Thorens TD-150, Rebuild & Modification Part Four


A tale of two turntables continues. Part One was a detailed introduction to the series, so if you haven’t read it yet I would encourage you to do so now. In Part Two, we built a turntable based on a stock Thorens TD-150MK2. In Part Three, we built the AAP12. And here in part 4 we’ll wrap up the series with some final observations, build notes and listening tests.

Setting The Suspension

When I embarked on this project, my biggest concern by far was the suspension which is a key part of these turntables’ design. I’d read many a story telling of the difficulty of setting one of these decks up to perfection, and many people (some who stand to profit from doing the work for you) advise against attempting this oneself. Surely they can’t all be wrong?

It turns out that with the right tools, due care and patience, and a copy of the dealer setup manual for an LP12, it is quite possible, if not easy, to adjust the turntable suspension. The idea is to adjust the 3 springs such that the chassis is as central as possible, the platter is level, and such that the platter is allowed to move gyroscopically (up and down) but in a way that lateral motion (swaying, twisting, juddering) is minimised if not eliminated. In truth, you’ll never eliminate lateral motion entirely without modification, for which there are a few theories which we may discuss in future.

Adjusting the suspension is achieved by twisting the springs, and their grommets, to minimise unwanted movement, improve the ‘bounce’ and centralise the sub-chassis. The spring nuts are then adjusted up or down to achieve a level platter. It can be fiddly, yes, but it’s not rocket science and it certainly isn’t beyond the capabilities of any DIYer of reasonable competence who knows one end of a spanner from the other. A set of box/tube spanners or a small ratchet and socket set are immensely helpful. The process quickly becomes easier with experience and is certainly not the art that Linn and their proponents would like you to believe.

When twisting the springs, it is important to move the entire spring and grommet assembly and not to simply introduce a twist in the spring itself, which will cause the suspension to quickly go out of tune. I also would not suggest lubricating the grommets as is often recommended, as any lubrication can also cause movement in the suspension over time or during transportation. Twisting the grommets is really not difficult despite the spring pressure. If you should accidentally introduce a twist in a spring, press up on the bottom washer with your finger and thumb, and then let go and allow the washer to snap back down against the nut. This will cause the spring to return to its natural position.

When adjusting the suspension, it’s important to have the platter mat installed (preferably the one you’ll be using) as well as the tonearm with its counterweight and cartridge. You’ll also want a sacrificial record, ideally one of the heaviest in your collection. This will ensure that the suspension is set to handle records of any mass. I recommend setting the stylus tracking force to approximately the correct value before you start, as this will place the counterweight in the correct position relative to the length of the arm board and will help to achieve an optimal suspension setting. You’ll also want a spirit level that is lightweight and accurate.

Adjusting the stock TD-150 is a little more fiddly owing to the mechanism used to move the belt to change speed. Adjust it too low and the platter will scrape against the speed lever on the 45RPM setting. Too high and the sub-chassis will contact the top plate. There is a very small area in which the suspension height is right, and getting to that point can be a frustrating process. It is important to set the suspension with the mat and a 180-gram record in place, as to set the suspension with a lighter record can cause scraping if the additional weight of a 180-gram record is placed on the platter and the turntable run at 45RPM. Many users simply remove the speed mechanism entirely, but if adjusted correctly it works just fine and is more convenient than having to manually move the belt.

Our stock TD-150 is fitted with some older specification Linn springs, which seem to be somewhat stiffer than the latest springs fitted to the AAP12. This was a common ‘upgrade’ performed by many TD-150 owners as the decks aged. Linn springs are longer and stiffer than the originals, and can therefore make adjustment more difficult due to their being approximately 1KG less mass in the TD-150 platter than the platter of an LP12. They therefore require longer bolts and require more care in setup, particularly if the speed mechanism is still fitted.

While we’re on the subject of suspension, let’s discuss for a moment the arm cable and its P clip. In the LP12, the arm cable is ‘dressed’ (held in place) by a nylon P clip, secured to the rear chassis bolt supposedly so as to minimise the possibility of the arm inhibiting free movement of the suspension. While it’s certainly true that a badly placed arm cable could inhibit the suspension, I feel that the use of the P clip has far more to do with using the arm cable as a quick and convenient way to correct one of the inherent weaknesses of the design. That weakness is lateral motion, Ann in effect the P clip is using the arm cable as a way to prevent too much lateral motion in the sub-chassis. While this does afford some stability it isn’t nearly enough. You’re far better off implementing another means of stilling the sub-chassis and dropping the arm cable straight down through a hole in the baseboard. Stabilising the sub-chassis also minimises the pull caused by the tension of the belt.

