Since I got my 2nd Lenco and my first stock plinth I decided to do some experiments. I know this would be far more fun with some pictures, but I'm lazy with the camera.
One Lenco, two plinths. Big one is 60x40cm, 3 layers of 18mm acacia sandwiched between 18mm layers of Bamboo, so 5 layers in total. Mounting of the turntable with 4 bolts that go through the whole plinth construction. For footers this one has 3 hockey pucks which rest on cradles made of rubberbands & little glass cups. (Think ash-tray feet like I hear DJs of old used to do with SL-1200).
Second plinth is the original one, but with some modifications. Springs were gotten rid of and replaced with small wooden blocks with some furniture rubber footers for a little bit of isolation from the bottom board. I also installed a cross-brace and damped the sides with acrylic mass. Both of these things helped a lot as far as knuckle test for plinth resonances go - it's not as dead obviously as a more substantial plinth, but it doesn't ring anymore and when rapped with knuckles produces a reassuring sounding little thud instead. I've seen several different variations of the stock plinth, this one seems a later production one which is made of chipboard with very thin veneer on top.
I recorded a lead-in groove and analyzed the spectrum for rumble. For first test the top plate was mounted like stock, so the 4 rubber grommets on corners and two screws on the pan. Now these figures don't follow any standard obviously and as such don't hold any meaning compared to anything published anywhere else, but they are comparable (within +/- 2dB or so) between each other. So here they are:
Big plinth
50Hz -59.6dB
100Hz -71.5dB
150Hz -74.1dB
Modified stock plinth
50Hz -59.9dB
100Hz -54.2dB (!!!)
150Hz -75.6dB
That 100Hz was clearly audible, not distracting while listening to music, but between tracks you could hear it, either very faintly or quite clearly depending on the volume Very disappointing result for the modified plinth in any case and removing the springs didn't seem to do much for the rumble even with the additional damping of the plinth. Sadly I didn't measure the TT in completely stock form, but using ears alone it was improvement, but a pretty slight one. I traced most of the rumble coming through the idler arm, so I damped it using electrical tape, rotated the rubber grommet 90 degrees just in case and in a different test (not directly comparable to this one, because platter wasn't installed and stylus rested in a resonating hollow plastic cap) that reduced the 100Hz rumble by 6dB and also spread the spectrum a bit. In the above test I doubt the difference would be as great, but probably a few dBs anyway. Next I altered the mounting and got rid of the rubber grommets at corners and the results were as follows:
50Hz -58.2dB
100Hz -64.3dB
150Hz -75.3dB
A whopping 10dB reduction @ 100Hz, which means with the rubber grommets in place (and idler arm undamped), the rumble @ 100Hz was about 3 times as loud. While the modified plinth still falls short of the big one (with better footers) the deck sounds splendid now and is practically silent.
I've heard some people be happy with the stock arrangement with the springs and all, but at least this one didn't sound good at all like that, plus it rumbled. The worst part seemed to be the rubber grommets though. I might try it with the springs yet just for kicks, though the upper plinth that rests on the springs is probably 2-3 kilos heavier now. Based on this, if you want a good sounding Lenco with minimum hassle, just getting rid of the springs and rubber grommets might get you quite far already with the stock plinth. However I noticed at high volumes it was prone to feedback, which doesn't happen at any volume with the big plinth and rubberband cradle footings. I attached two of self-adhesive rubber foam footers into each 4 stock feet on the bottom board and that cured it. I was just curious about what's the minimum effort and money to spend on a Lenco to get good results, and it's also further learning towards the eventual proper plinth.
I plan to try out what more solid mounting does to it - now the top plate is fastened to the plinth with only two puny woodscrews through the pan. I should've installed some additional blocks into the insides to faciliate bolting the deck down, but can't really do that easily anymore. Only easy option is to drill holes into the bottom board and bolt through it, but that means my little isolating rubbers in place of springs would become pretty much meaningless, but seeing they didn't work all that well to begin with, maybe that's not a bad thing.
Now, in stock form I don't think two 50 year old Lencos will perform exactly the same, there have been little variety in production and also the springs and rubber grommets, idler arm rubber grommet etc. have aged differently depending on the life of the deck. If after service the performance is not up to par in stock form, the above modifications cost me about 5 euros in the form of two 310ml tubes of the acrylic mass, and two pieces of scrap wood for the braces. These mods are still more or less reversable if wanting to go back to stock.
I don't know what maintenance (if any) has been done to this new-to-me Lenco, but since it came with a properly mounted Sumiko MMT and no extraneous noise is present I'm assuming at least the main bearing has been cleaned and oiled ... haven't actually opened it up yet, but I can tell it's not running dry. It came with an original 5-hole idler wheel. Just for kicks the results for my first Lenco, for which I've done motor and bearing maintenance and replaced the idler wheel with a Norbert one:
50Hz: -61.4dB
100Hz: -75.3dB
150Hz: -75.1dB
So a little quiter all-in-all, could be many things, probably mostly the idler.
What can be taken away from all this is resonances, the paths they take, damping, coupling/decoupling in mounting and the actual construction of the plinth do matter, not only in perceived sound qualities but in very easily measurable noise levels. Not exactly breaking any new ground here, but I wanted to have some actual numbers for myself to have some reference which is not dependent on my ears and how I happen to feel on a given day.
Btw. my hat off to people who make detailed threads with pics and all for tweaks and modifications they are doing, takes some real effort! I probably spent more time writing down and editing this post than I spent on swapping the Lenco between plinths, recording samples and doing the measurements ...
Next up some thoughts for a new plinth and things I'd like to experiment with.