Tape deck / turntable adjustable motors - how do they work? - Page 1
Forum section: electronics / general

author: Zbig
author_rank: Frequent Contributor
timestamp: May 28, 2022, 10:48:05 am
content: You see those integrated brushed DC motors with a hole or two in them for speed adjustment in countless cassette tape decks or entry-to-midrange record players. If you've seen at least a couple of VWestlife's or Techmoan's YouTube videos, you know what I'm talking about. Yet I have failed to find out how do they work internally, speed regulation specifically. So, you have that hole (or two) to poke your screwdriver through and you can adjust the speed this way. But how's the speed control actually implemented? Is it just a crude, open loop speed adjustment by means of varying the voltage going to the motor from a linear regulator? Or maybe some rudimentary closed-loop control? If that's the case, how's the timebase generated (simple RC circuit? Crystal oscillator?) and speed feedback realised (optical encoder? Hall effect sensor? Other?)

author: PKTKS
author_rank: Super Contributor
timestamp: May 28, 2022, 10:56:25 am
content: Vast majority K7 motors capstan drives i ve seen operate on DIRECT CONSTANT CURRENT DRIVE..Some very vintage turntables belt drive as well and some direct driven can use optical/hall sensors with leds to speed accuracy..Paul

author: NiHaoMike
author_rank: Super Contributor
timestamp: May 28, 2022, 07:22:23 pm
content: I remember taking apart a cassette player motor a long time ago and it had a centrifugal switch to regulate the speed. There was a fairly massive flywheel to smooth out the speed variation it would cause.

author: Benta
author_rank: Super Contributor
timestamp: May 28, 2022, 08:08:26 pm
content: @ZBig, I've absolutely no idea what you're talking about.I've never heard of VWestlife or Techmoan before, and your description of the motors is flaky.Please be more specific.

author: Zoli
author_rank: Frequent Contributor
timestamp: May 28, 2022, 10:52:05 pm
content: Quote from: PKTKS on May 28, 2022, 10:56:25 amActually, the typical cassette player motor control circuit has a limited NEGATIVE internal resistance(the current raises->the voltage raises - until the power supply allows) to compensate for the different loads(start and end of tape) to keep the rotational speed as constant as possible.

author: james_s
author_rank: Super Contributor
timestamp: May 28, 2022, 11:16:43 pm
content: These motors have a circuit board inside the end cap that has a potentiometer on it and that's what you're adjusting to adjust the speed. I've seen motors in older floppy drives that have a tachometer generator on them for closed loop speed control but I don't know whether cassette motors use this approach or just regulate the voltage or current. I've never reverse engineered one but I do know they have an electronic control built in.

author: Zoli
author_rank: Frequent Contributor
timestamp: May 29, 2022, 12:18:22 am
content: Quote from: james_s on May 28, 2022, 11:16:43 pmThe trimmer potentiometer has nothing to do with the characteristics of the circuit; it only sets the initial voltage. The beauty of the negative resistance approach is the lack of any(optical, inductive, whateva) feedback from the motor speed; however, it has only limited use(lack of precision).

author: Zero999
author_rank: Super Contributor
timestamp: May 29, 2022, 12:51:34 pm
content: Constant current drive would result in constant torque, with no speed control. The motor would continue to accelerate, until the maximum compliance voltage of the current source is reached.DC motors generate a back EMF proportional to the speed. The winding resistance appears in series with this voltage source. The speed is regulated by designing a voltage source, with a negative output resistance of the opposite value to the winding resistance. An op-amp can be used to achieve this, but there were ICs specifically designed for this application. Here's an article describing this linked below:https://www.precisionmicrodrives.com/ab-026Am application note from BB, now TI.https://www.ti.com/lit/an/sboa043/sboa043.pdf

author: tooki
author_rank: Super Contributor
timestamp: May 29, 2022, 01:35:46 pm
content: Quote from: Benta on May 28, 2022, 08:08:26 pmActually, his description is exactly right. If you’ve seen the insides of a couple of cassette decks, then you know exactly what he’s referring to, but even if you haven’t, it’s actually pretty clear: on some motors (the motor itself, not the control circuitry!) there’s a hole where you can insert a small screwdriver to turn a small adjustment control inside to trim the speed up or down. Looks similar to the compensation control on an oscilloscope probe.Techmoan is hands-down the authoritative channel on obsolete and obscure AV media formats, an absolute gem of a channel. VWestlife does a lot of repairs of old stuff, also a good channel.P.S. Flaky means “undependable” (as in a person who may or may not do something that they promised). It does not mean “imprecise” or “vague”.

