular disc with strobe markings but that does not necessarily work these days. Why not? Read on. P LAYING VINYL records has made a big comeback in recent years and many people are resurrecting their old turntables or buying new ones. But there are a few hurdles before you get the optimum result, such as making sure the cartridge stylus is not worn out and that your preamplifier provides the correct equalisation. On a more prosaic note, many turntables which have been out of action for decades may not necessarily operate at the correct speeds of 33.3, 45 62  Silicon Chip and 78 RPM. So you need to check that aspect. How do you do that? The old tried and true method was to use a circular card which had stroboscopic markings on it and run the turntable under mains voltage lighting; 230VAC 50Hz in the case of Australia, New Zealand and most of Europe or 120VAC 60Hz in the case of the Americas, parts of Japan, Asia etc. These stroboscopic cards have four or six bands of markings and depending on the speed selection, one of those bands would appear to be stationary. The reason for this was that incandescent or fluorescent lighting had a strong 100Hz or 120Hz component and this would act to make the relevant strobe band on the circular card appear to stop moving. The same method applies to those turntables that have strobe markings on the rim of the platter. But while the principle is still correct, it does not work very well in most homes these days. Why not? Because our political siliconchip.com.au masters have deemed that old-fashioned incandescent lights are “wasteful” and “bad for the environment”. At the same time, fluorescent lighting in most homes is now out of fashion, unless it is using those ugly compact fluorescent lamps (CFLs) with their unnatural hues and copious electromagnetic interference. So why can’t these modern lamps provide the same stroboscopic effect? The reason is that they run at much higher frequencies so that any residual AC component in the light output is very small. This applies to any lighting which uses electronic ballasts. Mind you, even when you are using incandescent or fluorescent lighting powered by 50Hz or 60Hz mains, the strobing effect is not particularly strong and it is even weaker with halogen lamps with their much hotter filaments. We will explain why later in this article. Turntable types Most good turntables are either belt-driven or direct drive. Cheaper turntables were driven from an idler wheel inside the rim of the platter. The belt-driven types usually have a small synchronous motor which can be assumed to be locked to the mains frequency, provided the belt is not slipping on the motor shaft. This could happen if the belt is perished, kinked or hardened. Idler-driven turntables typically have a shaded pole motor and they are not so tightly locked to the mains frequency (and because of the idler-drive, they are more likely to produce rumble). Direct drive turntables should run at the correct speed but again, that cannot be taken for granted. Also some direct drive turntables had or have a variable speed feature which allows the music pitch to be shifted over a a range of about a semitone. Again, how do you know what is the correct speed setting (unless you have absolute pitch)? Any substantial speed variation is liable to cause any music to sound off-pitch. And if you want to dance to records and the number of beats per minute is important, then again, the turntable speed should be correct. Our solution has been to design a white LED stroboscope which produces one millisecond pulses of light at a very precise 100Hz or 120Hz (ie, twice the mains frequency). But our recommendation is to use it at 120Hz siliconchip.com.au Turntable Speed Variations Turntables that rely on a 50Hz or 60Hz mains supply to drive a synchronous or shielded pole motor may not necessarily run at the correct speed. Typically, the 50Hz mains frequency can vary between 49.85Hz and 50.15Hz (ie, ±0.15Hz) over the course of a day. Typically, the mains frequency will be slightly low during periods of peak power demand and a little high at other times. That variation would mean that middle C could be as low as 260.841Hz and as high as 262.411Hz. Whether this is noticeable or not depends on how well you discern pitch. Further turntable speed problems can be present if an incorrect-sized pulley on the motor spindle is used to drive the belt. This could be because you have an imported turntable that’s been designed to operate from 60Hz instead of 50Hz (or instead designed to run with 50Hz instead of 60Hz). You may be able to supply the correct voltage for