Rabco SL-8E SL-8: Tangential Tonearm, Servo Control, Parallel Tracking, Functioning, Drawings, Construction, Manual.

See for the most complete manuals for SL8 and SL-8E at the end of this page.

EDISON The first tangential tone arm was devised by Thomas Alva Edison and could be found on the luxurious Edison Triumph Phonograph from 1877. Next to the cylinder with the membrane connected to the horn which read the sound groove, a second "cylinder" also with a spiral groove helps the pick up system to follow the groove of the music cylinder. When Emil Berliner proposed the "Grammophon", the technique of parallel tracking was abandoned overnight. Now the radial tonearm was the rule until several manufacturers (re-)discovered the benefits of parallel tracking and started to design arms which would open up the full potential of the Lp record, especially of the stereo Lp.
EXPENSIVE The only advertisement about the SL-8 that I found was a very small one in a Dutch magazine from 1969. The ad stated the benefits of the design and showed the price: DFL 750 which was a lot of money at the time. The arm that I bought had been ill treated and needed readjustment. So I took a closer look, made some pictures and drawings to explain the functioning and what to do if the tracking is not tangent and the servo and lift do not work correctly. The rest of the adjustment, disassembling, repair and reassembling can be found in the extensively illustrated manuals (see at the end of this page).
CUTTING LATHE But first some notes on tangential tracking. The playback of a record should mimic the cutting of the lacquer as closely as possible. During the cutting process the cutter diamond makes an angle of 90 degrees at all instances from the beginning of the groove to its end. Naturally the best way to read the groove is by using an arm which follows the groove in a similar fashion.
DIFFERENT ANGLE If a common, pivotal arm is used, only in two instances there is zero tracking error. The tracking error results in distortion because of the time difference there are actually two readings from the groove, especially when elliptical, shibata and other fine line tips are used. The human ear is very sensitive to irregularities in the time domain of any signal, be it digital or analog. A tangential arm has the least distortion because the tracking error is minimized to practically zero: the diamond tip of the cartridge reads the two groove walls at the same time without the delay. And there is another important factor: there is no centripetal action The tangential arm does not need side thrust (bias compensation). The drawing shows an exaggeration of the mistracking of a radial arm versus the action of the cutter diamond and the reading of the groove by the diamond tip in a tangential arm.
MARANTZ An early and nice specimen of a turntable with an integrated tangential arm was the Marantz SLT-12 (when Marantz was still Marantz). It was a 2-speed turntable with a lightweight cartridge without counterweight. It was also available with a heavier arm with balance weight and universal mount for playing with other (heavier) cartridges up to 10 gr. of weight. This edited picture is originally from an advertisement in High Fidelity of 1966, but the Marantz SLT-12 was already introduced in 1963.
HARMAN KARDON Rabco came up with a separate arm: the SL-8 which was superseded by the SL-8E around 1969. When Rabco had been bought by the Harman Group, various turntables with tangential arms were introduced: ST-4, 5, 6, 7 and 8. When the ST-7 was hitting the market, an advertisement explained the functioning of the arm, showing a cutting lathe on one page and a ST-7 tangential turntable on the following page. In the Harman Kardon turntables the arm was transported with help of the turntable motor. The arm in the ST-7, and also of the ST-8, was following a rubber belt instead of the bead chain as in the SL-8 and SL-8E.	Harman Kardon ST7

LENCO SWEEPER Designing and manufacturing a tangential arm is not as simple as may seem and only thorough engineering can result in a good functioning device. Proof is the only tangential arm Lenco could come up with and then even the geometry is not right. The cleaning brush does not follow the groove correctly because it is incorrectly positioned. Nothing to please Inspector Gadget.	Lenco's Tangential Sweeper
REVOX Revox too developed turntables with tangential arms: B 790, 795, 5 with direct drive motor. But since the arm was very short, it was only suitable for the high compliance moving magnet cartridges which were en vogue in the 1970's: ADC, Stanton, Pickering, Shure. GARRARD The British manufacturer Garrard produced the Zero 100 turntable. Its tonearm was a radial design but the headshell was not fixed. As the arm moved towards the spindle the headshell's angle in relation to the record groove was constantly corrected by a second "armtube" so that the cartridge always tracked the groove in a tangential way. ACOUSTICAL TANGENT ARM At right the prototype of a linear tone arm (designed by the engineers of the Dutch firm Acoustical), named "Tangent". It was first presented to the public in 1963! and was more or less the predecessor of the B&O tangential arms which were developed some ten years later.	Revox B 795 turntable with the ultra short 'Linatrack' arm, hidden in a rectangular housing which had to be "set aside" in order to be able to put on a record and to take it off. *Acoustical "Tangent", predecessor of the Bang & Olufson series.*
BANG & OLUFSEN Bang & Olufsen used tangential tracking since 1974 first in the Beogram 4000 and they refined the technique of parallel tracking each time they came up with a new model. There were many series of integrated turntables like the Beogram 3000 (the successor of the earlier 3000 which was a Thorens TD124), 1102, 4002, 5005, 6002, 8000 and 9000. At right Beogram 4002.
