Turntable Speed Check Made Easy

After refurbishing a 50-year old, Swiss-made Lenco turntable (see the illustration above) and, a little later, having repaired a slightly younger, portable, battery-operated Philips turntable, I thought it would have been nice being able to check them for correct speed. For this purpose, several stroboscope discs are available, either on the market or second hand, but I felt they are somewhat expensive for what they are. So I decided to try it by myself.

When looking at the picture of the Lenco, I realise in retrospect that it had a stroboscope disc supplied. This is sensible because this product has a continuously variable speed (with adjustable detents for the standard speeds). But without a matching stroboscope lamp even the best strobo disc is for the birds anyway...

So, to check for correct turntable speed(s), you need two things: A stroboscope disc matching your mains frequency (50 or 60 Hz) and a stroboscope lamp that flickers in the correct rhythm. Such a system is described here.

The short video that can be downloaded below is, by the way, what I received from the owner of the abovementioned Philips turntable after the successful repair :-)

A first disclaimer is required here, I'm afraid:

There is evidence that epileptic seizures are usually not random events. They are often brought on by triggers such as, among others, flickering light. People with epilepsy or at risk of epilepsy should refrain from using the stroboscopic light source described below.

I cannot accept neither responsibility nor liability for the information and materials contained in this report. Under no circumstances will I be held responsible or liable in any way for any claims, damages, losses, expenses, costs or liabilities whatsoever resulting from the use of the information given below.

Caution!

If you live in a 60 Hz country, your local mains voltage is probably 115 V. I assume that any wall wart that was sold to you is intended for use with this mains voltage. In the European 50 Hz countries (including mine), the mains voltage is 230 V, and the wall wart's primary voltage should match it. It wouldn't be amiss to check your wall wart's primary voltage anyway.

This is not a joke - in my junk box there is at least one wall wart with a 'wrong' primary voltage, and several of them can be switched over between 115 and 230 V. Needless to say that the selector switch has to be set to your local voltage.

In an online forum I found the formula for computing the correct number of bars per revolution. It is as follows:

B = 2 × f [Hz] × t [s] : n [rpm]

where

B is the number of black bars around the disc,

f is your local mains frequency in Hz,

t is the number of seconds per minute (60),

and

n is the platter speed in revolutions per minute (rpm).

You can easily see that the required number of bars not only depends on the current platter speed but also on your local mains frequency. Therefor there will be one ring of bars per speed on your strobo disc, and different strobo discs for 50 and 60 Hz, respectively.

Using the formula above, for a mains frequency of 50 Hz and

• 33⅓ rpm (revolutions per minute), you get 180 bars, spaced by 2° (degrees), because a full revolution is 360°.
• 45 rpm, you get 133.3333333333333 bars, spaced by 2.7° - which isn't realistic, because we deal with integer numbers of bars only. So the next approximation for 45 rpm is 133 bars, spaced by 2.706766917293° (the actual speed, when set with this disc, will then be 0.25% high).
• 78 rpm, you get 76.92307692307692 bars, rounded up to 77 bars, spaced by 4.68° (the actual speed will then be 0.10% low).

If however your mains frequency is 60 Hz, for

• 33⅓ rpm, you get 216 bars, spaced by 1.666666666666666666°
• 45 rpm, you get 160 bars, spaced by 2.25°
• 78 rpm, you get 92.30769230769231 bars, rounded down to 92 bars, spaced by 3.91304347826087° (the actual speed will then be 0.33% high).

In fact, that many digits after the decimal point are not really needed. A certain accuracy, however, is required, so that the ring of black bars closes without a gap. My calculator displayed them for free, and so I copy/pasted them as they were without losing any further thoughts on them.

What's more? You need to know the approximate diameter of your strobo disc. 90 mm is a reasonable value, because you may want to place it on the label of a vinyl record in order to check your turntable's speed while playing it. And the last information you need is the diameter of the center hole, which is standardised to 7 mm.