author: Benta
author_rank: Super Contributor
timestamp: May 29, 2022, 09:24:24 pm
content: Quote from: tooki on May 29, 2022, 01:35:46 pm@tooki, Thank You for correcting my imperfect language. Swiss precision rules!No, I haven't played with or repaired casette tape decks, my preferred tape player was a Revox A77.But the "negative impedance" viewpoint is interesting, I never thought about it like that.In the late 80s, I did a design for "constant-RPM-regardless-of-load" which is the same thing. The idea was to increase motor voltage when load current increased (current measurement simply with a series resistor). It worked extremely well.Target application was dental drills, specifically the KaVo MicroMotors (extremely impressive small motors with a lot of torque in a very small package).Done with a TL494, BTW. The two error amps were very helpful.

author: PKTKS
author_rank: Super Contributor
timestamp: May 30, 2022, 08:21:16 am
content: For the record..Capstan belt drivers run at fixed speedThe capstan mass is order mag biggerDedicated ics keep capstan belt at proper speed regardless tape usedNot true for VCRsPaul

author: tooki
author_rank: Super Contributor
timestamp: May 30, 2022, 03:09:07 pm
content: Quote from: Benta on May 29, 2022, 09:24:24 pmAmerican former technical writer/translator, to be precise! :p (But I agree, Swiss (and German) precision does rule!)Quote from: Benta on May 29, 2022, 09:24:24 pmGreat sound, no doubt. A bit bulky for pocket use, though!

author: tooki
author_rank: Super Contributor
timestamp: May 30, 2022, 03:14:53 pm
content: Quote from: PKTKS on May 30, 2022, 08:21:16 amNot in all cassette decks. Many use motors with integrated speed control. Others use external control. Only the best used quartz PLLs and whatnot to maintain precision speeds. Either way, the mass of the capstan flywheel serves to stabilize it.The tape used doesn’t have any effect on capstan speed, since the spool speeds are independent from the capstan speed. The capstan’s rotarion provides a stablelinearspeed at the point where the tape is pinched between the capstan and pinch roller.

author: Zero999
author_rank: Super Contributor
timestamp: May 30, 2022, 03:19:09 pm
content: Quote from: tooki on May 30, 2022, 03:14:53 pmHere's the data sheet for the type of IC used inside one of those motors.https://industrial.panasonic.com/content/data/SC/ds/ds4/AN6652_E_discon.pdfIn cassette players the motor needed to be reversed and run at different speeds, so an external controller IC was more commonly used.https://industrial.panasonic.com/content/data/SC/ds/ds4/AN6657__E_discon.pdf

author: tooki
author_rank: Super Contributor
timestamp: May 31, 2022, 08:24:52 am
content: Depends on the deck, but generally speaking, the capstan motor never changes direction. On cassette mechanisms without auto-reverse, there’s no need either way. And in auto-reverse mechanisms, generallybothcapstans are always turning (in opposite directions), with only one pinch roller being depressed.On the other hand, the reel motor is often controlled in wild and wacky ways (but its speed does not need to be controlled accurately at all). In most logic controlled cassette mechanisms, those motors not only wind the takeup reel at multiple speeds (for playback/recording and FF/rewind, often at multiple speeds to be gentle on the end of the tape, plus an extremely gentle low speed just to take up slack to prevent tangles before ejecting), but use pulsed high speeds in both directions to use centrifugal force to change directions, operate cams, etc. that (i.e. engage things that don’t at lower speeds), but also to overcome the friction in clutches and slip rings (i.e. to escape things that engage at lower speeds). This drives all the movements of the heads (including rotating the heads on auto-reverse units) and pinch rollers, reversing the reel direction, etc. (High-end decks use more motors and solenoids to make them faster.)In Walkman type portable players, they often use one single motor to do all of this. It’s absolutely fascinating that one motor can literally do it all. The downside is that it makes them rather slow, and you hear the various clicky sounds as they sequence the motions to reverse the playback direction at the end of the tape, or go from FF/rewind to playback, for example. (Attached is the relevant page from the service manual of my Sony WM-EX508 Walkman, which I’ve owned since around 1997, which vaguely explains how the mechanism works.)