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I bought a cheap record player, and it is spinning too fast. It is using a 12V 1Ainput with what I believe is a cheap Chinese 12V motor for turning the belt.

There is a pitch control on the turntable, which was a 10k potentiometer. When it's supposed to be spinning at 33 1/3 rpm, I believe it's at 36 rpm. I've swapped the potentiometer for a 100k one and it made a slight difference, but not much.

The record player is the Thomson TT600BT. What is the best way to reduce the speed of the meter closer to the desired speed?

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    \$\begingroup\$ did you try turning it down to 30 rpm so it runs at about 33 1/3? \$\endgroup\$
    – user253751
    Nov 26, 2020 at 13:56
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    \$\begingroup\$ They aren't that cheap! Check that the motor is DC by measuring the voltage across it with a DVM. \$\endgroup\$
    – Andy aka
    Nov 26, 2020 at 13:59
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    \$\begingroup\$ They aren't what I'd call cheap, but they do appear to be crap. The Amazon comments are all negative, and specifically mention the "turns too fast" problem. \$\endgroup\$
    – JRE
    Nov 26, 2020 at 14:19
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    \$\begingroup\$ It cost 170 Euros. If you bought in new, send it back instead of fiddling with it, then use the money to buy a better player. \$\endgroup\$
    – JRE
    Nov 26, 2020 at 15:53
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    \$\begingroup\$ A 10k pot can't control motor speed directly, so it must be connected to some kind of speed controller. Please show us photos of the electronics (pcb, motor etc.). \$\endgroup\$ Nov 26, 2020 at 16:05

1 Answer 1

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[edit 1] Note: as JRE points out in a comment, if the unit is still under warranty, try that route first. If so, see if you can reverse your potentiometer mod and bring it back. Even if there is no warranty, you may want to get an estimate on a proper repair. Then you can decide if you want to customize it.

My rule with taking apart electronics is that I will attempt to repair anything (excluding when there's a risk of fire or electrocution) as long as the owner already considers it a write-off.

[edit 2] I hadn't initially looked up the unit. As JRE and Bruce Abbot have said, it should already have a PWM controller of some sort. It would need to be a ridiculously cheap unit not to. It'd be possible to use a potentiometer as a simple voltage divider but, with the low cost of PWM controllers, that seems unlikely. Even in a rock bottom turntable (which this is not).

You may still need to add a PWM controller, but only if the original one is broken. And that would also require an unmodulated 12V source.

So, assuming return isn't possible and repair isn't reasonable, the value of my answer right now might be more in determining what you have. I couldn't find anything online about the guts of that thing. Do you have access to an oscilloscope?

Even without a scope, there are still some things you can test experimentally.

Is that pot doing anything?

  • Since it's already connected, start with your 100k pot.
  • Crank it to one extreme
  • Put a marker on the platter and Count revolutions over a
    reasonable interval like 3 or 5 minutes
  • Crank the pot to the other extreme and repeat.
  • Depending on your results, you may feel confident to shorten your counting interval.
  • Reconnect the original pot and try again.

When stuff like this fails, it's usually something mechanical. E.g. Ribbon cable pulled out of a socket, cold solder joint, missed solder joint, broken wire.

[Original answer]

PWM for Speed Control

If the motor has only 2 wires, I'd recommend a pulse width modulation (PWM) motor speed controller. Google "12V PWM motor controller" and you should get lots of hits. PWM works by briefly turning off the power for some percent of each cycle. The "pulse width" is the amount that it is on for each cycle. It does this really fast (should be >20kHz) so the individual pulses are not detectable. Fan controllers are configured for 25kHz so they are outside of the range of human hearing.

You'll should find reams of information if you replace "motor" with "fan" in your Google search. People with pumped up computers tend to have a lot of 12V fans and an obsession with controlling them optimally.

If you buy from somewhere like Amazon, they're probably at least a few bucks each. They'd be cheaper from somewhere like AliExpress.

Alternative Wiring Possibilities (3-wire and 4-wire)

If the motor has more than 2 wires, then it likely has a tachometer of some sort. You can still drive it with the same cheap PWM controller but you could get better accuracy with a device to adjust the PWM based on measured speed. You'd likely need an oscilloscope or a meter that reads PWM to figure out exactly what it's doing.

3-Wire

A simplified model of 3-wire is that the third wire emits pulses per rotation (probably 2) then a controller reads that information and adjusts the drive voltage to adjust speed. Reality is a bit more complicated because the power source for the output is the power for the motor/fan. Since the power source is modulated there will be times when a pulse will be missing.

4-Wire

4-Wire is like 3-wire but, instead of controlling the speed by modulating the voltage used to drive the fan, it has an extra wire that gets fed with a control signal. The control signal adjusts the speed and the drive voltage can stay at 12V.

Have fun and let me know what you figure out. :)

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  • \$\begingroup\$ Thanks for this! I'll buy one now and give it a try this weekend. \$\endgroup\$
    – Ollie G
    Nov 26, 2020 at 15:15
  • \$\begingroup\$ It has a speed control of some kind. That control is misbehaving. It is probably already using PWM internally to control the motor. Dinking with the motor will interfere with the existing control. \$\endgroup\$
    – JRE
    Nov 26, 2020 at 15:54
  • \$\begingroup\$ No problem. Last winter, I had a work laptop where the battery would bulge if it got warm and I got a bit carried away with cooling solutions. A baking cooling rack + Lego + glowing 4-wire computer fans and a PWM controller makes a pretty sweet laptop cooler. I'm only driving the fans 2-wire right now, but I have an Arduino kit and a cheap'ish PC Oscilloscope (Hantek 6022BL) waiting for me to have some time. Hopefully during the holidays. \$\endgroup\$
    – Naptha
    Nov 26, 2020 at 16:19
  • \$\begingroup\$ @JRE, I agree with that. I'll amend my answer. If something is valuable and in warranty, try that route first. \$\endgroup\$
    – Naptha
    Nov 26, 2020 at 16:22
  • \$\begingroup\$ !Here is a picture of the board, with poorly drawn labels from me The green circle goes to the motor. 1. Goes to the Start/Stop button 2/3. Go to the 33/45 buttons 4. Goes to the Potentiometer 5. Goes to the Bluetooth/light up thing 6. Goes to an LED somewhere I think 7. Goes to the needle \$\endgroup\$
    – Ollie G
    Nov 29, 2020 at 13:03

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