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Modifying an Idler Turntable

by Richard Elen on 11 Oct, 2016

in Audio Production

Modifying an Idler Turntable

Oné of my activ­i­ties is trans­fer­ring archive music library mas­ter tapes to dig­i­tal, so they can be made more wide­ly avail­able again. This is not always straight­for­ward, and it’s some­times nec­es­sary to trans­fer excerpts from disc if the mas­ter tape is dam­aged in some way. To do this requires a decent vinyl play­back sys­tem. This arti­cle is about how I put one togeth­er.

Some­times there are prob­lems with the old tapes — such as oxide or back­ing shed­ding, and in par­tic­u­lar when the back­ing binder becomes sticky and stops the tape pass­ing through the machine. Anoth­er issue is the adhe­sive used in splic­ing tape becom­ing sticky (although it is specif­i­cal­ly sup­posed not to) and this can result in oxide frag­ments being pulled off the front of a track result­ing in dropouts. And unlike the solu­tions for sticky binder and shed­ding (such as bak­ing the tape or run­ning it through a white spir­it or iso­propyl alco­hol-soaked pad) sticky splic­ing tape caus­ing dam­age is dif­fi­cult to avoid, even if wind­ing very care­ful­ly.

On more than one occa­sion, prob­lems like this, or major dropouts, tape dam­age and oth­er issues, mean that a (usu­al­ly short) sec­tion of the mas­ter tape is unre­cov­er­able. The solu­tion, then, is to try and find a copy of the library disc pressed from the mas­ter, cap­ture the appro­pri­ate sec­tion, match it in lev­el and oth­er char­ac­ter­is­tics and then edit it into the ver­sion trans­ferred from tape.

A bet­ter vinyl play­back sys­tem

To do this effec­tive­ly requires a decent record deck, and while the unit I’ve had for some time — a Numark TT-100, essen­tial­ly a DJ turntable — does a good work­man­like job, and has the ben­e­fit of 78rpm (which is some­times nec­es­sary) as well as 33 1/3 and 45, I though it worth spend­ing a bit of time and mon­ey acquir­ing a supe­ri­or vinyl play­back sys­tem.

There are basi­cal­ly three types of ways in which the motor can dri­ve the plat­ter in a turntable: Idler Dri­ve, Belt Dri­ve and Direct Dri­ve. They’re illus­trat­ed in the  dia­gram above. It should be not­ed that there is often more than one way of imple­ment­ing all three of these meth­ods: Direct Dri­ve — often found in DJ turnta­bles — can involve the actu­al plat­ter being part of the motor, rather than requir­ing the “intri­cate gears” sug­gest­ed above; with Belt Dri­ve the belt may go round the entire plat­ter and not a sub-plat­ter; and in the case of idler dri­ve the idler may be hor­i­zon­tal (as shown — Gar­rard used this) or ver­ti­cal (as in the Lenco designs).

The Lenco L75

I decid­ed on an idler design as these are high­ly-regard­ed for their sound qual­i­ty. While it would have been nice to have, say, a Gar­rard 401 tran­scrip­tion turntable, this was well out of my price range and I set­tled instead for a Swiss-made Lenco L75. I found one for a good price and a rel­a­tive­ly short dri­ve to Nor­wich. I have nev­er actu­al­ly owned one of these before, though I remem­ber one from the school music room, many years ago (they were com­mon in edu­ca­tion­al insti­tu­tions).

As soon as I got it home I reviewed it visu­al­ly, and all looked good, so I pow­ered it up and it ran fine, solid­ly at each speed. It had a rather cheap and nasty orig­i­nal plinth that (still) needs to be replaced with a prop­er, sol­id one. These decks per­form best with­out the ben­e­fit of the springs pro­vid­ed sup­port­ing it in the plinth, so I removed them.

Updates: V-blocks and wiring

Then I looked at the so-called “V-blocks” in the arm sus­pen­sion. NOTE that I didn’t use the orig­i­nal Lenco arm in the end, but this info may be help­ful if you are. The arm has a knife-edge bear­ing that allows it to swing up and down. The knife edge, attached to the arm tube, rests in two V-shaped blocks, one either side, and they are noto­ri­ous for degrad­ing. Sure enough, mine had decayed into sol­id lumps that looked like yel­lowed teeth. I care­ful­ly scraped them out, cleared the holes, and replaced them with a pair of “desmo” V-blocks sourced from eBay. The whole oper­a­tion was remark­ably straight­for­ward.

Next I reviewed the wiring. The audio cabling cen­tres around a ter­mi­nal block on the under­side of the deck plate and here the wires from the ton­earm head­shell meet the shield­ed cables going to the out­side world. The left and right sig­nals and their respec­tive ground leads need to by elec­tri­cal­ly sep­a­rate from the chas­sis ground (a yel­low wire also lead­ing out of the plinth): in my case they were, but I replaced the coax with mod­ern cable and the DIN plug on the end with two gold-plat­ed phonos. The met­al body of the arm is ground­ed to the chas­sis.

