If you feel that a variable-speed turntable like a Technics SL-1200GR is a bit beyond your means, Technics made a number of cheap, direct-drive turntables in the early 1980s which are suitable with modification to play shellac era discs at a wide variation of playing speeds.
WARNINGPotentially lethal voltages exist within record turntables. If you do not yet have the proper training in handling high-voltage electronics, please use the information on this webpage for information only. Whilst the author and Pspatial Audio have made every effort to ensure that the information contained here is accurate and complete, any of the information here contained should only be considered to be a general guide and not as the ultimate source of information. The information on this webpage is supplied without warranties of any kind, express or implied, and the author and Pspatial Audio assume no responsibility for any damage whatsoever to persons or property that may result from readers undertaking work on the equipment described here or to similar equipment. |
The circuit modification described here derives from an original article in Popular Electronics from nearly 40 years ago1.
The Technics SL-D1, the D2, D3 and D5 direct-drive turntables are all virtually identical and may all be modified in a similar way. These decks regularly appear on internet auction sites at very reasonable prices.
The ideal choice amongst these decks is the the SL-D1 because it is a manual turntable and this is an advantage when playing peculiar discs like the early discs from the Pathé Frères. These discs are centre-start and end in a locked-groove at the outside edge of the record. With an automatic turntable (D2, D3 and D5), moving the tonearm towards the centre of the record to place the stylus near the centre can trigger the auto-stop mechanism and this frustrates playing these unusual discs.
That said, the Pathé discs are an esoteric interest and the D2, D3 and D5 are all fine for playing the huge majority of shellac discs.
The technical background to the modification is given in the original Popular Electronics article. In a bid to avoid modifications to the plinth, in the original article, Mr. Bintliff purloined the 45 speed position on the deck speed-switch for 78s.
This has a number of disadvantages. Most obviously, the 45 RPM position is lost, which is a pity. Worse, the original article assumes that one 78 speed will be enough for all historic records and this isn't the case. So, the modification described here, rather than simply modifying the Technics speed setting circuitry, adds a further, independent network which may be used to control the speed from about 50 RPM to 110 RPM. The original circuitry is left entirely alone.
A full circuit diagram of the modification is available by clicking on the image (left). The modification involves adding a series combination of: 22kΩ resistor; a 100kΩ dual pot; and a SPST switch between the deck's 21V regulated rail and pin 24 of IC1 as illustrated. Note that the dual-pot is ganged so that the value of the potentiometer ranges from 50kΩ to 0Ω.
Calibration of the control is fun and uses the Technics' stroboscope system which, although calibrated for 33.3 and 45 RPM, may be used in exactly the same way to identify multiples of these speeds: 66.6, 90 and 100 RPM for example. A stroboscopic disc may then be used to calibrate a 78 position on the control-arc. The design is such that 78 RPM is attained around the 12 o'clock position of the control.
It's further possible - using the stroboscopic patterns for the region other than that the deck is operating (i.e. 60Hz patterns in a 50Hz region) - to identify speeds 20% above (or below) these values: for example 1.2 × 66.66 = 79.99 RPM; 1.2 × 2 × 45 = 108 RPM; and so on.
With patience, it's quite possible to produce a variable-speed control with a reasonably comprehensive calibration.
If perfection is sought (or a very precise speed is required), stroboscope discs may be printed for any speed (in 50 or 60 Hz regions) with a freeware program by Jean Vernet from Morges in Switzerland (see image right)2.
The automatic decks (D2, D3 and D5) offer somewhat less scope for the positioning of the speed control and switch because of the increased mechanical complication within the chassis. In the case of the SL-D1, the controls may be placed on the tonearm side of the chassis, near the main start/stop switch, as illustrated in a photograph of a modified unit (left). This is operationally very convenient and - although this involves quite long leads from the control circuitry to the controls - this does not seem to cause any problems or induce any noise into the audio signal from the cartridge.
These little direct-drive turntables from Technics were manufactured with the pinth formed from a one-piece plastic mold. Whilst cost-effective, this sacrificed the inertia of a well-damped, massive plinth like that found in Technics' more expensive models. Otherwise these are very good turntables and may be greatly improved by the simple means of adding mass in the form of car-panel damping-pads or something similar to the underside of the chassis; at least in the places where the free mechanism of the movement isn't compromised.
Their other shortcoming is the presence of the mains-transformer which is mounted on the hollow, plastic chassis in a (rather primitive) vibration mount. The magnetostriction (or physical buzz) of the transformer may thus be coupled into the plinth. Units vary - some are good, some are bad. All of them may be greatly improved by moving this transformer outboard and supplying the turntable with low-voltage AC; the appropriate safety precautions being observed.
Record players made in the 1950s and the 1960s often had four speed settings, 78, 45, 33⅓ and, the rather mysterious and rarely used, 16⅔ RPM. The unusual 16⅔ RPM records appear to have originated as far back as the 1930s but most discs cut at this speed were produced in the 1950s and 60s before disappearing by the early 1970s. The low speed meant that discs doubled the playing time of the more familiar formats so that a ten-inch 16⅔ RPM record would have an average playing time of 45 minutes per side - a remarkable achievement at the time in a world before the Compact Cassette.
The low speed meant that the recordings were of low quality and the format was mostly reserved for spoken-word records like plays and talking books (right); although a few record companies, like Prestige, experimented with the format for extra-long-play music records, featuring artists such as Miles Davis and the Modern Jazz Quartet.
All mono, these discs were cut with a smaller groove dimension (0.0005 - 0.0007") compared with the 1 thou' groove of the, then standard, mono LP. Most modern, stereo stylus shapes are suitable to play these discs, but finding a deck which rotates at 16⅔ RPM can be a real problem.
Happily, the Technics SL-D1, D2, D3 and D5 decks described above may be modified to run slower as well as faster. All that is required is more resistance in the feedback control chain. If the 22kΩ resistor described in the modification above is changed to 240kΩ, then the control range will encompass the range around 16 RPM. Once again, the stroboscope dots on the edge of the turntable may be used to identify the correct speed because 16⅔ is exactly half the speed of 33⅓.
One word of caution. Because the modification adds a parallel feedback branch to the existing Technics' speed-control circuitry, it is possible to select a standard speed (33 or 45RPM) and then further select the added network. The currect-control paths then add in parallel and this can result in very high rotational speeds. This should be avoided by paying careful attention that the original Technics' speed control is always left in the "OFF" position when operating the the variable-speed controls.
1. 78-RPM Records Live Again!, R. Bintliff, Popular Electronics April 1982.
2. Called STROBO.EXE, you can download the executable by right-clicking (or control-clicking) this link. This program runs on Windows but may be run on a Mac using Wine, or Boot Camp or a Virtual Machine in the usual way.
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