The Analog Dept.-----------------------------------Littering the web since 2002

-edible zone-

--The Story behind "Notes on the TD124" starts around 2006 or 7. It was near to that time when Art Dudley ( a well known Linnie) published his first TD124 article in the "Listening" column at Stereophile. I wasn't really looking for a TD124 but I got an email from a turntable hunter offering me a fixer-upper for ~$350 or so. (I can't remember exactly what I paid) It arrived not long after and that's how I came into possession of sn#2729. This was my first Thorens TD124. What I found with 2729 was that it was built to receive service. I documented my processes here on the site and that led to...  -- Notes On The TD124

--Original post date: 05/20/2010, revisions made 05/07/2014, Then more revisions in 2022. By now, the new page mission is to expand on the owner and service manuals with examples of the various processes. (please note that new material and edits to the page are still in progress at this time) 

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-- Above: Magazine ad scan from early in the period.

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-- Above: An original neon strobe bulb.  Type NE 48 or Phillips GL 1W

-- What it is: The TD124, it's a 4-speed belt-idler driven turntable produced in the early Stereo era.

  Features:

-- Die-cast aluminum chassis
-- 4-pole 10 watt shaded pole AC induction motor E-50
-- 9lb iron flywheel
-- 1 lb. Light aluminum clutch-able upper platter with mat.
-- Ø 14mm hardened & ground steel bearing shaft. Ball-thrust. Oilite Bushings.
-- Friction pads on flywheel.
-- A simple clutch lever which - when you push it forward - scissor-Lifts the light upper platter off the working flywheel. That stops the record spin. ---- Then, when ready, you pull the lever back your way to scissor-drop it back down into the working flywheel. Instant go. Like a DJ at a radio station would use back then.
-- Operates 50 or 60hz, depending on country.
-- Operates from 100 to 250 VAC, depending on country.
-- Neon lit Strobe
-- ± 3% Variable pitch speed control 

-- The Cast Chassis: Die-cast aluminum by Inca.

-- Casting revisions were made within the first version as noted below.

-- Below: three shots of sn#2078 An early first version. Arm board area. Note square corner where the arm board bed adjoins the chassis. Note also the center web / arm board bed.

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-- Below: sn#27520 shows revisions in two areas of the casting; -bevel at juncture between the arm board bed and chassis, -gusset reinforcement added to center web, 

-- The early version chassis have often been found to be bent in the arm board area as a result of customer handling. No doubt this prompted Thorens to make the above noted revisions to the casting. The good news is that bent arm board areas on early castings can be straightened.  

-- Bearing and Platter System

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-- Above image: A detailed section view of the platter and bearing system

It's a two-piece platter system. On top is the light aluminum clutch-operated platter. There are 6 rubber pucks located in a circular array adhered to the flywheel beneath it. The pucks provide friction to drive the upper platter while the flywheel is in motion. A simple clutch lifts the upper platter up off the flywheel when the operator pushes a lever forward. It drops the platter back down on the working flywheel when the operator pulls the lever back.

-- Below: Control map (re-annotated) from the owner's manual

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  Above: an alternate section view

  The Iron Flywheel: 

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  Above: iron flywheel showing traces of rust. In the beginning stages.

   rusty   clean

-- Above photos: TD124 cast iron flywheel with the bearing disassembled from it. When stored for lengthy times in humid conditions the iron will rust. In the above case the rust wasn't so bad. A spoonful or two of Naval Jelly then some judiciously applied elbow grease removes all the traces. Yet the flywheel is cast iron and iron rusts. The lesson to be learned time and again is don't store these outdoors where they will always rust. Keep them indoors in the home where you live and sealed away from weather.

-- Material: Cast Iron, weight: 9 lbs. (note the light upper platter weighs 1 lb, total wt. platter system is 10 lbs) Another note about the iron flywheels I've seen is that all surfaces of them have been lathe turned and trued. It's original design purpose was to produce a robust 'flywheel effect' with a high moment of inertia at the outer rim, and thusly achieve the wow/flutter readings they did.

-- The fit of the platter over the bearing shaft is very close, very precise...so precise, in fact, that you can remove the three mounting screws and remove the platter from the bearing while it resides within the player. Then replace the platter just as it was, re-tightening the screws just as they were, and there would be no change in concentric run-out between flywheel and bearing. It spins just as true every time. Unfortunately, the same can not be said of the lighter non-magnetic (cast zinc) platter. More about that below.

 -- Above photo: side-by-side underside of the two flywheel types used on the TD124. Left: the cast-iron platter (fully machined and trued on all surfaces). Right: The cast-zinc platter from the TD135. The zinc platter uses an adapter plate to fit the TD124 bearing. Note the difference in strobe patterns. The iron platter offers a strobe pattern for each speed and at either 60 or 50 hz. The zinc platter offers a pattern for 33-1/3rd rpm at either 50 or 60 hz. 

-- The Non-Magnetic Flywheel:

   Cast Zinc  

-- Material: Die-cast Zinc - adapted from the TD135 - part number CB 788 And also produced at the Inca foundry.

This flywheel weighs closer to 5 lbs and will display lathe-turned surfaces only where required for mechanical function. The main benefit of it* is that it does not attract the magnet within moving coil phonograph cartridges. (* as I see it)

The zinc flywheel uses an adaptor plate (spacer) in order to get it to work with the TD124 bearing shaft and in that chassis. It should be noted that when you remove and replace this flywheel from its bearing, it will be needed to align the platter to the bearing shaft when you re-assemble. If you don't you will see significant run-out between platter and bearing - it's quite visual - and there might be an audible wow. My solution is to do what every other machinist would do; get out the dial indicator and align the flywheel to the bearing while it is in the chassis. I'll add some photos of this process anon. Otherwise the zinc platter works fine and the audible difference in performance between that of the iron flywheel and the zinc is not huge. Unless the idea of using a moving-coil cartridge over the iron flywheel triggers significant mental trauma for the owner. (kidding, just kidding, I think) Otherwise, I feel the iron platter is the better sounding of the two. Greater flywheel effect. Slam is more robust.