You could now, of course, open your vector graphics software (such as Adobe Illustrator or Corel Draw) and draw your own disc by yourself with the information given above. But why invent the wheel again when I already did it for you? Just download the '90mm-50&60Hz-33&45&78.pdf' file given below. It contains both a 50 and a 60 Hz version on a single DIN A4 sheet.

Some time later, btw, I stumbled on a homepage where strobo discs for the four standard speeds 16⅔, 33⅓, 45, and 78.26 rpm, and some more other interesting information, can be found for download free of charge - look here: https://soundsclassic.com/ttindex.html. However the image definition is a bit less sharp than the one in my pdf file. In addition, the 78.26 rpm label indicates that this strobo disc is designed for a local mains frequency 60 Hz only (for 50 Hz this would rather be 77.92 rpm, and the number of bars for every speed would be different, as described above).

Make sure that your printer prints the true dimensions, and select 'Page scaling: None' in your Adobe Reader's print menu.

You may want to print on heavier card stock. It will be still more sturdy if you print it on plain paper and glue it on some heavier cardboard using a glue stick. In this case, I recommend glueing a second piece of blank printer paper to the reverse side of the cardboard, preferably in the same orientation, in order to avoid warping due to changes in air humidity.

Cut this assembly out along the circumference, and use a sharp(ie) knife for cutting out the 7 × 7 mm square hole in the center. There is no need to bother with a round hole; the square hole works well enough and is much easier to cut.

BTW, even if your disc isn't perfectly circular (as is the case with my own first prototype pictured above - it only has rings for 33⅓ and 45 rpm), this has no influence on the correct function.

If you have read some of my earlier Instructables, you will already know that I like doing things the cheap and easy way, preferably using free, harvested, or recycled/repurposed components.

One of these easy ways would be by using a small neon glow bulb, since these flicker in time with twice the mains frequency, i.e. 100 or 120 Hz. They were indeed used by turntable manufacturers for models with integrated stroboscope rings at the platter's circumference. Unfortunately these neon bulbs are rather dim, and the strobo ring isn't visible well in daylight.

Normal incandescent lamps, on the other hand, are a too lazy (i.e. slow) and cannot deliver a distinct flicker.

Somewhere fluorescent lamps are recommended. However, these have taken a back seat with reference to LED replacement tubes. Whether these replacements flicker at all, and if so, at which frequency, is totally depending on the manufacturer. Reliable predictions are impossible in this case.

The optimum solution is using LEDs. They turn on and off much more rapidly than the human eye can perceive, and they are by far fast enough for a distinct 100 or 50 Hz (or 120/60 Hz) flicker. Apart from that, they are very efficient, so they need little power for a rather high brightness yield.

My first thought was using one of the several white 2 W LEDs I recently had ordered from China, because they are for sure bright enough. For this an extra housing would have been necessary. Some more thoughts showed that for safe operation, together with a re-purposed wall wart, a rather high-powered series resistor and several other components would have been necessary (see diagram no. 1 below) - in addition to a housing that also serves as a heat sink, so a different solution was needed.

From an earlier project I had a cheap, modified LED torchlight that was the subject of one of my other Instructables (https://www.instructables.com/Long-Life-for-a-Cheap-LED-Torch-Light/). It is by far bright enough for the application.

For powering this LED, I wanted to use one of the many unused wall power supplies (aka wall warts) waiting in my junk box for future missions. However, it was important to find one of the old-fashioned types containing a mains transformer, because I wanted to profit from the AC (or pulsed DC) output that none of the modern, switch-mode wall warts supply. For more info on this subject, you might refer to my Instructable about repurposing wall warts (https://www.instructables.com/Reusing-Wall-Warts/).

