Lenco L-70 Plinth Construction

The Blue Rock

The Lenco L-70 was my first turntable - bought in 1967.  I have had other turntables including direct drive and belt drive turntables, and had  been thinking of buying a vintage Thorens, when I ran into the Jean Nantais thread and the Lencoheaven.com web site.  I figured it would be a cheap alternative to try and build a plinth for the Lenco and then revamp the arm, and just might be fun and productive.  SO --- after reading about everyone else's plinths, I began.

In the Beginning - the original Lenco L-70 on the factory plinth.

Note that the turntable sits low on the factory plinth because I removed the springs, and this lead to a problem requiring feet under the plinth to keep the motor from contacting the shelf.

The original plinth is a cheap box, with 1/4 inch hardboard for the mounting surface.

A cutting template for the new plinth was created by tracing the cutout onto some cardboard.

A constrained layer damping plinth with 7 layers was designed as follows.

The top 2 layers are birch plywood, glued together, after 4 inch carriage bolts are epoxied into recesses cut into the lower sheet.  The lower 5 layers are three layers of MDF, a layer of 1/8th inch rubber bonded to the MDF and a bottom layer of 1/4 inch lucite (tough acrylic).  The top 2 layers and bottom 5 layers are held together by the carriage bolts and nuts and joining nuts which also act as legs for the turntable.  Tiptoe style points can be threaded into the bottoms of the joining nuts to provide isolation from the base/shelf.

The preliminary stack of layers is shown after painting with melamine paint to provide the blue base for the project and to seal the wood from moisture, below:

As an homage to Jean Nantais, I decided to marble the top 2 layers (shown standing on the 4 carriage bolts), but rather than marbling the plywood directly, and taking my chances, I bought some hand-marbled art paper and used archival glue to attach it to the painted plywood.  The marbled paper glued to the top layers, before cutting out the central turntable opening is shown below

The ability to pre-select and control the colours and patterns of the marbling is a great advantage of this particular method, not to mention that it is easier than trying to dip a heavy plinth into a marbling vat.  The paper needs to be heavily sealed, in my case with 8 layers of varnish, which required progressively longer drying times between coats over two weeks.  The base layers were wrapped in 1/4 inch curly rock maple, and the corners were turned on a lathe to provide a rounded transition, and some protection for the paper top.

The turntable was mounted to the plinth using custom made 1/4 inch steel rods with M4 threads cut in the top 1/2 inch and 1/4-20 threads cut in the bottom inch (even though Canada is officially metric, most hardware is still Imperial/English).  The turntable is then directly bolted to the plinth with these stout rods, but is not directly in contact with the underlying shelf.  The result is spectacular in providing isolation for the turntable.  You can tap on the plinth or the underlying shelf, with no transmission at all through the cartridge sitting on the record surface.  The final picture of the turntable mounted on the plinth is shown below.

  I will have to modify the wall hung shelf to better accommodate the enlarged plinth, which is heavier, deeper, wider, and higher than the original.  Following the lead of the other plinth builders, I have named the turntable the Blue Rock and have created a logo which I will have engraved into a small brass plaque (shown at bottom).

Total cost to this stage was about $50 Canadian ($40 US).  The turntable is still using its original tonearm, because there are few tonearms currently made that are dimensionally even close to being replacements for the L-70 tonearm, which is longer and has a greater spindle to bearing distance than that of the L-75 (247 and 230 mm compared to 227 and 212 mm).  The closest available tonearms are one model of Thorens tonearm, which would have to be sourced from a vintage turntable and the Hadcock 242 (very expensive ~$1200 US).  I have worked up a spreadsheet to calculate the geometric changes needed, and may try to modify a Rega RB-250 to match the needed dimensions.  The Rock turntable reference above is due to the marbled plinth as well as my plan to incorporate a DiskTracker damper to the extended headshell of the modified tonearm to damp resonances.  More on the tonearm later.