On the mains side, the cir­cuit is sim­ple: Live and Neu­tral come in, one leg goes via a switch to one side of the motor and the oth­er side goes to the motor. This might have been fine 40 years ago but today, with old elec­tri­cal sys­tems, we prob­a­bly want a bet­ter approach. The sug­ges­tion in the Lenco Heav­en forum — where all the experts on the sub­ject of these turnta­bles hang out — is to fol­low the wiring shown below, drawn by Stephen Clif­ford:

lenco_earthing_main

Not shown above is the fact that the yel­low (chas­sis ground) lead is extend­ed out of the plinth to be con­nect­ed to the appro­pri­ate con­nec­tor on a phono pre­amp if required.

Impos­si­ble hum

Hav­ing car­ried out all the re-wiring, I installed a car­tridge and ran it up. And it hummed, bad­ly. Now you do not need the yel­low lead con­nect­ed to ground on the phono pre­amp and the ground con­nect­ed in the mains plug as it will cause a hum loop, but in this case I could not get the hum to go away, what­ev­er I did. I tried clean­ing the head­shell and arm con­tacts, dif­fer­ent earth­ing schemes, dif­fer­ent car­tridges and even dif­fer­ent pre­amps, but to no avail.

It seemed like­ly to me that the prob­lem lay in the wiring to the head­shell con­nec­tor but this seemed fair­ly hard to address. In addi­tion (and no doubt purists will hate me for say­ing so), I found the orig­i­nal arm rather clunky. So, even though I had car­ried out the task of replac­ing the V-blocks et al, I decid­ed to replace the tone arm.

The Orto­fon AS-212 as a replace­ment arm

as_212_vintage_page-2There are only a cou­ple of tone arms that will slot more or less straight into a Lenco, ie they are the right length etc to fit. The one that appealed to me was an arm made by Dan­ish man­u­fac­tur­er Orto­fon (famed for their pick­up car­tridges) the AS-212. But where to find one? Hunt­ing around net­ted me a gen­tle­man in Ger­many sell­ing a Tele­funken S600 deck — these were fit­ted with this arm — at a good price.

Sad­ly, when it arrived, the rear of the arm had dis­ap­peared and the lid of the turntable was cracked — a result of the ship­ping com­pa­ny mis-deliv­er­ing it and the erro­neous recip­i­ents open­ing it.

Not only that, when I men­tioned my inten­tions on a Face­book group I belong to spe­cial­is­ing in vin­tage equip­ment, they were hor­ri­fied. The Tele­funken S600 was an excel­lent belt-dri­ve deck, they said, prob­a­bly out-per­form­ing the Thorens decks of the time, and should not be van­dalised and left ‘arm­less’. So I decid­ed to repair it, and see if I could find a spare AS-212 arm for the Lenco, then keep the one I pre­ferred and sell the oth­er. The Tele­funken sto­ry is for anoth­er arti­cle.

Imme­di­ate­ly up came an offer on the Vinyl Engine forum of a com­plete AS-212 arm, boxed: a replace­ment arm for a Tele­funken. At the same time I received an offer of a replace­ment arm­tube, bear­ing and coun­ter­weight. I could use the for­mer on the Lenco and the lat­ter to repair the Tele­funken.

Prepar­ing the arm for fit­ting

The new-old-stock com­plete AS-212 assem­bly duly arrived, and I acquired a mount­ing base for the new arm to fit the Lenco deck­plate hole — the Orto­fon is a dif­fer­ent diam­e­ter and thus needs a dif­fer­ent fit­ting. These are avail­able on eBay: I bought a sil­ver-coloured one.

Before fit­ting to the Lenco, the new arm need­ed some dis­man­tling. I decid­ed to use the Lenco arm lifter — pret­ty much oblig­a­tory, in fact, with an AS-212 designed for an S600, as the Orto­fon arm comes with an oil-damped lift­ing cylin­der with just a bot­tom pin that is sup­posed to fit into the S600 lifter mech­a­nism, a clever Belden-style cable arrange­ment: thus it does not include a com­plete lifter sys­tem. So I removed the lifter cylin­der and arm rest, leav­ing a 10mm hole in the body of the arm, which I decid­ed to fill with a suit­ably-sized cir­cu­lar bub­ble spir­it-lev­el, secured with the exist­ing set-screw. Adja­cent to it in this pic­ture is the AS-212’s nat­ty no-con­tact mag­net­ic anti-skate sys­tem. The lit­tle hole for­mer­ly took a pin on the lifter to stop it rotat­ing. I found a use for it lat­er.

img_2156

I also removed the arm clip from the AS-212 (the rod to the left of the above image is the back of it) so as to use the Lenco one, which is the cor­rect diam­e­ter to hold the arm secure­ly.

Next step was to mount the arm col­umn in the new base. This was eas­i­ly done. I set the height up by attach­ing a car­tridge and adjust­ing the height so that the arm was hor­i­zon­tal with the sty­lus rest­ing on a disc. I lined up the body of the arm to be par­al­lel to the edge of the deck-plate and it looked great. I tight­ened the set-screw and there it was.