the motor using a transformer but the frequency will not be correct. For precision speed from a synchronous motor drive, an electronic driver circuit could be used to produce a suitable sinewave source for the motor. This could be a low-powered crystal locked sinewave inverter such as for an uninterruptible computer supply. Modified sinewave inverters may not be suitable since the square wave supply may introduce noise into the motor and cartridge pick-up leads. Why Is This White LED Strobe Necessary? In the “olden days” the usual method of providing a strobe light source involved using an in-built Neon discharge lamp powered from the 240VAC 50Hz or 120VAC 60Hz mains supply. The neon would produce light pulses at 100Hz or 120Hz and this would give a stationary pattern for the set speed. However, using the mains supply is most unlikely to give a completely steady strobe pattern when you are using a crystal-controlled direct-drive turntable unless the mains frequency is precisely 50.000Hz or 60.000Hz. Even a slight error will cause the strobe pattern to rotate slightly. Of course, with a belt-driven synchronous motor turntable, you would never be aware of these speed errors (unless you build our Turntable Strobe). to give the most accurate speed indication with a strobe card. So why is that? Funnily enough, a lot of strobe cards are not necessarily accurate and if you want the most accurate speed indications at 33.33 and 45 RPM, you should use a strobe pattern designed for 60Hz operation. Interestingly, as far as 78RPM records are concerned, it is not possible to get an absolutely accurate speed indication at 100Hz or 120Hz but 100Hz is the more accurate, with a speed error of 0.1%. Because of these issues, we have also designed a PCB strobe disc that you can place on your turntable to check its speed. It is just the right size to fit on the record label and will not cover the playing area. Since it is precisely etched and machined, it will not have the common fault of some printed strobe discs which can be slightly off-centre or the centre hole is a little over-size. A turntable rotating at the correct speed will have one band of the strobe disc markers remaining stationary. If the markers drift clockwise, then the turntable speed is fast and if the markers drift anticlockwise, the turntable speed is too slow. Any slight wavering forwards or backwards of the markers will be due to irregular speed variations and significant variations of this nature and may be audible as “wow and flutter”. What can be done about a turntable that doesn’t run true to speed? More information on this is detailed in the above panel. Our LED Turntable Strobe is built on a small PCB that fits into a small plastic utility box. Alternatively, the PCB can be installed inside the turntable cabinet and the strobe LED can be mounted to illuminate strobe markings on the platter’s rim. It can be powered with a 9V battery, an external DC supply or a 5V supply via a USB connector. Circuit description Fig.1 shows the circuit and it is based on a PIC12F675 microcontroller (IC1). The microcontroller vastly simDecember 2015  63 S1 D1 1N4004 CON1 A 9-12V DC IN 1N4004 REG1 78L05 K OUT IN A GND 100 µF 470 µF GND 1N5819 16V 16V + 78L05 K IN OUT K A D2 1N5819 9V A – +5V 100nF 1k 1 2 3 4 5 USB MICRO‘B’ SOCKET K 4 CON3 2 X1 4.0MHz 33pF 33pF 3 1 Vdd GP3/MC GP5 68Ω A GP1 6 LED1 (WHITE) GP2 C 470Ω 5 Vss 8 EXTERNAL LED λ CON2 K 7 IC1 PIC12F675 GP0 GP4 68Ω B JP1 IN: 120Hz OUT: 100Hz Q1 BC337 E BC 33 7 LED SC 20 1 5 TURNTABLE STROBOSCOPE K A B E C Fig.1: the circuit is based on a PIC12F675 microcontroller (IC1), with 4MHz crystal X1 used as the reference clock. Pin 7 of IC1 drives transistor Q1 to flash white LED1 while jumper JP1 sets the strobe frequency to 120Hz or 100Hz. plifies the circuit, compared to using a separate crystal oscillator and dividers. In addition, the microcontroller makes it easy to incorporate 100Hz and 120Hz operation. IC1 uses a 4MHz crystal as the reference clock for its program to run the strobe. The un-calibrated accuracy of the crystal (typically 50ppm) is sufficiently accurate for the strobe. IC1 internally divides the 4MHz frequency by four, so that the program runs at 1MHz. Single clock instructions of the program are therefore 1μs in duration. As already noted, the strobe LED is driven with 1ms pulses and this gives a duty cycle of 10% at 100Hz or 12% at 120Hz. This will ensure that the strobe disc markings appear quite sharp. Longer pulse durations will cause noticeable blurring of the strobe pattern as the markings move further during the on-period. This is a distinct advantage of