The greatest strength of the suspended turntable is also its greatest weakness. That suspension is surprisingly bouncy when you get it set correctly, and the slightest movement in the supporting service can set it off. These turntables are not ideal for use on a springy suspended floor, bouncy isolation platforms or wobbly wall shelves. In short, they need to sit on a solid structure that is entirely free of movement if they are to be unaffected by environmental factors such as footfall or even disturbances in other parts of a building. Concrete floors are ideal, and solid shelves fixed to equally solid walls (preferably brick) are as good if not better. You can certainly get away with situating one of these decks atop a rack on a suspended floor, providing the floor is solid and has no sags, dips or areas where footfall will cause movement in the floorboards.

Grounding

Grounding is an area where the TD-150 and possibly the LP12 can be improved, though I haven’t had the opportunity to look at the grounding scheme used in a current specification LP12. The TD-150 had the sub-chassis earthed to the top plate via a thin, flexible cable. I grounded both of my turntables to mains earth – in the case of the stock TD-150 via the top plate, and in the case of the AAP12 via the T structure which also served to earth the top plate. Thin flexible wire was used to ground both sub-chassis, which in turn grounds the main bearing and platter. The arm earth is kept separate and grounded via the amplifier earth terminal. This results in zero hum, no interference from the motor or electronics, and absolutely no static when lifting records from the platter, even with the deck spinning and using a felt mat.

Listening Tests

In terms of sonics, the first thing I noticed on spinning up the AAP12 was that is it is surprisingly quiet. The bearing is in perfect condition, almost as if it had just left the factory. But I didn’t expect it to perform quite so well in this area. No doubt the bearing and motor damping helps as the stock TD-150 is good though not quite on par, though most of the TD-150’s noise can be attributed to the motor which is considerably noisier without the thrust bearing fitted.

Sonically the two decks are similar in character though naturally quite apart in capability. They’re very full sounding turntables, with excellent voicing in the mid-range and d a very coherent musicality which is quite enjoyable. I’m used to the rhythmic adeptness of a Technics and belt drive turntables tend to sound somewhat lethargic in comparison, as is certainly the case here. Speed stability is excellent however and there’s a nice sense of three-dimensionality to the stereo image.

Future Improvements

In terms of future improvements, I think my first port of call will be to add some mass to the AAp12 platter. Mark of Vinyl Passion has suggested the use of self-adhesive wheel weights which is something I may experiment with. Bonded steel or brass layers is also an option, as of course is an entirely new platter in a heavy material or composite of heavy materials.

That power supply and power switch have got to go. Electronic speed switching is a must as far as I’m concerned, and I’m sure that improving the power supply can bring a further significant reduction in motor noise.

And I’m also toying with the idea of rebuilding the sub-chassis in a thicker, heavier aluminium plate with either an integrated arm board or a separate arm board in aluminium, stiffened with support struts for greater rigidity and support across its length. I took the opportunity to create a template of the original sub-chassis during the build process, so this may well be a modification to consider sooner rather than later. I’ll reuse the existing bearing as I think it is a fine unit, and I’m told that it is easy to remove, being press fit into the sub-chassis under heat at the factory.

This rounds out part four and for now the TD-150 modifications series. I can’t say when or if there will be another instalment, but I do hope to be revisiting the AAP12 at least in future articles. I also hope to share more builds this year as and when I have the time and means, so do stay tuned if they are of interest. Until then…


About Ashley

I founded Audio Appraisal a few years ago and continue to regularly update it with fresh content. An avid vinyl collector and coffee addict, I can often be found at a workbench tinkering with a faulty electronic device, tweaking a turntable to extract the last bit of detail from those tiny grooves in the plastic stuff, or relaxing in front of the hi-fi with a good album. A musician, occasional producer and sound engineer, other hobbies include software programming, web development, long walks and occasional DIY. Follow @ashleycox2

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