At some point Thorens management did admit that there were one - or maybe two - cartridges that they could not recommend the use of while playing over the original iron flywheel. According to Joachim Bung in his book "Swiss Precision" 2nd ed. these stand-outs were the Decca FFSS and the Neumann DST. Those two, it was decided, had magnets within them that were just too strong to be used over the iron flywheel. Evidently it was considered ok to use any version of the Ortofon SPU cartridges, and those are known to have very strong magnets within them.

From my own experience I can add that many moving coil type cartridges do have magnets within them that will demonstrate some downward magnetic pull while tracking the record over an iron flywheel. This can be compensated for by adjusting vertical tracking force to accommodate it. I have written more about that further down the page and also have produced an additional page to illustrate one method for making this compensation.

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-- The Power Unit:

    The E50, a 4-pole 10 watt shaded pole AC induction motor. It's behavior has influence on the character of the machine. Sixty years of experience has taught us that this, along with other parts, will need regular maintenance. The good news is that it is built to receive regular maintenance....and you can get the parts you'll need.

 -- The E-50 drives an elastic (rubber) belt that turns a 4-speed magnetically damped stepped pulley. The stepped pulley serves as the capstan which turns the idler wheel that turns the inner rim of the 9lb iron flywhee. It's a 'belt-idler' drive. The four speed range includes 16, 33-1/3rd, 45 and 78 rpm. The magnetic damping occurs via an eddy brake which produces enough drag against the belt for a variable speed pitch adjust system. The measured belt-drag also serves to damp motor cogging action somewhat. The shaded pole design of the motor manages to produce a smoother running motor, while the eddy brake dampens the flow of power a bit further.

(note: A similar shaded pole/eddy brake design was used on the Garrard 301 players of that era, but the Garrard didn't use a belt.)

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-- Above 4 photo sequence; Photo 1 shows position of reflector to achieve negligible eddy effect. Photo 2 shows position of reflector to achieve maximum eddy effect. Photos 3 and 4 indicate the pitch control knob that is turned via your fingers to adjust this.

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-- It's a Four-Speed Transmission

-- Above sketch indicates point of adjustment (via F1029 shims) to align the idler wheel relative to the stepped pulley that drives it.

-- More on the motor - E50 (ME 380)

-- motor rpm: 60 hz: 1740 (average)

-- motor rpm: 50 hz 1450 (average)

-- Note: motor rpm is variable ±3% by means of the eddy brake pitch adjuster, which increases or decreases drag on the motor at the belt/stepped pulley.

-- Measuring rotor shaft for wear

-- measuring bushing ID (used bushing)

-- measuring bushing ID (new bushing)

-- Measure coils for uniform ohms readings

-- Clean

-- Polish rotor shaft when needed - but don't reduce its size or alter its form.

--  Check the Running clearances shaft to bushing: I've determined that a desired bushing to shaft running clearance will range between .0008" and .0010". (inches) Less than that puts drag on the motor at the bushings and that no amount of upper to lower bushing alignment will alleviate that much drag.

Over the past few years I have been taking micrometer readings for wear on E50 rotor shafts. These are identified by the turntable serial numbers.

-- Project - SN#59805

  E50 rotor prior to clean-up

-- Measuring the rotor shaft for form and size on chassis sn# 59805.

Short end: Ø .18660 to .18670 The .0001" of taper variation is measurable and repeatable. Checking for roundness indicates zero deviations.

Long end: Ø .18695 to .18685 The variation seen is due to slight wear (.0001"). Btw, these measurements I resolve to the nearest 10-thousandth of an inch. (While the instrument reads out to 5 decimal places.)

Differences in size between long end and short end, I attribute to manufacturer's process variation. Manufacturers work to within a tolerance specified within their own process control. And now we can see an average difference between the long and and the short end of .0003 inches. That does not surprise me (coming from the manufacturing world myself) and likely fits within factory allowed tolerance.



Note: for this rotor shaft I simply cleaned the shaft ends with acetone. The result was a finish on the shaft ends that exhibited no need for polish. Size remained the same. Nice specimen, actually. 

  Left: using the mic to check the rotor shaft diameters.

    Left: Mahr split-ball type bore gage

-- Note:  About the Mahr spit-ball gage. (above image) It gets its internal read by means of a two-point 180° contact. With this type of gage it is possible to evaluate any irregularities in the roundness and cylindricity of the bore. To measure the bore for roundness one simply rotates at the same length location and take the read again.

To measure for taper, (that is any difference in diameter over the length of the feature), one takes reads at several points along the length of the feature. Then report all of the differences in reads found within the feature bore.

Note also the set-ring in use for this feature size. This is the calibrated reference in use for the read. As an additional reference measure, once set the Mahr gage can also be checked with the micrometer

After a bath in lacquer thinner, the bushings are checked for cleanliness then the bores are measured using the Mahr bore gage.

In this instance a set ring in use has as its stated diameter .1869 inches. The set ring is used to calibrate the Mahr gage to read zero at .1869 inches. Then reads are taken within the bushing bores and the difference in read between the bushing bores and set ring determine actual bushing ID size.

One bushing measured .1876 and the other .1877 inches in diameter. Both checked round. Same for taper. No taper and no out of roundness on the bushings. This is better than I thought it might when I first viewed the bushings after disassembly.

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-- Above photo: the setup to check the new bushings for inside diameter size and form. 

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