So, which wall wart to choose? The torchlight mentioned in step 4 consumes about 200 mA from a 4.5 V DC supply (e.g. from three AA batteries connected in series). I selected an old Nokia mobile phone charger (type ACP-7E) specified for 3.7 V at 355 mA pulsating DC current (the corresponding symbol on its type label is '---'). Under no-load conditions its output rises to around 8 V. I used this charger/wall wart for several reasons apart from sufficient output: it can be opened without using brute force (ok, you need a tri-wing screwdriver or some other trick), and I'm sure that several people have one of these Nokia chargers lying around from one of their first mobile phones. Remember the time when mobile phones were not smart yet?

Looking into it, you find a mains transformer and a small PCB containing four 1N4001 diodes in a bridge rectifier configuration (converting the transformer's 50 or 60 Hz AC output to pulsating DC with 100 or 120 Hz), and a disc-shaped component in series with the output which I assume is a PTC serving as short-circuit protection. (This part gets hot when excess current flows, it then increases its resistance in order to limit the current and avoid damage).

Please note that the windings of so small a transformer are made from very thin wire. Be careful not to break any of the wires since reconnecting such a broken wire is next to impossible!

My 1st idea

was to use only one (the positive) half-wave of the wall wart's AC output, resulting in a 50/60 Hz flickering light. This required several additional thoughts - first of all, all the components had to be removed from the PCB. I re-used two of the diodes and added 3 resistors for allowing current flow also during the negative cycle of the output voltage 'around' the LEDs, since transformers (small ones in particular) hate half-wave operation and need both the positive and the negative current flow to avoid overheating. One of these resistors was required for additional current limiting to the LEDs since the supply voltage was higher than the original 4.5 V battery supply. Apart from that, most LEDs can only withstand about 5 V in reverse direction, so one of the diodes had to be in series with the LEDs to block the negative voltage. Figuring out the new circuit and how to place the components on the original PCB took quite some time. There was not sufficient room in the enclosure for leaving the output cable in its original position, so it had to be placed somewhere else. Anyway, all this proved unnecessary in the end since I could show that a 100/120 Hz flicker frequency works as well and is much easier to obtain. The circuit diagram (no. 1) and the component placement drawing of this first idea are, nevertheless, shown above - in case you should be interested.

The 2nd, better idea

came up, unfortunately, a little later. It fully complies with the KISS principle (keep it simple, stupid!). In order to compare it with the first idea, I modified a second ACP-7E Nokia charger that was, at a first glance, identical with the one I used for the 1st idea. When looking a little more closely, you can spot some differences (see picture). And after opening it, it looked rather different inside. The PCB is much smaller while accommodating the same circuit and components - a mains transformer, four 1N4001 diodes connected as a bridge rectifier, and a PTC in series with the output. Since I wanted to have 100/120 Hz pulsating DC instead of the 50/60 Hz half-wave from the 1st idea, the only thing I had to do was replacing the PTC by a suitable series resistor and was done. The much simpler circuit diagram (no. 2) as well as the corresponding component placement drawing are shown above - however, drawn for the PCB of the first charger.

That was easy: I took the torchlight mentioned in step 4 and inserted the cable from the Nokia charger through a hole in the torchlight's case. I soldered it to the battery holder respecting the correct polarity (white is +, black is –); the batteries must, of course, be removed. The battery holder with the attached cable then connects to the internal contacts. A knot in the cable inside the torchlight's case helps against pulling the cable out and is used as a makeshift strain relief.

BTW: My modified torchlight from the Instructable mentioned above already had, for each of the eight LEDs, individual current limiting series resistors. But the supply voltage is now a bit higher than with normal battery operation, so a single series resistor had to be added in order to avoid overloading the LEDs. This resistor is placed within the wall wart's case.

Assuming that the charger/wall wart according to the 2nd idea above is used, the torchlight's switch can remain in place and must, of course, be switched on when using the torchlight. Only when using it together with the 1st version, the switch must not be switched off while in operation because the transformer then is asked to deliver DC, which it doesn't appreciate. I bypassed the switch with a soldered-on piece of thin wire, in order to be on the safe side in case I should forget to switch it on while using the 1st version supply.