MITSUBISHI In the early nineteen eighties, many manufacturers discovered the linear tracking principle. Not so much for its correct technique, but rather creating a new incentive for music lovers to buy yet another turntable. Technics designed the SL 10, SL5 and SL3. Pioneer marketed the PL-L800 and PL-L1000 models. Sony built a top model in the PS Series. JVC also joint with stylish designs. Mitsubishi manufactured several models: LT 20, LT 30, etc., LT meaning Linear Tracking. The LT 5V was one of the most spotted linear tracking Mitsubishis. It is a belt drive turntable. It has an optical sensor which measures the diameter of the record. It has two speeds: 33 and 45 RPM. The speeds can be adjusted individually. BALANCED PLATTER The fun of the machine is that it is a vertical record player (V=vertical) breaking with the rule that a record should be placed securely and evenly on the turntable mat. To keep the record in place on the thick LT 5V turntable mat, it is held in the center by means of a light clamp at the end of a horizontal bar. The platter itself is balanced by a heavy disc on the other end of the spindle (shaft) which is hidden with the electronics and motors, the same as in modern washing machines. The belt runs around this cast disc, the periphery and diameter of which are precisely machined. THREE BELTS AND ONE THREAD The arm works perfectly well. It is moved by a small motor via a strong thread and a worm wheel. When the arm leaves the 90 degrees position the motor is activated by a micro switch. There is a second motor for the operation of the arm lift. This turntable has three belts and one thread. VERTICAL POSITION It is regrettable that this turntable needs to be operated in the vertical position. That is the only position in which the arm works. This construction has the advantage that there is only one rail on which the arm carriage moves. The down force functions on the basis of this vertical position. Near the end of the arm there is a round weight with a calibrated scale. By turning the weight (knob) the weight protrudes more or less. The more it sticks out, the higher the down force is. This uncommon design does not take fully advantage of the high end principle of linear tracking.	The clever engineering of the belt drive Mitsubishi with tangential arm precisely driven by a strong thread.
PIONEER To give a new boost to the sales of turntables, knowing that the Compact Disc would soon be introduced, many manufacturers came up with newly designed tangential turntable. In 1980 Pioneer presented a very clever tangential arm system for their PL-L-1000. The arm is transported electronically by the interaction of magnets and a long coil: electro-magnetical suspension. Exactly like the Maglev train (Magnetschwebebahn), a technique initiated by the German engineer Hermann Kemper in 1922.
TECHNICS Also Matsushita launched a series of tangential turntables: SL-5, 6, 7 and 10. In these turntables the arm was incoprorated in the lid. At right the SL-10 which was introduced in 1980.
