[DIY] Fixing and tweaking the original Lenco Tonearm

Product name: DIY Lenco Tonearm Fixes
Manufacturer: not for sale, TNT-Audio DIY design
Authors: Roger McCuaig - TNT Canada & Pierre Lurné - TNT France
Published: June, 2020

Introduction

Part 1 of DIY Lenco Tonearm Fixes discussed modifications that were implemented on some of the many Lenco turntables owned over the past few years. Part 2 will discuss some ideas for further tweaks that are the result of a series of emails exchanged with Pierre Lurné who's great ideas and technical comments created the need for a second look at the subject. Although the writing was done by Roger, the reader should consider this report as a joint effort by Pierre and Roger. The report will be subdivided in the same way a Part 1, headshell, pivot and counterweight, however, some new sections have been added to deal with other topics of interest. The quotation marks in the text refer to direct quotes from Pierre's emails to me.

Vibrations have turned out to be a recurring theme throughout this project, and rightly so as stray vibrations are the worst enemy of vinyl record playback. Here is what I wrote on the subject of vibrations vs turntable design about 2 years ago:

Headshell

The easiest path for stray vibration energy to follow in a Lenco turntable (especially one with a rebuilt, massive plinth), is into the tonearm. Therefore, one of the major tasks that a well-designed tonearm must perform is to minimize the lifespan of stray vibration energy, effectively, to find a way to attenuate or to bleed the vibes out of the system. The headshell is the closest component of the tonearm to the source of this energy, the stylus/cartridge, therefore, it has a key role to play. This is, of course, one of the weaknesses that motivated the replacement of the original Lenco headshell with the aftermarket unit as discussed in Part 1. It is worth noting that some of the later versions of the Lenco headshell were made of a slightly thicker material, however, the added mass was offset by putting 9 holes in the top surface. The mass removed is insignificant from a performance standpoint whereas the holes so close to the cartridge can only add to vibration problems. "Holes cause a vibes mess." This holey version of the heashell is even less desirable than the original design.

The sides of the original Lenco headshell do add rigidity, a design feature lacking in the aftermarket model. However Pierre has been experimenting with some glue-on wooden reinforcements to add rigidity to the headshell and has promised some sketches of this tweek some time in the future. Post Covid-19! ".... the combination of the wood and aluminum .... give excellent rigidity and damping results". Finally, the aftermarket headshell is quite long. It is, in fact, 2 mm longer than the original. This doesn't sound like a lot but the original was is already quite long. The new model could easily be shortened, which would improve its vibration characteristics. One could saw off 10 mm, possibly more. This would, of course, require drilling new holes for the cartridge mounting and trigger the need for revised alignment settings because the pivot to spindle distance will have changed. The reader may want to reread the recent review of alltheprotractors.com. Keep in mind that if cut so short that the pivot to stylus distance (effective length) becomes shorter than the pivot to spindle distance (mounting distance) then alignment will not be possible. In otherwords, there must be some overhang. The top left photo below shows the approximate cut-off line with masking tape. The second photo is the jig setup used to guide the metal cutting blade.

Based on the email exchanges with Pierre bracing pieces for the under side of the headshell were prepared. These are maple, 2 mm thick, glued in place with Superglue. Cutting off the end of the headshell left open ended slots. Not a good idea from the aspect of vibration control. Imagine 2 little tuning forks! These openings were closed off by glueing (Superglue) a small piece of leftover headshell material back on the end and then filing it down to give it a neater look. The soft aluminum is easy to file to the desired shape. (refer to the bottom left photo of the group of 4 below)

The Pivot

Part 1 stated that there are many different offerings of aftermarket V-blocks available on the market. The best results with respect to draining off vibration energy can be obtained by installing V-Blocks of harder material. "... a good part of the nasty vibes will be able to escape there." The original V-blocks and some of the aftermarket products are made of a very soft material which therefore allows the vibration energy to continue along its happy way up and down the arm wand and into the counterweight. Pierre suggests that the best material for the V-blocks would be the same material as the pivot points (or slightly softer). The reader may recall that in the TMIOTT DIY article it was stated that the best material for a platter mat would be the one that best duplicates the acoustic characteristics of the vinyl record. The same appies here, duplicating the acoustic properties of the pivot material in the V-blocks. These are made of metal, it appears to be steel, so possibly brass V-blocks would be best. A set of brass V-blocks is now on order from a USA source for about $40. Can.