I had one torchlight and two different supply units now. For easy changeover while testing I used two female DIN speaker plugs on the cables from the wall warts, and a male DIN speaker plug on the cable to the torchlight.

Interestingly enough, both wall wart/torchlight combinations, i.e. the 50/60 Hz and the 100/120 Hz versions, work fine with the strobo disc produced in steps 2 and 3 above. So it is clear that version 2 is preferred, since it is much simpler and easier to make.

When placing the strobo disc on a rotating platter (or a vinyl disc while playing it) and illuminating it with the flickering torchlight, the matching ring of bars either seems to stand still or rotates veeeery slowly.

Unfortunately this cannot be nicely shown in a video because the picture repetition rate of the camera and the flicker frequency do not match. However a short video clip giving an impression of the flickering torchlight only can be downloaded using the link given below.

The image you get with the pulsating light is somewhat blurred. It could be much better defined by using a light source flickering with very short, square-wave pulses. To get that, some more, sophisticated electronics including a quartz-controlled time base would be needed, resulting in some overkill, imoho.

This system is, in fact, not suitable for absolute turntable speed measurements.

A lot of turntables are equipped with a synchronous motor, so the speed of the platter is intimately linked to the mains frequency - as, of course, also is the flickering rhythm of your strobo system. This means that you can only detect speed deviations caused by a slipping drive element, such as a worn-out rubber belt or idler wheel, excess friction in the platter's bearing - or perhaps braking of the platter due to increased friction within the groove during loud music passages.

Quartz-regulated turntable drives are usually more precise and constant than the ones with a synchronous motor, but any speed deviation can be detected with our system as well.

The vintage portable Philips turntable I was repairing, however, had a DC motor and was equipped with a simple but rather sophisticated speed regulation circuit, designed with largely varying battery voltage in mind; the nominal speed value of this drive might indeed have profited from our strobo system.

Since both your torchlight and your wall wart will probably be different, you will have to adapt some components to your specific application. But measuring the current flow through one or several LEDs, determining its/their forward voltage and computing the voltage drop across the required current limiting resistor isn't rocket science.

In case of doubt, you find information on this subject here: https://www.instructables.com/Long-Life-for-a-Cheap-LED-Torch-Light/ - there's no need to repeat all that here.

Of course, a wall wart allowing easy access to its entrails is preferable. If however the case is not screwed but glued together, you will need patience, imagination, and creativity for opening it, sometimes even brute force. A utility knife or a hacksaw can be used for cutting the seams; these tools are best applied in the corners of the case only. Take care to only make very shallow cuts in order not to damage anything inside. The cuts can then be used for inserting e.g. a screwdriver as a wedge for prying/breaking the case shells apart.

For the current application, you need to remove any smoothing capacitors from the wall wart (in case there should be some electrolytics included). The best location for installing any series resistor is also within the wall wart - which might make you think a bit. But some brain training is not bad at all, right?

In case you wish to know a little bit more about AC, different kinds of rectification and the corresponding results, you can find three different waveform diagrams above.

The first diagram shows the normal (AC) output voltage of the mains transformer. This is a plain sine wave with one half of every period being a positive voltage, the other half a negative voltage.

The second diagram shows the mains transformer output voltage after passing a single diode (a kind of unidirectional valve, similar to a one-way road), also called half-wave rectifier. In this case, only the positive part of the transformer output, the eponymous 'half wave', is provided, while the negative part is cut away. In diagram no. 1 in step 5, the diode in question is D10. In order to achieve the same average brightness as in the next example, the LED current has to be twice as high, since the LEDs are operating only for half the time.

The third diagram shows the mains transformer's output voltage after passing a bridge rectifier (a tricky circuit that consists of four diodes), also called a full-wave rectifier. Here, thanks to the clever arrangement of the four diodes, the positive part of the transformer output is provided, and the inverted negative part is added. In diagram no. 2 in step 5, the diamond-shaped arrangement of the diodes D9-12 is the bridge rectifier.