GOLDMUND Goldmund developed their belt driven servo arms T3 and T5 more or less akin to the Rabco SL-8 and SL-8E and they certainly were inspired by the technique of sensors developed by Bang & Olufson. Like B&O, Goldmund does not use micro switches as Rabco does, but applies detection by a photocell. In the Goldmund T3 and T5 arms the measurement is done at the position of the cartridge and is translated into the movement of the carriage. The slightest deviation is measured at both sides of the cartridge. At left the motor for the linear movement of the arm can be seen. Just behind the arm there is the motor for lifting and lowering the arm assembly. Today there are many designers of tangential (=parallel tracking) tonearms which can be fitted on a variety of turntables.	Goldmund T3F Automatic Tonearm with 2 separate motors for lift and transport. A second arm which is fixed at the base of the carriage, at an angle of 90 degrees, has a photo electric cell at its end which senses the slightest deviation.
In the DIY department there is Poul Ladegaard's air bearing arm which keeps inspiring many a hobbyist to build a tangential arm. In the audiophile section there are the arms of Clearaudio, Forsell, Rockport and Airtangent. MUSIC MAKER CONDUCTOR The newest high end product is the 'Conductor' tonearm, designed by Len Gregory, 'The Cartridgeman'. This air bearing pick up arm uses a simple high flow, low pressure aquarium based air pump system.
It is marketed by HIAudio in the UK. On the web site it says: 'Forget the issues of cartridges mounted on carriages on wheels, or high pressure air pumps through ceramic bearing housings'. It is amazing in this age, in which the digital format is constantly reinvented, that the sound reproduction of analog recordings is brought to an even higher level.	The Cartridge Man's Conductor Tonearm.
HIGH END / HIGH PRICE Prices of the high-end tangential arms are in most cases far beyond the budget of most audiophiles and music lovers. But if you have read this entire page and discovered the technology of the Rabco SL-8 and SL-8E, you might as well try to find one of those vintage arms and restore and optimize it. Or modify it by exchanging the chain for a belt for the transport of the carriage and make the arm wand lighter. Or... you may check out the expensive, but beautifully working high end arms, if you can afford one of them that is.	HiAudio's Martin Bastin-Garrard 401 The Cartridgeman's Conductor arm and Music Maker III cartridge.

The most important aspect of a tangential arm is put in this question: How does the arm follow the groove? The Rabco SL-8E has two "sensors" (micro switches). One for activating the motor for lifting and lowering the arm, and another for activating the motor for the lateral movement of the arm. The motors of the SL-8E are small and efficient. In the "up" position of the arm the carriage can be moved freely. Only in the down position the carriage will grip firmly onto the bead chain. The SL-8E is servo controlled. When the carriage motor slowly turns, the chain transports the carriage which holds the arm. If the arm is well calibrated the maximum tracking error is 1/6 of one degree.

TWO MOTORS The SL-8E has two motors. A cast aluminum housing hides the carriage motor and the 1.5 Volt battery. The second motor is the carriage motor. This is a stepping motor which can turn just a fraction of a degree. The image of the rear shows the RCA female connectors which of course can be replaced with up to date items from Cardas or WBT or any other high quality brand. The signal wires are bound together with the electrical wire and do not obstruct the transport of the arm nor is there any influence of the electric lead on the signal wires. Arm wire and signal wire of course can be replaced by Cardas phono wire, although I did not do that (yet).
LIFT MOTOR The ingenious lift motor only comes into action when the yellow lift/lower switch is pressed down (for a second or two) and automatically at the end of the record.
SERVO MOTOR This drawing shows the principle of the servo system which works with micro switches. The servo motor is activated by the movement of the arm. When the arm follows the groove, the Vertical Contact Pin A (black spot at the end of the side bar) moves also and will eventually touch the Carriage Motor Contact Strip B. The carriage motor comes into action so that the angle between arm and groove is 90 degrees again. At that moment A ceases to make contact with B. Every time when there is a slight deviation A will come into contact with B and the carriage will move until the position is corrected (A does not touch B any longer). Contact Strips B and C are flexible. At the end of the record the movement of the arm is sudden and generally over a much larger distance (wider angle). The result is that Vertical Contact Pin A not only touches B but also touches the Lift Motor Contact Strip C. Now the lift motor is activated and the arm is lifted and stays in the upward position. No deviation can be detected any longer because the arm rests in the middle of the diabolo shaped Arm Fixing Knob and is in the 90 degree position. If the carriage motor still continues turning (without moving the arm of course because it is in the up-position) it means that 1) the arm is not parallel to the sides of the chrome carriage, 2) Contact Pin A was adjusted with a too narrow tolerance and constant contact is made between A and B..
