Lenco L75 based Turntable Project

[Qwin]

Lenco Based Turntable Project

I've heard some great idler drive turntables over the years and always wanted to try one myself. The likes of Garrard and Thorens have got to very high prices recently and more than I wanted to spend. I came across various articles singing the praises of the Lenco L75 (GL75 in UK) and after some research over at the Lenco Heaven website, I decided this model would be a good way into the world of idler drives.

This was the donor deck I purchased off eBay, no arm and a rough looking plinth, but fully working.






I built a new plinth from three 40mm slabs of Bamboo worktop. My purchased material is A1 quality, with no voids or filler, but expensive. I found a kitchen supplier selling A2 offcuts at reasonable price, I think these were the cut outs for double basin sinks etc, so scrap.






I used the PTP6 stainless steel top plates available from the Netherlands and an upgrade bearing made by SPH (Malaysia), I built an additional bottom bracket to support the bearing both ends.








With a good bearing, these heavy platter decks can take over a minute to come to rest when the motor is turned off. This can be a pain when changing records, so I designed/built a brake, which brings the platter to rest in less than one revolution.





I added a spirit bubble and feet, adjustable from the top face, to make it easy to set up and finished the deck in Satin varnish.
















Tonearm

Similar to my hankering for an idler drive, I've always wanted to try the original Mission 774 tonearm and I managed to obtain a very nice example from a member of this parish. Just like this:






Anyone who has followed my projects knows I like to do things my own way, so modifications were bound to happen. The arm is very well engineered, so no fear of me touching anything that would upset that. However, the base plate has no way of adjusting the Spindle to Pivot distance and the method of holding the arm stub, in a relatively thin plate, with a single grub screw is not the best, so I made my own base for the arm to drop into.









I rewired the arm with “The Missing Link” and my cartridge of choice was the Van Den Hul ONE Special. This is a good compliance match for the arm and the cartridge has a healthy current output, important for my fully balanced current amplifying AQVOX 2Ci phono stage.






Fitting the arm was not without its problems.
There is conflicting and confusing information on the internet regarding the set up for this arm, some of it from Missions own documentation. The only dimension I could trust, was the effective length of 229mm which is in the specification info that comes with the arm. I set out with this in mind and using the other dimensions for offset and S to P being quoted on the net. This was the result.






As you can see, the cartridge leads exit the arm wand under the holding clamp. If it were tightened in this position it would trap and probably damage the wires.
Something is seriously wrong here, the effective length seems about 10mm to short for my wand. I was confident it was the original Mission badged wand that came with the arm. Made with a double wall tube, most replacement wands are single wall. With all the conflicting information I decided to create my own geometry, using the alignment program on vinyl engine.

First step was to measure the offset angle of the mounting block on my specific arm wand, as there was a variety of dimensions being talked about on various forums, from 20.5deg to 22deg. I created a measuring template, which other owners might find useful and is linked here:
https://www.jkwynn.co.uk//774/Pics/L...fset_Angle.pdf

My mounting block has a measured offset of 21deg, so this would be locked into the settings of the alignment program as a non variable and the Spindle to Pivot and effective length, which are adjustable, would be tweaked in the program. The following results and comparison with the presumed Mission, Lofgren A and Stevenson geometry were produced.










As can be seen my Geometry seems to have better results than the presumed Mission dimensions, in terms of tracking error and distortion. The cartridge sits square in the mounting block and the wires now exit the wand in front of the clamp, like this:






It's early days, I used a basic set up, with the stylus pressure in the middle of the range, Azimuth and VTA were handled with a spirit bubble on the mounting block, so I haven't dialled in the sweet spot or tried the paddles/damping oil, but it all sounds very nice regardless. A very black background and lack of surface noise (on a good record) tends to suggest the alignment is a good one.

My wand is 247mm long, measured along its centre line, to the middle of the front face of the cartridge mounting block. Should anyone else have the same issues I faced, with a wand of similar length and offset, you might want to try my alignment. The only down side, is my S to P distance has increased by 9.6mm, not a problem for my adjustable base, but the stock base will need re-mounting at the new distance. The values are:

Spindle to Pivot = 225mm
Effective Length = 239.5mm
Offset Angle = 21deg

The protractor I created can be found here: https://www.jkwynn.co.uk//774/Pics/L...Protractor.pdf



I'm just enjoying the way this combination of turntable arm and cartridge work together.



