Plinth

I've made plinths from Concrete, Birch Ply and MDF in the past, I wasn't sure what I would use for this one, but wanted it to look a bit special.

I came across a YouTube video, of someone using a pair of IKEA resinated Bamboo Butchers blocks, as an isolation platform. I loved the checker board effect of the laminated Bamboo strips on the edges and wondered if I could use several glued together, to make my plinth. While Bamboo is not the last word in terms of damping properties, its also a lot better than some other materials popular with DIY builders. With a damping factor of around 0.2 its ten times better than MDF for instance. It turned out the IKEA boards were a little on the small side for what I had in mind, but it got me thinking. After an extensive search, I found a UK supplier selling A2 size pieces, of 40mm thick resinated Bamboo Kitchen Worktop.

My plinth will use three of these 40mm sheets, so a total thickness of 120mm. The picture below shows what the pattern will look like, you have to use your imagination here and picture it sanded and polished, but I think you will get what attracted me to it.






The A2 dimensions are about right for a deck with a 12" Arm, but a little large for decks with 9" arms (my choice), so I had to cut the boards down slightly, to give a Plinth size of 500 x 380 x 120 mm. I then pegged them together using 6mm diameter alloy dowels. This will allow accurate alignment when drilling holes for threaded inserts, cutting pockets for the motor etc. prior to gluing the layers.



The locations I used for the dowels coincide with the positions for the four feet I will be fitting. These holes will be enlarged after the boards are glued together and used to access/adjust the threaded feet.
No more reaching round the back with a spanner, trying to find the adjustment nuts, just drop a screw driver down the holes, to adjust from the top. I'll fit a spirit bubble to the top face to aid leveling.



PTP6


The key components of the Lenco decks are the good quality Motor/Bearing/Platter, the chassis is ok, but only made of 2mm thick mild steel, so not the most rigid platform to mount a bearing and heavy platter. The Plinth is quite lightweight in construction and uses materials and techniques typical of the period. The stock Tonearm is quite good, but can easily be bettered, a no brainer decision for me, as my deck came without one.
On the LencoHeaven forum, I soon discovered a kit was available for those without the facilities to fashion a new chassis. It's called the PTP6 (Peters Top Plate version 6) and is made from laser cut 4mm thick stainless steel.
The kit comes with all the parts required to mount the Motor, Bearing and Control Mechanism from your donor deck, on to a new solid Plinth of your own design. This is the promotional shot for PTP Audio's kit.





I cracked on with the Plinth and added the M5 threaded inserts for Tonearm location.



I didn't use "T" Nuts for this build, these inserts are Zinc Alloy, so are non feromagnetic.
Four M5 inserts were fitted to the underside of the top piece of Bamboo and the heads were countersunk, so they wouldn't prevent the slabs clamping down flat when glued.



The cut outs will be cleaned up further, once the slabs of Bamboo are glued together.


The PTP6 kit arrived from the Netherlands.



Everything lined up with the cutting template.
One thing I'd noticed on other peoples builds, was the edge detail on the speed adjuster platform, that sticks out from under the platter, likewise for the portion of the motor plate that protrudes at the back. The laser cutter leaves vertical striations on the wall thickness and a square top edge, neither of which looks particularly pretty and certainly not high end.
I decided to clean up the vertical faces and add a top edge radius before going any further, I only did this for the visible portions and feathered it out beyond these areas. Note that the cutting process case hardens the metals surface and its difficult to file, once through this outer layer the Stainless Steel underneath is much softer. I've indicated where I did this with the Red arrows, on the pictures below.



Another detail which was like a poke in the eye for me, was the rear corner of the motor plate, it's just crying out for an additional screw fixing, if only to balance out visually with the other one that's there. So I added the one indicated by the Yellow arrow. When marking out the location for this new fixing, it's very close to the edge of the motor cut out, so I tweaked the shape of the cut out slightly, to create more room. I checked against the motor and there's space to do this.



I fitted M4 threaded inserts to the underside of the top slab, to locate the PTP6 plates and made sure they were countersunk.
For the motor aperture, this was cut a slab at a time. A 55mm dia Forstner bit made the circular cut for the bearing. I used a 10mm dia drill at each corner of the shape and then cut the straight line between each with a jig saw.
40mm thick Resinated Bamboo is tough to cut, you just have to take your time.
The aperture will be trued up after the slabs are glued together.






Bearing


My upgrade bearing arrived from SPH (Malaysia)



At 118mm, from the top of the mounting flange to the base, Its considerably longer than the original on the Left and runs on grease as apposed to oil. My 120mm dimension for the depth of the plinth was partly to accommodate this longer bearing.






I didn't like the top profile of my standard SPH bearing (on the left).
Sien at SPH has created a top, finished with a cap head screw. This can be removed and an extension screwed in to cater for "stacked platters". While I can see the logic in this, in creating a one size fits all bearing, I feel it compromises the quality of the basic one platter application. I change records a lot and anything which detracts from this experience, or makes it more fiddly is not good in my opinion.
I modified the top as per the picture on the right. To do this, I first
removed the spindle from the bearing and placed it in a poly bag, with just the last 30mm protruding. I wound masking tape around the end of the bag to seal it and prevent the greased spindle from being contaminated. I removed the top bolt and ground the end of the shaft to remove the chamfered lead-in around the screw thread. Then de-greased the threads and screwed a longer stainless bolt in all the way, with super glue applied to the threads. The excess bolt sticking out, was then cut off with a hacksaw and the end shaped to a dome on a fine grinding wheel. I used progressively finer grades of abrasive paper to smooth it and finished with some metal polish.
When completed, the bolt down the center just disappeared and in my opinion it looks much better. Records self center on the dome and slip on to the spindle much easier now.
It's a minor thing, but it improves the overall experience of using the deck.