MAXIMUM ERROR The maximum lateral tracking error is dependent on the distance between Pin A and Strip B. The least error is achieved by keeping the distance between A and B as small as possible. If during play the arm shows a deviation before its position is corrected, it will be necessary to adjust the Vertical Contact Pin A and check if Contact Strip B is in place or has a bend. The correction of Pin A is not done by turning the nut which is firmly attached to the metal. Trying to turn it does easily bent the soft metal structure to which it is attached. The service manual and trouble shooting manual indicate that by turning an allen type set screw which should be turned either clockwise or counter clockwise. The allen screw uses a .050 wrench. In my case a very small adjustment was necessary of about 1 mm. Care has to be taken because the pin could get loose. So it is certainly not advised to adjust Pin A nor is it advised to change the position of the fragile Contact Strips which probably can bent or can break if roughly manipulated. Consult the manuals for the proper alignment and adjustment in order to attain minimal tracking error. The materials of Strips and Contact Pin are well chosen and provide full electrical contact even after more than 30 years. They are located in a compartment of the carriage and are well protected. The round hole gives enough room for movement of the bar with the Vertical Contact Pin. The bar has a pivot and there is a spring attached to it which draws the bar against a bolt.
SWITCHES By using a magnifying glass you can check if the strips are intact and if the contact pin is positioned correctly. Adjusting strips and pin will only be necessary if a former owner did change their positions or if the arm was ill-treated. If you buy a Rabco SL-8 or SL-8E, take a magnifying glass with you and check if pin and strips have not been manipulated or damaged. Distances between A and B and between B and C should be about 1 mm. If the arm is lifted during play, it means that the distance between A and C is too small. This means that also this distance has to be adjusted. The chrome part of the carriage is the arm lift. In the up position the carriage can be moved freely, but the arm is held firmly by the lift assembly. In the down position the carriage grips to the chain and the arm is free from the lift assembly and can move freely in both the horizontal and the vertical plane.
These drawings show the cleverly designed device for adjusting the maximum deviation of the arm. By turning the bolt the distances between Pin A and the Strips B and C are increased or made smaller. The bolt has been set by the factory at the time of assembling and it is not advised to try to adjust the bolt, unless the former owner did turn the bolt. The bar can move independently but is held in place against the end of the bolt by a spring. The carriage motor comes into action at intervals that vary in relation to the way the record was cut. Loud passages are mostly cut with more land in between the actual groove and the motor is activated every 2 or 3 seconds. The intervals can be 6 seconds or more if there is less land which is mostly the case if the modulations have low dynamics. (I plan to insert a small light bulb in the wire connecting the carriage motor that will light up every time the carriage is moved.) If the carriage motor keeps on turning after the arm has been lifted and rests on the diabolo shaped knob, this of course means that Pin A still touches Strip B. If you have adjusted the arm for zero tracking error then the diabolo shaped knob could need a final adjustment. In the drawing at right you see that the thickness of the washer determines the position of the arm when resting. By placing a thinner washer in between the knob and the arm, the distance (Y) becomes shorter (=X) so that Pin A does not touch Strip B any longer. One criticism of the Rabco arm is that the effective arm mass is relatively high and not suitable for high compliance cartridges. The mass can be reduced by making that part of the arm on which the cartridge is mounted, out of balsa wood. The use of balsa would makes it also possible to increase the effective length of the arm in case 16" records should be played.
