Printable turntable strobe disc for 33⅓, 45 and 78 RPM

Choose your mains frequency and a size, print the page, cut out the disc and put it on the platter. Under a lamp that flickers with the mains, the ring for your speed stands still when the platter is at that ring's speed and creeps when it is not.

I develop Stillspin, an iPhone strobe tachometer, and one section below is about it. The disc, the arithmetic and the instructions all work without the app.

Print it and cut it out

1. Print at 100%, Actual size or no scaling, not Fit to page. Use your printer's best quality, because the finest bars on the label-size disc are about half a millimeter wide. Heavy paper or card lies flatter on a platter, and you can glue a thin print to card.
2. Check the print against the rulers (next section).
3. Cut along the outer line and cut out the center hole. Start at about 7 mm and trim until the disc fits your spindle snugly with no play.
4. Put the large disc on the platter, or the label-size disc on a record's label. Measure your label first, and check that the disc clears the tonearm before you lower the stylus.
5. Let the turntable run for a few minutes, dim the room and hold the lamp close to the disc.

What the scale check is for

The sheet has two 100 mm ruler lines at right angles, an inch scale beside each, and a box the size of a standard bank card. Measure them, then measure the disc's cut line across and top to bottom.

Being a few percent off in size does not change the reading. The speed depends on how many bars pass per turn, and a print at 97% still has the same number of bars. Size matters for fit: a label-size disc that prints too large may not clear the tonearm. What the check really catches is a printer that stretches one direction more than the other. That turns the disc into a slight oval, and the pattern wobbles back and forth twice per turn. A spindle hole that is off center does much the same once per turn.

How to read the rings

Each label names the ring just outside it, so the label nearest the center belongs to the innermost ring.

• If the bars stand still, the platter is at the speed printed on that ring.
• If they creep in the direction of rotation, the platter is fast.
• If they creep against the rotation, the platter is slow.

To put a number on it, count how many bars pass a fixed point and time them:

difference in % = (bars passed ÷ seconds) ÷ (2 × mains Hz) × 100

For example, on 60 Hz mains, 2 × 60 is 120. If the 33⅓ ring creeps forward one bar every 5 seconds, that is 0.2 ÷ 120 × 100, or about 0.17% fast, which is about 2.9 cents of pitch. On a ring marked "still at", the difference is measured from that speed and not from the nominal one.

The bar counts and the arithmetic

A mains-powered lamp flickers on both halves of the AC cycle, so twice per mains cycle. The bars look still when one bar spacing passes between flickers:

bars = 2 × mains Hz × 60 ÷ RPM

Three of the six do not divide evenly, and a ring cannot have a fraction of a bar, so each uses the nearest whole number. The ring is labeled with the speed at which it really stands still. A platter at exactly the nominal speed shows this:

• 45 RPM on 50 Hz: the bars creep backward one bar every 4 seconds.
• 78 RPM on 60 Hz: the bars creep backward one bar every 2.5 seconds.
• 78 RPM on 50 Hz: the bars creep forward one bar every 10 seconds.

The 78.26 and 77.92 figures are the ones record collectors have long quoted for 78 on 60 Hz and 50 Hz mains. They have argued about why. If your deck is set up for a plain 78.00, use the creep rates above.

Lamps that will not work

This is a common reason a printed disc fails. The bars only stand still under light that really flickers, and LED and many other modern lamps may not flicker at the mains frequency at all, because their drivers smooth the current. iFixit's turntable speed guide says most LED lights will not work. A battery flashlight does not flicker with the mains either, and daylight drowns out whatever flicker there is.

If a ring stays an even gray and never shows bars, suspect the lamp first. An audioXpress article on stroboscopes reports that neon lamps pulse cleanly on AC, and so do white LEDs fed directly from a low-voltage AC transformer, as in the author's own lamp, while filament and fluorescent lamps give fuzzy edges. That is not the same as an ordinary LED bulb with its own driver. A filament lamp flickers only faintly, so expect low contrast in a dim room. Do not build anything that runs at mains voltage unless you are qualified to.

Some cheap LED lamps flicker once per mains cycle, not twice. The rings still stand still at the same speeds, because the bars then move exactly two spacings between flickers. The formula above still uses 2 × mains Hz for them.

What a disc cannot tell you

• A disc checks only average speed, not wow and flutter. A pattern that swings back and forth may come from wavering speed, an off-center hole or an oval print, and a disc cannot reliably tell them apart. If a record sounds unsteady but its average speed checks out, you need a test record and a wow-and-flutter meter.
• The reference is your mains. Mains frequency wanders and only averages to its nominal value over time, so a small drift seen over a few seconds may not belong to the deck. A deck with a synchronous AC motor follows the same mains as the lamp, so a change in the mains moves both together.
• The large disc runs with the stylus up. The label-size disc lets a record play, so it includes the drag of the stylus in the groove.
• It has not been compared with a reference. I checked the bar counts by calculation, and checked PDFs from Chrome's print engine by counting the bars and measuring the rulers with a script. I have not compared a printed disc on a real deck against a reference tachometer. Your deck's manual is the place to look for its adjustment procedure and speed tolerance.

Other free discs

This is not the only one. Instructables has a 90 mm A4 PDF with both mains frequencies and the same bar counts, and Tindie's turntable strobe page links separate 50 and 60 Hz PDFs. Use whichever prints best for you.

No flickering lamp?

The black block at the rim of the disc is a single marker, and a single marker needs no special lamp. For the stopwatch method you only count 100 turns of it.

Stillspin's free strobe uses the same single marker with the iPhone's rear torch: choose 33⅓, 45 or 78 to set the flash target, time the drift, and the app works out the difference. It does not read the rings on this disc. Its flash comes from the target you choose and not from the mains, and its marks-per-turn setting stops at 64, below every bar count here. No Stillspin reading has been compared against a reference tachometer, and the target is a setting, not a measured platter speed.

Stillspin is free to download for iPhone with iOS 17 or later. The torch strobe, turntable targets and drift timing are free; a one-time unlock adds the live camera turntable screen, the optical tachometer, sound RPM and Compare. There is no subscription. It does not measure wow and flutter either.