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    Mitsubishi LT-5V

hobbyists's views for hobbyists
Rabco SL-8 / SL-8E
Tangential Tonearm

© Rudolf A. Bruil. Page first published January 8, 2004.

Servo Control, Parallel (Linear) Tracking,
Minimal Lateral Tracking Error

Edison - Rek-O-Kut - Ortho-Sonic - -BJ - Marantz - Pierre Clément - Harman Kardon
Garrard - Acoustical - Revox - B&O - Goldmund - Mitsubishi - Cartridge Man - Rabco

© Rudolf A. Bruil. Page first published January 8, 2004.

Jump to:


Linear Tonearms

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


"A Rabco SL8 is a perfect arm... that is... if you happen to have one which works perfectly well."

This is what the sales manager of a Dutch audio import company said to me, many years ago. You may have heard a similar statement. And although the sales manager is a knowledgeable man, I always doubted his criticism concerning the Rabco. If the arm does not work perfectly well, you will have to make it work perfectly well. The sales manager's audio company does not exist any longer. However the Rabco SL- 8 and the SL- 8E (which are in essence the same except for the improved electrical circuit for the SL 8E and the omission of a lift switch button above the motor compartment) are still alive and kicking in the dedicated systems of a select group of die-hards. Maybe it is working on a second turntable. Or just as the perfect match to the main turntable.

If the arm you have or just recently bought is not functioning well this page may give you the incentive to investigate and find out how the arm works and how to adjust it in order to obtain practically zero tracking error. Or you may order the documents explaining trouble shouting, maintenance, philosophy, etc. shown at the end of this page.
Maybe you have a SL-8 or SL-8E which needs adjusting or just needs cleaning. Or you may want to buy one at an auction or from a dealer or private person and start to enjoy the benefits of the tangential arm. Let me warn you: Once you have heard the sonic performance of a good and well adjusted tangential arm, there is a possibility that you will be hooked. I have used the Rabco for several years now on one of my turntables and I am still pleased with its functioning and soundings.
But first a bit of history, and then some alternatives and explanation.


The first tangential tone arm (straight line tonearm) 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 in the LP era and started to design arms which would open up the full potential of the LP record, especially of the stereo LP.

When I started my research in 2003, 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, probably around US$ 163.
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.

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.

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.

The utmost care must be taken to mount the pick up cartridge with precision. The needle tip must be at a 90 degree angle to the line perpendicular to the spindle.

If this is not the case, it makes no sense to use a tangential arm. Although the deviation will be less severe if compared to the result of a radial arm, one should use the tangential principle to the full.

The Rek-O-Kut Company proposed a portable professional disc recorder and playback phonograph for 33 1/3 and 78 rpm disc cutting. An accessory idler wheel could be mounted for cutting at 45 rpm speed. A constant speed, hysteresis motor was the driving force of the table and arm. It had interchangeable leadscrews for standard as well as microgroove recordings. The amplifier measured a frequency response of +/- 1 dB from 30 to 20.000 cycles. It had controls for bass and treble. Recordings could be made from radio tuners, tape recorders, and other signal sources. At right part of an advertisement which appeared in 1953 High Fidelity magazine.

Added on July 23, 2014:

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.

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.

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


Be on a Straight Line with Trueline


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.

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.

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. The arm had no servo system and therefor did not always function as desired. This edited picture on the right is originally from an advertisement in High Fidelity of 1966, but the Marantz SLT-12 was already introduced in 1963.
Very ingenious and quite modern at the time was the tangential arm devised by Pierre Clément from France. He received a patent in 1968 from the French National Institute of Industrial Property: "Procédé et dispositif de lecture d'un disque de phonographe le long d'une trajectoire rectiligne." The use of the light bulb and the photoelectric cell has inspired many a designer.
As soon as the arm moves out of the 90 degrees position, the light beam will no longer be interrupted by the small plate at the end of the arm. Then the motor will receive current and starts to turn the worm on which the carriage rides. The carriage is moved towards the spindle until the light beam is interrupted again and the arm remains at 90 degrees. See The Archéophone website.

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.

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.

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. Watch Watch udio Expo's video about the later Rabco ST Turntables on YouTube.

Harman Kardon ST7

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.

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.

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.

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.



Garrard ZERO 100 SB

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 too developed turntables with tangential arms: B 790, B 791, B 795 and B 291, all with a 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, Goldring.