OPTIMUM LENGTH OF ARM There is however an optimum length of the Rabco arm. Making the arm longer postpones the moment of correction. The error increases with the length of the arm. This means that the distances between A and B and A and C have to be narrowed. The result should be that the angle is corrected in time. If this does not work well the mechanism with the micro switches should be placed away from the rail, quite a distance towards the spindle, on a bar next to the arm. But this option is for the knowledgeable hobbyist. It is possible that the gliding of the carriage (with its 4 wheels) on the aluminum rails (profile) is not going smoothly. It is also possible that the chain has come too loose or that the wheel at the right and the wheel on top of the motor at left need cleaning, Maybe the chain needs a good clean as well. You can clean all by using cotton wetted with Kontakt Spray and degrease them with blue spirited alcohol. It is not difficult to take of the motor with the battery compartment and then clean whatever needs cleaning and assemble chain and motor again. See to it that after assembling the chain is not hanging, but straight. The chain should also not be too tight. But if the mechanism has not been manipulated by someone before, the chain will be straight and well moving when the motor is activated.
Taking full benefit of the tangential tracking needs perfect adjustment of the arm. The manual gives the instructions, It is not too difficult to choose the best down force and to have perfect azimuth. However most important is to have minimal tracking error. Only if the sensors (micro switches) are perfectly adjusted, you will hear a perfect image, you will notice that coloration that you attributed to the speaker, the cartridge, the record, etc. has disappeared. The harmonics in the music are real and the sound is clear. Only then you will know that the arm is perfectly working. At right an early example of a Rabco SL-8 mounted on a Thorens TD 150 turntable. The installed cartridge was a Shure V15 Type II. Since the TD 150 had a wooden arm board as it was copied by Linn for their LP12, the Rabco SL-8/8E could still be mounted on a Linn LP 12 today. But the Linn's springs have to be exchanged for sturdier ones. (The two images of the TD150 supplied by Don Coffman)
If all mechanical parts and connections are in order but the arm does not work when the battery is inserted, it could be that the transistor in the electronics of the arm is defect or there could be a bad contact. That would mean interchanging the transistor with a replacement or equivalent. WIRING A disadvantage of the construction of this design is that the signal path consists of several wires and connections. First there is the arm wire that connects to the large round plug fitting the specific socket. Then there are the wires from there to the actual wire that leads to the connectors at the far end of the bridge. And from there the phono cable carries the signal to the phono stage I have been contemplating to bypass this path by using the very thin, single Cardas phono wires which will not obstruct the smooth movement of the arm. Another method is to rewire the arm (it is not easy to connect new wires to the plug) and change the wires that travel from the carriage to the end of the bridge. These options have not yet been tried by me. Maybe you will find a good way to ensure purer signal transmission. EXTENSIVE DOCUMENTATION If you own a Rabco SL-8 and/or SL-8E I can provide * the Template for positioning the arm on the turntable, * the Owner's Manual, * the Design Philosophy, * the Service Manual, * the Troubleshooting Instructions, and * a Review from 1969, I can provide these in neat, well printed copies. The Service Manual contains all descriptions, instructions for disassembling and reassembling the arm, adjusting the arm, repairing the electric/ electronic circuits for the SL-8 and 8E. These brochured documents are printed from carefully scanned manuals, the pages were edited and enhanced in order to achieve the highest quality, readable brochures. 68 pages in total. $30 / Euro 20.00. Click here to send a mail for more info.	Drawings, pictures and edited images on this page Copyright Rudolf A. Bruil

Technics SL1100 with Rabco SL-8E and Universal Record Stabilizing Ring (RSR) plus Universal Stabilizer Weight.

The Rabco came well packed in a large box with the Serial number written on the top.
At right the relatively short but in length sdjustable arm can be seen, the wooden strip (shim) and two triangular wooden parts
to be used when the Rabco SL-8 or 8E will be mounted on a Thorens TD-150 or
TD 125 or later model of which the armboard is too short to accommodate the arm
without being obstructed by the plinth.