The Counterweight

The Lenco tonearm should perform better with elimination of the decoupling between the armwand and the counterweight shaft. Pierre has provided an extensive analysis on this topic which is tempting to insert here however this is probably something that is better left to Pierre to explain in detail in some future article. Briefly, the arm moves up and down with the warps of the record and the floppy connection that is always present in old, unmodified, Lenco arms allows the counterweight to move sometime in unison and sometimes independently of the armwand depending on the warp amplitude and frequency. It becomes possible for the effective mass, the VTF and the resonant frequency to vary continuously. "The system ceases to be neutral and tends to have an ever changing behavior ...". Which leads to an important recommendation, use a record clamp/weight. Pressing the record down on the platter can reduce warps by as much as two thirds, which significantly reduces variations that the tonearm has to deal with whether decoupled or not.

One of Pierre's basic tonearm design principles is to simplify and minimize the pathways available for stray vibration energy to follow. Every additional pathway adds to the complexity of the vibration signal. "A correctly designed arm aligns cartridge + headshell + tube + bearing housing + rear stub + counterweight along the same general axis." This simplifies the task of eliminating this parasite from the system. The explanation of this is complicated physics. I have not done the math, as Pierre has, and am not qualified to give a detailed explanation of the science. The Lenco arm has an off-center hole in the counterweight, thus making the center of mass of the c/w far below the center line of the arm wand. This design has become, rightly or wrongly, quite common in unipivot arms as it provides a simple way to implement a correcting force to bring the stylus back to vertical. (like a pendulum) The concept has gradually leaked into other non-unipivot designs as is the case for the Lenco wiich is, of course, a dual pivot design. As a result, the center of gravity of the whole tonearm is moved below the axis of the armwand and the pivot. The result is that the vibration energy travelling back and forth along the axis of the armwand - pivot, now transfers vibrations to a mass attached like a pendulum. The resultant vibration system certainly can be imagined to be more difficult to manage than a setup where the vibration energy simply runs on a path that is aligned with the centre of mass of the tonearm components. As stated earlier, the math exists and Pierre has published some technical papers on this subject if the reader cares to look them up.

So the ideal design would be to have the hole in the center of the counterweight. Using a larger weight in order to move the weight as close to the pivot point as possible also carries benefits which will not be discussed here as this idea has been around for years. Pierre suggests using a lead weight instead of the stock Lenco weight as lead is more acoustically "dead", which is always a good thing with respect to reducing the lifespan of vibrations. "Lead can be considered as a 'magical' material for our phono purposes thanks to it's high density, excellent Q damping and Acoustic Impedance." No attempt has been made at this time to find a local source for lead rod. Ideally, the c/w should be attached to the shaft on top rather than on the side, again with the purpose of minimizing sideways or off-centre pathways for vibrations. The counterweight can be encouraged to work more like a "single point" by making the hole a bit bigger than the rear stub, placing some felt in the space and not tightening the connection too much.

Interconnect cable

Whether or not you believe that the design and materials used for an interconnect cable has an effect on the sound does not preclude the importance of good connections. It is worth while replacing the original interconnect cable as the connections at both ends are very old, possibly frayed, and probably corroded (see photo below). This involves a bit more DIY work than some readers may be interested in doing, it's up to you. The arm wires run down through the center of the arm pedestal are are soldered to a terminal board attached to the underside of the turntable top plate. New interconnect cables can be soldered to this board or the arm wires can be extended out from the connection board to a pair of external connectors to allow the use of standard interconnect cables. Ordering new wiring to replace the ones in the armwand is possibly a bit too much of an investment in dollars and in effort for an old Lenco arm.

Platter vibrations

This item, of course, is off the topic of tonearm fixes however it is such an easy and effective tweak that it was hard to pass over. In the course of a long exchange of emails with Pierre we ended up on the subject of platter material and design vs vibration management. Pierre mentioned that one of the design flaws of the Lenco turntables is that the long, thin outer rim of the platter makes it act like a bell. The light bulb lit immediately and a quick search of my workshop turned up some automobile noise damping material. This is a dense and somewhat pliable material about 2 mm thick. It has one sticky side for application to fit the curvature of the inside of a car door. This makes it ideal for application to inside of the platter rim. Setting two platters side by side on the workbench, sitting on a piece of metal to simulate the spindle, a tap to the rim of each produced a significantly different result. On the platter treated with noise dampening material the ringing sound was greatly reduced. Although not completely gone, it was still more like a thud than a ring. This modification will certainly improve the dampening of any vibration energy that gets into the platter.

Conclusions

This DIY stuff is fun and rewarding. Staying at home in order to stop the progression of Covid-19 has generated a lot of free time. So one morning, after reading Pierre's email of encouragement, the DIY tonearm project that has been percolating for years, finally got under way. The next DIY article is in the works.

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DIY Lenco Tonearm Fixes - Part 2