The Lost Worlds of Lenco . . .

Disarming the Lenco

You've just taken delivery of a Lenco L75/78. Hopefully it was packed well - the platter removed and packed separately, maybe the motor transit screws were tightened if you were really lucky? And the counterweights removed from the arm?

They weren't, were they?

This might be your first Lenco and you're wanting to check it out as soon as, and with its existing arm, before committing yourself to upgrading. Only one problem, the arm feels loose and worse, the headshell appears tilted to one side, viewed from the front.

Your V-blocks are shredded, my friend!

Lenco experimented with different materials for these bearings. The spongy yellow ones were particularly disastrous and are better off replaced. Either way, these V-blocks will perish with time, 5 years according to T&G, or instantly as the result of rough transportation - i.e.with counterweights in place.

New V-blocks are available from Technical & General, or you could fashion some yourself, Blue Peter style. Dimensions here.

But first, you've got to get the old ones out . . .

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Quick and dirty L75/78 V-block bearing replacement

There are two ways to do this: the proper way, which involves de-soldering the arm's signal leads from the tag-board, or there's my way, which involves trying to get away without doing this, relying on enough slack in the arm's wiring to give you enough room to manoeuvre,

You will need a small flathead screwdriver, and some new V-blocks . . .

Here we go.

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1. Remove all weights, bias assembly and headshell.

2. Remove the screw (and the washer it holds) that secure the bearing cover

 

The screw holds a semi-circular washer in place. If it doesn't fall out (probably between a crack in your floorboards) when the screw is removed, remove it anyway. Don't lose them!

 

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3. Gently lift up the bearing cover.

There should be enough slack in the arm's wiring to lift the cover the half inch required to clear the top of the bearing housingg, and then sideways, to reveal each v-block in turn.

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Don't force anything!

If there isn't quite enough slack, you could loosen the arm-heigh adjustment screw, and reset the arm in its highest position - this should get you another centimetre of slack (maybe we should have done this first!)

If there still isn't sufficient slack, you'll have no choice but to unsolder the arm's wires from the tag-board. However I haven't had to resort to this on the half a dozen L75 arms I've repaired.

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4. The V-blocks can then be prized from the housing with a small screw-driver.

Here's one of the little blighters!

Replacement V-blocks are available from Technical & General. They are the black rubber variety, and definitely an improvement over the spongy yellow ones that made there way into a few generations of the L75/78's.

Alternatively, you could consider making some of your own.

Here are T&G's instructions for V-block replacement:

 

Reassembly

is the reverse of disassembly. Take particular care not to trap the signal wires when replacing the bearing cover. The best approach is to gently pull the wires through the bottom of the arm post whilst simultaneously pushing the bearing cover home, making sure the arm knife-edges are sitting correctly in the V-blocks.

All done?

Don't forget to reset the arm height - the arm tube should be parallel to the record's surface.

Finally, you can check the overhang with the Lenco protractor.

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Hope for the L75 arm?

The rear part of the arm - where the main counterweight sits - is 'decoupled' from the main part of the arm: the two sections are held together by an internal high tensile steel wire which was intended to act as a dynamic vibration absorber (a device pioneered by J.P. Den Hartog in the 1920s.) Over time the rear part tends to sag slightly. If it breaks then it is irreparable. If it sagging starts to impede the free movement of the arm, then it is also done for.

The decoupled counterweight, like sprung chassis and suspended sub-chassis, are really relics from another time - when acoustic isolation of the turntable unit was paramount, as few sonic concessions were made to the turntables siting. It wasn't unusual for a turntable to be mounted in the same cabinet as the amplifier and speakers, perched on legs which often rested on a springy wooden floor. Under these conditions, it was all too easy to fall victim to acoustic feedback, (aka 'howl round') and obviating this eclipsed other considerations.