If you want a blow by blow account of the plinth build or arm mods/alignment, find the Project on my web site here: https://www.jkwynn.co.uk/Lenco/Lenco_Thumbs.html

[walpurgis]

Looking very good Ken. I can see a lot of thought and care has gone into everything .

[Ali Tait]

Yes indeed, look forward to hearing it.

[Qwin]

Thanks Geoff, Ali
The last deck I'll build, so ease of set up and use was a high priority.

Need to thin out my Turntables now.
My Techie/ProJect hybrid with Terminator arm may have to go.
As will the Techie with Rega arm.
Just don't have the space since I moved to Carlisle.

[Barry]

Excellent write up Ken. Greatly admire your new arm base and height adjustment attachment. Well done!

The Van den Hul One Special is a very good cartridge. I use one in a Breuer Dynamic 5A arm.

All tonearm alignment prescriptions are a compromise, and often the manufacturer's instructions seem to conflict with what is found in practice; the instructions for the Breuer arm are certainly confusing: https://theartofsound.net/forum/show...-5A-pickup-arm. I ignored them and set it up to correspond to Baerwald (Lofgren 'A'), and it sounds perfect to my ears.

[Qwin]

Thanks Barry, I have no milling facilities, so the base was made by hand with a hacksaw, drill and files.
Yes its amazing how many of these old arms seem to conflict with the instructions that came with them.
A friend has a Grace G-707 and he can't get it to work with the supplied template, the slots in the head shell just aren't long enough to align the stylus. He still gets it to sound good though, using trial and error.

The Silicon fluid for the damping trough arrived today, I had no idea it was so thick and gloopy, I thought it would be like thick oil, but its more like treacle (30k and 50k cSt).

[Barry]

Apropos the viscosity, in general for pickup arms the following apply:

100,000cSt for unipivots

30,000cSt for damping troughs

20,000cSt for lift/lower devices.

So your fluid at beween 30k and 50k cSt sounds about right. Do you have an assortment of paddles to use with the damping trough? With the SME version, they provide three paddles of various sizes. I have found that the smallest one is quite sufficient, but then the damping fluid supplied by SME is 200,000cSt (!)

One can always thin or dilute the silicon fluid, but the thinners are not very pleasant (acetone, naptha, toluene, etc) and may be difficult to obtain. If you can't get hold of acetone, try nail polish remover. Alcohol (isopropanol) is probably the safest and easiest solution (no pun intended), but I haven't tried it.

[Qwin]

Thanks for that information Barry, very useful.

The 774 comes with three paddles, I have the large and med size, the med looks to be the one for my cartridge compliance, based on the chart in the arms manual.

I have the dimensions for all three paddles and posted these on my web site, the smallest can easily be made from 18swg (1.22mm) stainless wire using pliers. Plus there is a guy on eBay selling new sets of three.
I will try the med paddle and 30,000cSt first and see how that works out.

200,000cSt ! that's a solid

[walpurgis]

I'd generally start with the smallest paddle, especially if the silicone damping fluid is thick. You'll need to experiment depending on cartridge used, some cartridges need no damping. If you want, you can thin the fluid with silicone oil, the type sold in aerosols as a general lubricant.

[Patrick Dixon]

It looks very nice. Always good to get a plumbing fitting into a build too ;->

I have two Mission 774 arms, an early type (purchased Dec 79) and a later type. AFAIR (I can't find the earlier one ATM) they both have the same headshell offset.

The later one (which I have mounted on a Thorens TD124 II with a Decca SC4E) has the arm wiring exiting the tube around 30mm from the end and it's aligned so that there is about 8mm of arm tube protruding behind the clamp. The original template says the spindle to mounting position is 8 9/16" (217.5mm) and the original alignment protractor has the null point at 61mm. I can't recall what I set mine up to, but it sounds OK to me.