A few mod­i­fi­ca­tions

An ini­tial prob­lem was that the arm wiring was not as long as the orig­i­nal Lenco, so I moved the audio con­nec­tion tag strip to some­where near­er to the arm so it reached, and under the deck plate instead of on top.

img_2157

This pic­ture also shows the revised pow­er wiring men­tioned ear­li­er. I made a new hole in the plinth for the audio cables to exit so that they didn’t run par­al­lel to the pow­er cable.

The Lenco lifter actu­a­tor lever is quite long, and actu­al­ly fouled the arm when at rest, so I short­ened it. Actu­al­ly, I was going to bend it out­wards but the top bit snapped off. Ooops. It’s still easy to reach and use: a short piece of black heat­shrink tub­ing and it looks the same as the orig­i­nal, but short­er.

img_2166

The biggest chal­lenge was get­ting the lifter to work. The Lenco lifter arm is quite deep, and when low­ered rest­ed on the top of the Orto­fon plat­form long before the sty­lus was able to reach the record. I thought this could be solved sim­ply by short­en­ing the lifter arm to avoid the edge of the plat­form, but this was a Bad Idea as the arm could drop down and hit the deck plate between rest and the start of a disc, and tend­ed to fall off the end of the lifter. The solu­tion instead was to file the under­side of the end of the lifter arm where it over­hung the plat­form to about half its depth. This allowed the lifter to drop far enough to allow the sty­lus to reach the record. All the ele­ments of the arm are able to be adjust­ed for height: the lifter arm, the arm rest, and of course the arm col­umn.

It’s worth not­ing that there is a small caveat here. The Lenco arm rest, which I’m using, allows the arm to be unclipped by mov­ing it ver­ti­cal­ly. It is pos­si­ble, once unclipped, for the arm to swing out­ward, where­upon it will fall off the lifter arm and could drop down and clout the car­tridge on the pow­er switch or the deck plate itself. I made this impos­si­ble by insert­ing a thin rod into a hole left by part of the orig­i­nal AS-212 lifter mech­a­nism and bend­ing it over and above the arm to stop this from hap­pen­ing. You can see the hole in the close-up of the bub­ble lev­el a cou­ple of images up. While I was at it, I added a cut-down self-adhe­sive foot to the right-hand front of the plat­form so that the arm couldn’t drop if it went back­wards. Anoth­er approach would have been to rein­stall the Orto­fon arm clip, which opens towards the turntable and is thus less like­ly to allow the arm to go back­wards. How­ev­er this would require remov­ing the Lenco arm-rest, leav­ing a hole in the deck plate.

Next I need­ed to mount the car­tridge more accu­rate­ly. The AS-212 needs a 16mm over­hang — ie if you swing the arm across to be over the cen­tral spin­dle, the sty­lus should be 16mm to the left of its cen­tre. This proved to be quite dif­fi­cult to do: the slots in the head­shell only just allowed it with the car­tridge as far back as it would go. But it worked, and I was also able to set up the null points suc­cess­ful­ly with a pro­trac­tor, with the car­tridge par­al­lel to the groove at both points. The cor­rect way of fit­ting the car­tridge is to use the two thread­ed rods on the under­side of the man­u­al lifter prong, but in my case, although I have sev­er­al Orto­fon head­shells, none of them would actu­al­ly hold the Shure M97xE, either because they were too long, too short or there wasn’t room for the nuts. So I mount­ed the car­tridge with a pair of bolts and fit­ted the man­u­al lifter sep­a­rate­ly (see below).

Play­ing some records

giWith that done, I car­ried out a final check of the set­tings, includ­ing: check­ing that the arm real­ly was hor­i­zon­tal while play­ing and thus the Sty­lus Rake Angle was cor­rect (I think this is a bet­ter way of look­ing at it than by address­ing the Ver­ti­cal Track­ing Angle, and I don’t have a micro­scope); set­ting up the track­ing weight for my Shure M97xE; and adjust­ing the AS-212’s ele­gant mag­net­ic anti-skat­ing set­ting.

And then I played a record for the first time — but not a ter­ri­bly excit­ing one. It was the B-side of a KPM Music Library test press­ing of pieces by Richard Har­vey, con­sist­ing sole­ly of a 1kHz tone. I was able to lis­ten to the tone qual­i­ty at var­i­ous points across the disc and was very pleased with how pure the tone was at all points.

Then to play some actu­al music: the con­tent side of the same disc. I was imme­di­ate­ly very impressed with the wide fre­quen­cy range appar­ent on play­back, and a good tight feel­ing to the bass end. The over­all sound was very clear, clean and detailed, and the stereo imag­ing nice and sta­ble. Excel­lent.

My view is that this is an excep­tion­al com­bi­na­tion of arm and deck and I am very pleased with the results so far, though I need to give it a lot more crit­i­cal lis­tens. But in the­o­ry, all that’s need­ed now is a new plinth that does the deck jus­tice.

I have, inci­den­tal­ly, kept all the Lenco bits I’ve removed, includ­ed spares of items I mod­i­fied (eg the lifter actu­a­tor lever and the lifter arm) so that if it ever needs to be restored to its orig­i­nal spec, this can be done.

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{ 1 comment… read it below or add one }

Rasto September 17, 2017 at 15:27

Excelent job ! Thanks for sharing. I do same project and IT helped to me a lot.

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