OK, you're thinking, but what does the L75's decoupled counterweight have in common with the famous 3-point suspended sub-chassis? Is this a completely gratuitous sideswipe at the Linn Sondeks and Thorens of this world?

Both these suspension systems are constantly in motion, and effectively present the stylus with a low frequency signal - or modulation - unrealated to the musical signal, but which will inevitably interact with it. Not good.

Most Lenco L72/75/78's chassis were either suspended on springs attached to a plinth, or rigidly attached to a plinth (so far so good . . ) which featured an innovative sprung baseboard. It's a relatively trivial matter to disable these springs and actually hear the benefit this creates. Of course, this will decrease the turntable's immunity from external acoustic and physical interference, but proper siting will obviate this. (If you have wooden floors then rigid brackets attached to a brick/stone wall is the only way to go.)

Many tone-arms implement counterweight decoupling, including models from Audio Technica and SME (3009) to the Acos Lustre and OEM 'd Acos arms that graced many 70's turntables, such as the Pioneer PL12D and the early Rega Planars. The decoupling typically consisted of a rubber plug between the rear (counterweight) arm tube and the main arm tube. Some enthusiasts have found sonic improvements from rigidly coupling the two arm sections - by replacing the rubber plug with an inflexible one. (The rubber-plug approach also has some beneficial damping properties, so in removing it, we might want to compensate by adding damping by some other method, a topic for further discussion.)

Some Vinyl Engine discussion.

"Of late, the attention of tone arm designers has been focused especially on the Low-frequency resonance which is caused by the inertia moment of a tone arm and its stylus tip compliance. They have found that such resonance often results in a kind of frequency modulation of the audio signal . . . " [Moeller] AES 1978

The situation with the L75's arm is more extreme (i.e. worse) because the wire-coupled counterweight exhibits none of the damping properties of rubber, and acts more like a resonator - with almost any movement of the arm - stylus cueing, external knocks or record warps, for example - causing the counterweight to wobble around sympathetically - even enthusiastically. Add to this the relatively high mass of the arm, and the low resonant frequency it exhibits paired with a fairly high-compliance cartridge, and you've got a mini-disaster on your hands - if not your records - and a stylus that jumps out of the grooves.

And there's more.

Current thinking (and common sense) would conclude that the stylus should be rigidly coupled to the record. The L75's rubber V-block bearings exhibit some flexibility at the best of times and this will inevitably result in some loss of detail.

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A 99% Solution?

I think the biggest improvement would result from disabling the flexible coupling. With this in mind I 'separated' the two sections of a spare L75 arm - i.e. I broke them. (Once these two parts are separated there's no way to put them back together in the intended way - think curiosity and cat - i.e. time to get a new arm.)

My intention was to re-join both parts with a non-flexible coupling.

Unfortunately I have thus far been unable to remove the brass inset from the counterweight tube, and have actually abandoned this part of the project.

I am defeated.

So if anyone else want to have a go, I'm happy to send you the arm parts photographed below.

 

 
Rear part of arm tube. Note steel wire which was previously attached to the centre of the brass insert (right) Main arm tube with brass insert.

V-block D.I.Y.

If you're feeling adventurous, you might like to try a really hard material.

And if you're feeling really generous, you might like to send me a pair of your creations to check out. :-)

A solitary V-block displays its vitals

Other Lenco arms

The L75 arm was also sold separately (I once acquired one attached to a Goldring 99 once) and there were other arms such as the P77. The arm Lenco fitted to the L69/72/B55 was similar to the L75 - they share the same V-blocks and the decoupled counterweight design.

Earlier L69/72/B55's didn't have bias compensation but later models saw this added. Unlike the L75/78's weight-on-a-thread design, a spring mechanism was used; one end of the spring attached to an extension to the arm's horizontal bearing, and the other end attached to the underside of the bias amount slider.

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Spring tension providing bias compensation on later L69/72/B55's

 

 

Any corrections or comments always welcome.

Lost Worlds Of Lenco