Air bearing tangential arms do have a completely different "connection" to the turntable (i.c. earth). There is a thin air cushion between the arm wand and its support and the base from which the air flows. Such an arm does not act as a pivotal arm like the Rabco does. By making the arm wand of any tangential arm low in mass, the fundamental resonance of the arm and cartridge combination (system) will be relatively high, probably above the preferred band of 8 to 12 Hz. Does not matter if it is a air bearing design or another construction like the Revox and Beogram. The Rabco engineers were well aware of that and gave the arm a rather high mass of 24 gr. In radial arms such a high mass would be questionable for most cartridges. Also in case of the Rabco it is imperative to find the best compatible cartridge. Synergy is the key word. That is why later lighter arm wands were conceived by technicians and hobbyists who wanted to track their records with high compliance cartridges. The reviews said that with the Rabco SL-8 and 8E it is possible to track with a down force much lower than is indicated by the manufacturer of the cartridge. The Rabco was constructed in the nineteen sixties and the first reviews were from around 1968. It was introduced on the market when it was common to mount Shure V15 II MM and Ortofon SL 15 cartridges.

The bead chain and the carriage on four nylon wheels make all the difference in this design if compared to other tangential arms. By this method of transportation the arm is "grounded" in the sense the designers of Goldmund taught us for amplifiers and loudspeaker cabinets. The design of the Rabco comes therefore close to the stability and "firmness" of the pivotal tone arm design. A rubber belt for the transportation of a tangential arm wand will come second to the bead chain of the Rabco.

The motors of the SL-8E are small and efficient. In the arm's "up" position the carriage can be moved freely from left to right, or to and fro depending on your position. Only in the arm's down position the carriage will grip firmly onto the bead chain and then the arm acts as a pivotal arm. That is an essential feature of the Rabco which many tangential arms do not have. When the carriage motor slowly turns, the chain transports the carriage which holds the gimbal suspension of the arm firmly. If the arm is well calibrated the maximum tracking error is 1/6 of one degree.

The most important aspect of a tangential (straight line) arm is put in this question: How does the arm follow the groove? The SL-8 and SL-8E are servo controlled. Therefor the Rabco SL-8E and SL-8 have two "sensors" (micro switches). If the first switch is on, the motor for the lateral movement of the arm is activated. If the second one is on, the motor for lifting the arm at the end of the record is activated. This will be explained later.

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.

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 arm has no servo system. The arm base rests on a thin cushion of air and this does not generate friction. The arm just moves by itself when following the groove.

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.

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.

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.

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. It is an uncommon design that after so many years needs some tweaking. See Mitsubishi LT-5V .

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.

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.

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.

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.

In pick up arms the pivot is designed in such a way that there is the least play and the least friction (1). The problem with the Garrard ZERO is that it has three more pivots (2, 3, and 4). These extra pivots should not add friction. Therefore they are adjusted in such a way that they have free play.

The arm had its flaws. The solid mounting of the cartridge in the head shell was not possible as there are too many pivots allowing the wand and head shell to adept during play. At these points resonances occur, already at relatively low frequencies. The result is that the harmonics of these resonances are generated and do influence the frequency characteristic. In short: there is increased distortion.

The arm of the ZERO 100 worked along the same mechanical principle as the earlier arm designed by BJ (Burne & Jones). The ZERO 100 was introduced in 1971 and eventually came in various versions: C, 92, SB, Z.

GARARD ZERO 100 SB The British manufacturer Garrard produced the Zero 100 turntable. Its tonearm was a radial design but the headshell was not fixed. While the arm moved towards the spindle the headshell's angle in relation to the record groove was constantly corrected by a second "arm tube" so that the cartridge always tracked the groove with the least tracking error.

ADVERTISEMENT When the ST-7 (shown at right) 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 opposite page. In the Harman Kardon turntables the arm was transported with help of the turntable motor. So it would be difficult to dissect the turntable and mount the arm on a different record player/motor deck. The arm in the ST-7, and also of the ST-8, was transported by means of a rubber belt instead of the bead chain as in the SL-8 and SL-8E.

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 in the 1970s, more models with tangential arms were introduced: ST-5, 6, 7 and 8.

RABCO ST-4 TURNTABLE This turntable is from 1971 and it's a Rabco. The correction of the arm is a very ingenious procedure. The long horizontal shaft turns continuously at a constant speed. The rear of the arm has a rubber idler wheel that is set in motion by this turning shaft. Every time when the arm is forced to move while following the groove of the record, the idler wheel will seek the normal, 90 degrees position.