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.

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.

Acoustical "Tangent", predecessor of the Bang & Olufson series.

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.

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.

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.

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.

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.





The clever engineering of the belt drive Mitsubishi with
tangential arm precisely driven by a strong thread.

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 turntables. 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: electromagnetic suspension. Exactly like the Maglev Train (magnetic levitation) a technique initiated by the German engineer Hermann Kemper in 1922
and first applied for the Magnetschwebebahn.
Also Matsushita launched a series of tangential turntables: SL-5, 6, 7 and 10. In these turntables the arm was incorporated in the lid. At right the SL-10 which was introduced in 1980.

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.


At right is the 'Conductor' tonearm, designed by Len Gregory, alias 'The Cartridge Man'. This air bearing pick up arm uses a simple high flow, low pressure aquarium based air pump system.

The text explaining the benefits of the arm read: "'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 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.

The Cartridge Man's Conductor Tonearm.

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.

Martin Bastin's Garrard 401, The Cartridgeman's Conductor arm and Music Maker III cartridge.

Top of Page

The Rabco SL-8 / SL-8E

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.

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 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.

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 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).
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.
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.

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 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.


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.


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.


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 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.)



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.

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.


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.






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."




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.

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 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.

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 assembly, the distance (Y) becomes shorter (=X) so that Pin A does not touch Strip B any longer. These adjustments are in principle not necessary if the arm is in the condition it was initially sold. But in time the precise functioning may not be so precise any longer. So checking and verifying is necessary.

Rabco Arm Wands

It is not clear whether the SL-8 had a one piece arm as a drawing in a magazine review showed. Could be that the two part design was omitted in the drawing. Whatever. The arm is a two part adjustable arm, the connector being the third element.

The measurements are given for those owners of Rabco arms who want to restore an original arm tube or reconstruct an alternative for modified arm wands while using the original connector.

The measurements are given for those owners of Rabco arms who want to restore an original arm tube or reconstruct an alternative for modified arm wands while using the original connector.

How to go about if you want to make an armwand which has less mass and would better suit the cartridge you use better as far as weight and sound are concerned? There are several suggestions for using balsa wood. But just pure balsa is in my opinion not suitable. It is soft and therefor less firm. My experiments have shown that the combination of aluminum strips and balsa make an arm or armwand stronger and does influence the frequency characteristic and transient response in a positive way. I do not guarantee that the suggestion pictured in the drawing at left gives the results you desire. But the drawing indicates a possibility and shows along what lines you could think. The amount of aluminum (large width or narrow, weight and thickness) determines the effectiveness and the final sound quality and dynamics. So some experimentation should be done.
At left is an image of the arm wand proposed by David Shreve built of two layers of balsa and a magnesium block which holds the nylon threaded bar. - Picture Copyright David Shreve.
This drawing explains the construction of the arm wand proposed by David Shreve. It is made of a thin upper layer and a thicker base of balsa wood. The lift wire has to be adjusted carefully. The original connector is omitted and makes place for a magnesium block with threaded rod of nylon. - Drawing Copyright David Shreve.

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 methylated alcohol. It is not difficult to take of the motor with the battery compartment and then clean whatever needs cleaning and reassemble chain and motor. 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. 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 TD-150 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.

SL-8E on early Panasonic SP-10
The picture at right was send to me by Gerhard Evers from Germany. It shows the earliest SP-10 direct drive turntable from National Panasonic with Rabco SL-8E. This solid combination was bought in 1972 by his father and - so he writes - it is still working very satisfactorily today almost 40 years later. The motor is mounted on a heavy, stone plinth. As you can see the arm wand was changed and has less mass if compared to the actual Rabco arm wand weighing 24 gr. Although the original wand does not obstruct high compliance cartridges, with the light wand high compliance cartridges can be used while maintaining the proper resonance frequency of the arm-cartridge combination for optimum tracking.

With this arm the Vertical Tracking Angle (VTA) cannot be corrected on the fly. On the other hand, a precise adjustment is of course possible by changing the height of the side pillars. Changing the height will also change Azimuth of the cartridge.

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.

By adjusting the height of the side pillars, azimuth and vertical tracking angle can be maximized.

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

© Rudolf A. Bruil. Page first published January 8, 2004.

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 adjustable 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.

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