our LED strobe compared to the light from an incandescent lamp powered from a 50Hz or 60Hz mains supply, with the resultant display being quite indistinct by comparison. Designing The Strobe Disc We have designed our strobe disc to suit 120Hz operation for 33.33 RPM and 45 RPM. We have also provided a strobe band for 78 RPM at 120Hz but it will produce a speed error of -0.325%. To counter that, we have also provided a 78 RPM strobe band for 100Hz operation and this will have a speed error of -0.1% (close but no cigar). Mind you, precision speed setting at 78 RPM is not so important because most records from that era were not cut at a precise 78 RPM. Note that there are lots of strobe disc patterns that can be down-loaded from the internet but most are incorrect. They may be correct at one speed, say 45 RPM, but incorrect at 33.3 RPM or 78 RPM. As an example, some patterns are designed for 33 RPM, not the correct value of 33.33 RPM. If you already have a strobe disc, how do you check that the pattern is correct? It’s a simple calculation. Just multiply the strobe frequency (100Hz or 120Hz) by 60 to convert to pulses per minute. Then divide the turntable speed in RPM into this number. So 33.33 RPM requires 100 x 60 ÷ 33.33333 or 180 bars for a 100Hz strobe or 216 bars for 120Hz. It’s not possible to obtain a correct pattern for 45 RPM at 50Hz, since the number of bars is not an integral number; it is 133.333. So any card with 133 bars is doomed to error. If you want to be sure of the result, use our strobe disc. 64  Silicon Chip The white LED (LED1) is driven via transistor Q1 and a 68Ω resistor connected to the +5V supply rail. Q1 is switched on and off by the GP0 output of IC1, using a 470Ω base resistor. The LED is driven at a nominal current of 29mA, assuming a 3V drop across the LED. Connector CON2, a 3.5mm jack socket, is provided so that an external LED can be connected. We have provided several options for the power supply: a 9V battery, a 9-12V DC plugpack via CON1 or 5V via a micro-USB “B” socket. If using a 9V battery or a DC supply via CON1, the 78L05 3-terminal regulator (REG1) provides 5V to the micro. Alternatively, if you are using a 5V USB supply, this is fed to the micro via Schottky diode D2. If you intend using a USB power source exclusively, you can omit the other supply components such as CON1, D1, switch S1, REG1 and the 100μF capacitor. For those interested in the effects of the strobe flash length, you can select a 2ms flash duration by tying pin 6 of IC1 to pin 8 using a short piece of wire under the PCB. This will set the strobe to flash for 2ms but it will still run at 100Hz or 120Hz, as selected with JP1. This change needs to be done while power is off. A return to a 1ms flash duration will only occur when pin 6 is disconnected from pin 8 with power switched off and on again. siliconchip.com.au TOP OF CASE (NO LID) 12mm 12mm + 13mm B A + A = 5mm dia. B =- 6mm dia. TOP OF CASE (NO LID) 24mm 10mm 5mm C + + E C = 6mm dia. D = 9 x 5mm E = 5 x 9mm 10mm D 9mm + Fig.2: the two end-panel drilling templates. They can either be copied or downloaded as PDF files from the SILICON CHIP website. The program checks the GP2 input level and produces the 100Hz strobe signal when this input is high at 5V. It produces a 120Hz signal when the input is low. The GP2 input is pulled high via an internal pull-up resistor in IC1 when JP1 is out and is pulled low when jumper shunt JP1 is inserted. The jumper setting can be altered while the strobe is operating and the strobe frequency will change immediately. Drilling the case The Turntsable Strobe is housed in a UB5 plastic utility box (83 x 54 x 31mm) with holes cut in one end for the LED and the external LED socket (if fitted) and in the other end for the on/ off switch, the DC socket and microUSB socket. It’s necessary to drill and cut the case before installing any parts on the PCB. There are a few options here, though. First, if you will be running the unit from battery power only, then there’s no need to cut holes in the case for the DC socket and the micro-USB socket and these two parts can be left off the PCB. Alternatively, if you will be supplying power via the DC socket or micro-USB socket only, then the battery and on/off switch can be left out and there’s no need to cut a hole for the switch. You could also leave out either the DC socket or the micro-USB socket, depending on the external supply. At the other end of the case, you can leave out the 3.5mm jack socket if you don’t intend using an external LED. By the way, the micro-USB input siliconchip.com.au does not have to connect to the USB port