BURNE - JONES In an advertisement in The Gramophone of May 1957, British manufacturer Burne-Jones of BJ Arms explained the benefits of tangential tracking and the types of arms they manufactured. There was the cheaper Standard Arm and the 90 Super Arm. They also made a protector to measure the tracking accuracy of all tonearms, cartridges and plug in headshells.

ORTHO-SONIC V/4 An early example of a tangential arm is the Ortho-Sonic V/4. The image is from an advertisement in High Fidelity Magazine of September 1956. A quote from the advertisement: "The cartridge is supported on a multiple ball bearing trolley. Lateral friction is virtually illuminated. While the conventional arm, due to the inertia, throws the stylus against the side of the groove, Ortho-Sonic V/4 keeps the stylus in the center of the groove. This adds up to perfect, distortion-free reproduction so that worn records sound as when they were new." This may have been true though we must admit that we have heard rather unfavorable demonstrations of high end tangential arms in the 1980s where the needle tip alternately touched the left and right wall of the groove and it was clearly heard! There were three versions of the Ortho-Sonic: Model #100, Model #200 (Transcription) and Model #300 (Binaural), costing $ 35.95, $ 44.50 and $ 59.50 respectively. The arms were not cheap.

The ad lists all the advantages of linear tracking. The price for the Binaural Tonearm was $ 49.50. The Monaural Arm sold for $ 7 less at $ 42.50. See also Cook Livingston Binaural Recording System

At right you see the ad. The TRULINE is a tangential tonearm that came in two versions, a binaural one and a single (one cartridge) version. It had been on display at the New York Audio Fair. There the COOK LABORATORIES Exhibit showed the Binaural version. And at the ELECTROVOICE exhibit the monaural (single cartridge version) was used to demonstrate ElectroVoice's NEW Model 84 Cartridge.

Today I discovered in a newly acquired edition of High Fidelity Magazine, January 1956 edition, an advertisement by a company named Audio Specialties, 13167 Steel Avenue, Detroit 27, Michigan. They were the manufacturers of the TRULINE Reproducer Arm. It is an advertismeent that I did not come accross in earlier editions nor in later ones. So it is not sure if the product was sold in large enough quantities to establish itself on the market.



TWO MOTORS

The SL-8 and SL-8E have two motors. One for lowering and lifting the arm wand.

The second motor, which ois the carriage motor is hidden in a cast aluminum housing together with the electronics and the 1.5 Volt battery. This motor 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 (shown at left in the photo) are bound together with the electrical wire feeding the lift motor. These wires do not obstruct the movement of the arm nor is there any influence of the electric lead on the signal wires. Arm wire and signal wire of course can also be replaced by a modern OFC 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 at right 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 towards the Carriage Motor Contact Strip B (as I call these for convenience) and will eventually touch that strip and make contact. Now the carriage motor comes into action and moves forward while the cartridge needle remains in the groove. The carriage is moved until 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.

POWER-LIFT

The lead-out groove at the end of the record is cut over a wide area. There the movement of the arm is sudden and generally over a much larger distance. The result is that Vertical Contact Pin A not only touches B but immediately touches the Lift Motor Contact Strip C as well. Now the lift motor is activated and remains 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 and not firm with the bead chain) it means that:

1) the arm is not parallel to the sides of the chrome carriage and the pins still make contact, or

2) contact Pin A was adjusted with a too narrow tolerance and constant contact is made between A and B. This pdf from the vaults of the Library of Congress says, The servo is capable of moving the carriage faster than required for tracking 33, 45 or 78 RPM records, but is slower than that re­quired to follow the run-off groove. In other words, The movement to lift the arm (A-C) has to be significantly larger than following the groove (A-B) of the disc. Only if the distance between the micro switches for correction and that for lifting the arm (B-C) are too close the arm will lift during normal play.

WHAT TO DO FIRST?

