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Darren-UK · 19th Oct 2011, 10:06 pm

Turntable speed and the (mains powered) motor

Your record player should run at a speed within +/- 1 or 2RPM (Revolutions Per Minute) of the record speed it's been set to. Any greater deviation can be detected by those with a sharp ear and, indeed, even a 1 RPM deviation can be detected by those determined to do so, depending on the record being played.

Typically, your domestic mains-only record player will be fitted with what's known as a 'Shaded Pole' motor. These motors are, to a degree, susceptible to load variations and the ups and downs of the mains frequency. The latter, though, isn't such a nuisance nowadays as it tended to be at one time - not that it was ever really a major nuisance.

However, with reference again to the importance of correct lubrication/maintenance and cleanliness, your record player should do its job perfectly acceptably and with no real cause for complaint. This means clean and properly lubricated motor bearings, idler wheel bearing and turntable bearing. The turntable should not be fouling on anything (for example, something dropped down between turntable rim and deck plate) and the stylus tracking weight should be correct although a gram or two will make no difference in this context.

If you wish to check the speed for accuracy, you can do so with a 'strobe disc'. This link provides one you can print out and others can be found via a web search. You need to ensure the disc is for use with the mains supply frequency for your country (eg 50Hz in Europe, 60Hz in the USA and Canada) and you also need a mains powered filament light bulb operating at fairly close proximity when you use the strobe disc. In use, fix your eyes on the strobe disc and if the speed is correct the disc will appear to be stationary. Any deviation from the set speed and the disc will appear to move slowly in one direction or the other.

Caution: If you suffer with Epilepsy it might be wise to avoid using a strobe disc. I'm not medically qualified so don't know if any risk exists or not, but I feel it's as well to mention it.

Finally on the subject of speeds, be aware that not all 7" records run at 45RPM (45.1 to be pedantic), some are 33RPM (33.33 to be pedantic again) and a few 16RPM (16.66) examples also exist. Be aware, too, that what you assume to be a 78RPM record might not be; at one time all sorts of odd speeds existed, 74RPM, 80RPM to name just two. The uninitiated can easily be caught out by this, believing incorrectly that a fault exists with the record player.

Moving specifically onto the motor now; as previously mentioned, your record player is most likely to contain a 'shaded pole induction motor' (for convenience, "SPIM"). "Induction" means the motor does not make use of carbon brushes. SPIMs are a type of, but not a literal, synchronous motor. Motor speed (which is constant irrespective of turntable speed) is governed by the frequency of the AC mains supply. For this reason (different voltages aside) motors designed for 50Hz supply are not suitable for use on 60Hz supplies and vice versa. See the next post for just such a situation and the usual solution.

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^ Not the best of images, apologies for that, but here we can see the familiar BSR SPIM as fitted to a later BSR deck (C129 I think it is). The plastic housing (1) contains the field coil, which is nothing more than a hollow bobbin carrying a winding of insulated copper wire. Through the bobbin passes the approximately horseshoe-shaped pole pieces (2). Although this is seen as being one component, making "pieces" something of a misnomer, it is in fact several thin plates rivetted or bonded together. Problems can occur here with plates working loose and causing unwanted high-pitched rattles, almost a buzz, although this is extremely rare with record player motors. It's more common with transformers which use a similar construction method - usually known as "laminations". Should you encounter the problem, a generous application of, say, varnish usually sorts it out.

Upon the poles is mounted a bracket (3). On this bracket is located one of the motor bearings at (4). This image shows the underside of the motor; there is a similar bracket and bearing on the topside but whereas the underside bearing is blind, the topside bearing isn't. This is to allow the rotor shaft to extend through for the fitting of the stepped drum from which the idler wheel is driven. The rotor is just visible behind the bearing (4). Note that this type of motor has a 'rotor' and not an 'armature'.

For readers familiar with synchronous clocks, the layout of the motor above will be familiar as it is similar to motors found in many such clocks, notably Metamec and Ferranti. Smiths clocks, at least until their later years, mainly used a different type wherein the coil (stator) completely surrounded the rotor.

See also post #13 regarding motors.