on a computer. Any USB output from a 5V plugpack or power board can be used to supply power. Some modern turntables even include a USB port on the turntable plinth. The first job with the case is to remove the internal ribs on each side and this can be done using a small pair of sidecutters. You can then finish off by using a sharp chisel to remove any remaining rib material. The next step is to use the PCB as a template to mark out its three mounting holes in the case. That’s done with the PCB sitting inside the case and pushed hard against two of the side pillars (see photo). The PCB is then removed and the mounting holes drilled to 3mm. Countersink these holes on the outside of the case using an oversize drill. You now have to cut and drill the holes in the end panel and that’s done using the templates shown in Fig.2. These templates can either be copied from the magazine or downloaded in PDF format from the SILICON CHIP website and printed out. Once you have the templates, cut them to size and attach them to the end panels using adhesive tape. Be sure to attach the correct template to its panel – the template with the two circular holes must go on the end that matches the LED end of the PCB. It’s now just a matter of drilling and cutting the holes in the panels as required. The square cut-outs for the micro-USB socket and switch S1 can be made by drilling a series of small holes in a row, then joining them and filing to the required shape. Note that it’s a good idea to always use a 1mm pilot drill to start the holes (to ensure precise location) and then enlarge them to the required size using successively larger drills. PCB assembly All parts (except the battery) are mounted on a PCB coded 04101161 and measuring 79 x 31mm. Fig.3 shows the parts layout. Begin by soldering the surface mount micro-USB socket (if used) to the underside of the PCB, then flip the board over and install the resistors on the top side. Table 1 shows the resistor colour codes but it’s also a good idea to check each one using a digital multimeter before soldering it into place. Follow with diodes D1 & D2, making sure that the 1N5819 is used for Parts List 1 PCB, code 04101161, 79 x 31mm 1 set of turntable templates (see text) 1 UB5 case, 83 x 54 x 31mm 1 4MHz crystal (X1) 1 DIL8 IC socket 1 SPDT vertical slider switch (Altronics S 2071) (S1) 1 2-way header (2.5mm pin spacing) (JP1) 1 pin header shunt 3 6.3mm tapped Nylon stand-offs 3 M3 x 5mm countersink head screws 3 M3 x 5mm machine screws 1 Micro-USB type B socket (CON3) (FCI 101035940001LF) (au.element14.com – Part No. 2293752) 1 PCB-mount DC socket (CON1)* 1 9V battery* 1 9V battery snap connector* Semiconductors 1 PIC12F675-I/P microcontroller programmed with 0410116A. hex (IC1) 1 78L05 regulator (REG1)* 1 5mm white LED (LED1) 1 BC337 NPN transistor (Q1) 1 1N4004 diode (D1)* 1 IN5819 Schottky diode (D2) Optional external LED parts 1 5mm white LED 1 switched stereo 3.5mm PCBmount jack socket (CON2) 1 mono 3.5mm jack plug 1 length of single cored shielded cable 1 100mm length of heatshrink tubing (1mm and 5mm) Capacitors 1 470µF 16V PC electrolytic 1 100µF 16V PC electrolytic* 1 100nF MKT polyester 2 33pF ceramic Resistors (0.25W, 1%) 1 1kΩ 1 470Ω 2 68Ω *Note: omit DC socket CON1, diode D1, switch S1, the 100µF capacitor, regulator REG1, the 9V battery and the battery snap connector if the unit is to be exclusively powered via the micro USB socket. December 2015  65 REG1 78L05 100 µF100nF 470 µF + 33pF D2 CON1 IC1 PIC12 F675 33pF 4MHz 4004 9V 1k X1 D1 + Turntable Strobe 470Ω Q1 BC337 JP1 5819 CON3 68Ω + 68Ω S1 JP1 out 100Hz JP1 in 120Hz 04101161 © 2016 revB LED1 A WHITE CON2 K MICRO USB-B T S + R + FROM 9V BATTERY CLIP Fig.3: follow these two parts layout diagrams and the photos below to assemble the PCB. The micro-USB socket (CON3) should be soldered to the underside of the PCB first, after which the remaing parts are installed on the top side. Left: inside the completed unit. The battery and switch S1 can be omitted if the unit is to be powered only via the DC socket or microUSB connector. Similarly, CON2 can be left out if you won’t be using an external LED. D2. Make sure also that D1 & D2 are correctly orientated. The DIL8 socket can be then installed, followed by the 100nF capacitor and the two 33pF ceramic types. Crystal X1, transistor Q1 and REG1 are next on the list but don’t get Q1 & REG1 mixed up. The two electrolytic capacitors can then go in, along with the 2-way pin header (the header’s shorter pins go into the PCB). Once the header is in place, install the jumper shunt (ie, to short the pins) so that the unit will