What to do first with that Rabco that you found in a second hand store or that you bought on the internet and it is not working properly? If on first glance the arm does not show that it has been modified or tampered with, we may assume that the arm is basically in the same state when it left the factory. That means that the micro switches have the correct distances as originally set in the factory, decades ago. Before you start dismantling the arm and adjusting whatever you think is necessary to make it function allright (if you have the appropriate instruction manuals), first do the following. Put in the C cell battery. Lower the arm by pressing the yellow switch. Take out the battery. Now you can move the arm freely in the vertical and horizontal plane. Take the complete arm assembly off the base and turn it upside down. You can spray the micro switches with the special Tuner Contact Spray (used for the variable condenser) which does not leave a residue. Move the arm wand several times in the horizontal plane. This will "clean" the micro switches. Put the arm assembly on the base and put in the battery. Check if the arm will function as it should. If not, repeat it one more time. In many cases this procedure will help. The micro switches will make contact and the arm will behave as it should. If this does not help, you may want to disassemble the arm. check the functioning of the micro switches, clean all parts at the same time, and adjust the bead chain, motors, etc.... if necessary. But for that you would probably be better off if you had the trouble shooting manual. WHAT TO DO FIRST?

C CELL BATTERY

Sometimes an owner asks me how to make the lift working more effectively. There can be some problem in the electronic circuit which has to be checked. But most of the time the cause can be corrected in a simple manner. The time it takes to make a lateral correction and the lifting speed of the arm lift both depend on the power of the C cell battery. When the motor lift works slow which specifically results in a sloppy behavior at the end of the record, nine out of ten times the battery has lost too much power over time. Replacing it with a fresh one, often is the cure. If you stop playing records, see to it that the top of the battery compartment is moved to the right so the battery's top does not make contact. This will prevent leaking.

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 (see earlier drawing).

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.

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. 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.

THEORETICAL

These drawings show the cleverly designed device for the initial adjustment of 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 at the end of the bolt by a spring.

The carriage motor comes into action at intervals. These 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.)

OPTIMUM LENGTH OF ARM

Making the arm longer postpones the moment of correction. The error increases with the length of the arm. A longer arm than advised would mean 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. In that case the mechanism with the micro switches should be placed away from the rail, closer to the cartridge, that is quite a distance towards the spindle. This can only be done by adding a bar parallel to the arm as Goldmund and B&O did. But such a construction is for the knowledgeable hobbyist only.

MASS OF ARM AND LENGTH OF ARM WAND

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 the adjustable part of the arm on which the cartridge is mounted (the wand), of balsa wood. The use of balsa would make it also possible to increase the effective length of the arm in case 16" records should be played. In that case as maximum distance between spindle and arm 20.5 cm is advised. The least tracking error is obtained with the shortest distance possible: 16.5 cm. Using any distance from 16.5 to 20.5 cm is of course right. See the drawing at right.

UNIQUENESS

The advertisement at right is of the SL-8 published in AUDIO (The Authorative Magazine About High Fidelity), March 1970 edition. It shows that the second version, SL-8E, was not yet on the market. The headline reads: "A Major Breakthrough In Sound Reproduction". The excerpts of reviews show how well the unique design was received. The ad states ACCLAIMED BY EXPERTS. REVIEWS

The reviewer of AUDIO wrote: "Without question, the Rabco arm does what it is supposed to do, and does it nicely. ...minimized record and stylus wear, superb reproduction." The following paragraph is from the review in an issue of High Fidelity Magazine: "Extremely well engineered, unprecedented low tracking force, no skating effects, minimum groove wear. ...ruggedly constructed: after months of continuous use the SL-8 remains as responsive and foolproof as when first installed."

JONAS MILLER SOUND

Various experts have proposed alterations and optimizations of the Rabco SL-8 in order to reduce the mass of the arm, to suppress and avoid resonances in the metal housing, and thus achieved a better reading of the signal. Jonas Miller of M & K (Miller and Kreisel) is known for the skeleton arm, the modification of the counterweight and at the same time addressing the problem of resonances. At right the Jonas Miller Rabco mounted on a Technics by Panasonic SP-10 Direct Drive turntable.

AUDIOETC

Another famous name linked to the Rabco SL-8 is that of Dean A. Slindee of AUDIOETC. The modifications of AUDIOETC addressed the mass of the arm by using redwood, the damping of resonances occurring in all materials and parts, the operation of the arm, the silicone encapsulated carriage bearings, the counterweights.

The modified arm was mounted on a 4 point spring suspended Oracle turntable. The original Rabco cartridge shell weighed 29 gr. The Audioetc just 9 gr. The related mass of the counterweight was reduced from 142 gr. to 45 gr. DAVID C. SHREVE

David C. Shreve offered a service for rebuilding the Rabco SL-8 and SL-8E. His modifications included a balsa wood arm wand, miniature precision instrument bearings, he made several counterweights available so cartridges of different mass could be installed. The many mods he proposed made the arm extremely stable.