19th Oct 2011, 10:06 pm	#8
Darren-UK Retired Dormant Member Join Date: Apr 2003 Location: Blackpool, Lancashire, UK. Posts: 4,061	The deck, specifics Part 4. Turntable speed and the (mains powered) motor Your record player should run at a speed within +/- 1 or 2RPM (Revolutions Per Minute) of the record speed it's been set to. Any greater deviation can be detected by those with a sharp ear and, indeed, even a 1 RPM deviation can be detected by those determined to do so, depending on the record being played. Typically, your domestic mains-only record player will be fitted with what's known as a 'Shaded Pole' motor. These motors are, to a degree, susceptible to load variations and the ups and downs of the mains frequency. The latter, though, isn't such a nuisance nowadays as it tended to be at one time - not that it was ever really a major nuisance. However, with reference again to the importance of correct lubrication/maintenance and cleanliness, your record player should do its job perfectly acceptably and with no real cause for complaint. This means clean and properly lubricated motor bearings, idler wheel bearing and turntable bearing. The turntable should not be fouling on anything (for example, something dropped down between turntable rim and deck plate) and the stylus tracking weight should be correct although a gram or two will make no difference in this context. If you wish to check the speed for accuracy, you can do so with a 'strobe disc'. This link provides one you can print out and others can be found via a web search. You need to ensure the disc is for use with the mains supply frequency for your country (eg 50Hz in Europe, 60Hz in the USA and Canada) and you also need a mains powered filament light bulb operating at fairly close proximity when you use the strobe disc. In use, fix your eyes on the strobe disc and if the speed is correct the disc will appear to be stationary. Any deviation from the set speed and the disc will appear to move slowly in one direction or the other. Caution: If you suffer with Epilepsy it might be wise to avoid using a strobe disc. I'm not medically qualified so don't know if any risk exists or not, but I feel it's as well to mention it. Finally on the subject of speeds, be aware that not all 7" records run at 45RPM (45.1 to be pedantic), some are 33RPM (33.33 to be pedantic again) and a few 16RPM (16.66) examples also exist. Be aware, too, that what you assume to be a 78RPM record might not be; at one time all sorts of odd speeds existed, 74RPM, 80RPM to name just two. The uninitiated can easily be caught out by this, believing incorrectly that a fault exists with the record player. Moving specifically onto the motor now; as previously mentioned, your record player is most likely to contain a 'shaded pole induction motor' (for convenience, "SPIM"). "Induction" means the motor does not make use of carbon brushes. SPIMs are a type of, but not a literal, synchronous motor. Motor speed (which is constant irrespective of turntable speed) is governed by the frequency of the AC mains supply. For this reason (different voltages aside) motors designed for 50Hz supply are not suitable for use on 60Hz supplies and vice versa. See the next post for just such a situation and the usual solution. ^ Not the best of images, apologies for that, but here we can see the familiar BSR SPIM as fitted to a later BSR deck (C129 I think it is). The plastic housing (1) contains the field coil, which is nothing more than a hollow bobbin carrying a winding of insulated copper wire. Through the bobbin passes the approximately horseshoe-shaped pole pieces (2). Although this is seen as being one component, making "pieces" something of a misnomer, it is in fact several thin plates rivetted or bonded together. Problems can occur here with plates working loose and causing unwanted high-pitched rattles, almost a buzz, although this is extremely rare with record player motors. It's more common with transformers which use a similar construction method - usually known as "laminations". Should you encounter the problem, a generous application of, say, varnish usually sorts it out. Upon the poles is mounted a bracket (3). On this bracket is located one of the motor bearings at (4). This image shows the underside of the motor; there is a similar bracket and bearing on the topside but whereas the underside bearing is blind, the topside bearing isn't. This is to allow the rotor shaft to extend through for the fitting of the stepped drum from which the idler wheel is driven. The rotor is just visible behind the bearing (4). Note that this type of motor has a 'rotor' and not an 'armature'. For readers familiar with synchronous clocks, the layout of the motor above will be familiar as it is similar to motors found in many such clocks, notably Metamec and Ferranti. Smiths clocks, at least until their later years, mainly used a different type wherein the coil (stator) completely surrounded the rotor. See also post #13 regarding motors.