operate at 120Hz. As explained earlier, DC socket CON1, jack socket CON2 and switch S1 are optional. CON1 is required if you are using a 9-12V DC plugpack (ie, one with no USB output) to power the unit, CON2 if you are using an external Table 1: Resistor Colour Codes   o o o o No.   1   1   2 66  Silicon Chip Value 1kΩ 470Ω 68Ω 4-Band Code (1%) brown black red brown yellow violet brown brown blue grey black brown LED and S1 if you are using battery power. If you are using a DC plugpack to power the unit (via CON1) but will not be fitting a battery, switch S1 can be replaced by a wire link. LED1 is installed by first bending its leads down by 90° exactly 10mm from its plastic body. Make sure that it is correctly orientated before doing this though (the anode lead is the longer of the two). The LED is them mounted with its leads 4mm above the PCB (use a 4mm thick spacer to set this height), so that the centre of its lens lines up with the adjacent jack socket. The last part to connect is the battery snap. Feed its leads through the stress relief holes as shown in Fig.3 before soldering them to the PCB. If you intend using an externally connected LED, this can be now wired to a length of single-core shielded cable. Connect the centre lead to the LED’s anode and the shield wire to Table 2: Capacitor Codes   Value µF Value IEC Code EIA Code   100nF 0.1µF   100n   104  33pF  NA   33p   33 5-Band Code (1%) brown black black brown brown yellow violet black black brown blue grey black gold brown siliconchip.com.au These two views show the completed unit. Note that some of the holes in the end panels can be omitted, depending on the options chosen when you build the unit (see text). the cathode. The other end of the cable is then terminated in a 3.5mm jack plug, with the centre lead going to the tip contact and the shield to the outer sleeve contact. Final assembly Now for the final assembly. First, attach three M3 x 6.3mm tapped Nylon stand-offs to the PCB mounting holes and secure them using M3 x 5mm machine screws. The PCB assembly is then installed by angling it down into the case so that LED1 and CON2 pass through their respective holes, then squeezing the sides of the case together and pushing the other end of the PCB down until the switch and micro-USB socket go into their panel cut-outs. The PCB is then secured in position using three M3 x 5mm countersinkhead screws which go through the base and into the stand-offs. Once it’s in place, fit the battery snap to the battery and slide the battery into the case as shown in the photo. Fig.4: this screen grab shows the waveform at the GP0 output, pin 1, of IC1. In this case, the circuit is set for 100Hz operation (JP1 out). The LED is lit for 1ms at a 10% duty cycle. Ignore the error of the displayed 100.032Hz which is because the oscilloscope frequency calibration is not particularly precise.   Dataflex/Datapol Labels (1) For Dataflex labels, go to: www.blanklabels.com.au/index. php?main_page=product_info& cPath=49_60&products_id=335 (2) For Datapol labels go to: www. blanklabels.com.au/index.php? main_page=product_info&cPath =49_55&products_id=326 SILICON CHIP siliconchip.com.au Testing Now for the smoke test. Apply power and check that there is 5V (4.85-5.15V) between pins 1 and 8 of IC’s socket (or 4.5-5.2V if using USB power). If this is correct, switch off and install IC1 (watch its orientation), then reapply power and check that the LED lights. If it does, then your Turntable Strobe is working and you can attach the lid which now becomes the base of the unit. If you now move the unit rapidly from side-to-side with the LED viewed side-on (ie, not looking directly into the lens), it should be seen to light in several different positions. That indicates that the LED is being flashed siliconchip.com.au on and off. By contrast, if you look directly at the LED when it is stationary, it will appear to be continuously lit due to its 120Hz flash rate. Finally, if you have made up an external LED cable, plug it in and check that its LED also operates. Front panel label The front-panel label is available in PDF format on the SILICON CHIP website. It’s just a matter of downloading it and printing it out onto an A4 sized synthetic Dataflex or Dataplex sticky label (see panel). This label is then attached to the top of case (ie, not the lid), as shown in the photos. Turntable Strobe Fig.4: the front panel label can be downloaded as a PDF file from the SILICON CHIP website. Alternatively, you can print out a paper label and attach this using doublesided tape. That’s it – your Turntable SC Strobe is ready for use. December 2015  67