I decided to make a bottom bracket to steady this very long bearing.



A 3mm thick Stainless Steel plate, fixes across the motor well, on the underside of the plinth.
A modified 30mm diameter "C" clamp, attaches the bearing to the plate using a pair of M10 Stainless bolts.
This bottom fixing will make the bearing absolutely rock steady and should help couple the thrust pad to the mass of the Plinth.







Test assembly of the bottom bracket.
The top mount was fully tightened, then the bottom bracket was assembled. I left enough wiggle room in the fixings, so it could be aligned and tightened without applying any side load to the bearing.
There's a 32mm diameter hole in the plate, which the bearing end cap pokes through. In theory the bearing could be disassembled, re-greased and put back together, while still in situ on the deck.






I assembled the speed control rod and slide mechanism.
I added an additional nut on the back of the adjustment knob, which only allows it to be released by a couple of turns.







A router was used to form a channel for the speed control rod to run in. It was also used on the centre slab, to form the passage way for the switch to motor cable. This channel will be hidden when the bottom slab is glued over it.
The other passageways, for the mains cable and tonearm cable, will be drilled from the rear face after all three slabs are glued together.




Brake

It's reported that with an upgraded bearing, the platter can take a couple of minutes to come to rest after the motor is switched off. This makes changing records difficult and the thought of stopping several Kilo's of platter with my hand, detracts, from what is otherwise, a high end component, so I decided to make a brake.
I don't have milling or turning equipment so the trick was to design something that can be made from stock profiles, using a hacksaw and file.



I ordered some "T" section Aluminium, 2.1/2" x 2.1/2" x 1/4" and cut my shapes from this..






The pivot has not yet been drilled for the Stainless Steel pin, but you get the idea, a simple see-saw acting on the underside of the platter rim.






The friction material used is PVC hose, it's cheap and readily available.
A similar, but much smaller tube, is used on the stock GL75 brake. It's not very effective, due to the very small force applied by the spring, in its automatic application.
The end profile of my lever, was shaped to accept, ID 12 x OD 15mm tube, as a nice tight push on fit. When it wears out, I'll simply remove the platter, cut a new piece of hose and slip it over the end of the lever.






The cut out for the brake.
The blind hole on the left is for a return spring.





And the brake installed for testing, it worked better than I could have hoped, bringing the platter to rest almost instantly.
I don't have a CAD package, but I knocked up this sketch which is what I worked from.






I plugged all the threaded inserts with Play-Doh, to stop the glue going up the threads.





Finally the three slabs come together, I used hide glue to stick them.






I made an output plate for the 4 pin XLR and grounding post.
The plate is 3mm thick Aluminium and sits on the back face of the plinth, in a 2.5 mm deep recess, machined into the Bamboo.







The machining/sanding was completed and inserts fitted.
I used a Router to add a 5mm radius around the top and bottom edges. With a 6mm half round bit used to create a channel on the underside.
I gave it a final rub down with 400 grit and applied a generous coat of Danish Oil (Natural) with a lint free cloth.
This brings out the colour and seals the surface ready for the Polyurethane (PU) varnish.
Danish Oil is a mixture of Tung Oil and around 20% PU varnish, so it penetrates then sets.
Three coats of Clear Gloss PU varnish, thinned with pure gum terps, were applied by spray gun. I de-nibbed between coats and the 4th and final coat was using Clear Matt PU varnish. The Matt varnish is not as clear, due to the matting agent used, so you build in Gloss and save the Matt for the final coat, this gives maximum transparency to the finish.







On the underside, the channel I machined close to the edges, forms a boundary to paint up to.
Satin Black Acrylic paint was used, 2 coats applied by brush to the intricate areas and using a small gloss roller on the flat bits. The 55mm diameter tilt/swivel feet, will eventually have 3mm thick Sorbothane pads stuck to their bases.
This is a bit of an experiment, but I can fit any M8 threaded foot or spike.





Motor




The motor, as removed from the original deck, was dirty and ran a bit of noisy.
The cone drive cleaned up with some fine Scotch-Brite plastic abrasive, I applied this while the motor was spinning. It was dirty rather than corroded and has no groves or dings in it.
I then completely disassembled the motor.
The internal workings were dusted off with a dry tooth brush and the chassis was cleaned using a tooth brush soaked in white spirit. The bearings were removed and all parts cleaned in white spirit. When re-assembled, the floating Bronze bearings had grease applied to their seats and a drop of sowing machine oil was applied to each end of the spindle when in situ.
Once reassembled, I adjusted the running clearance on the nose end. To do this, it's best tackled while the motor is running on a bench. If you do this, be careful what you touch, as it's mains voltage. Slacken off the lock nut and carefully wind the adjuster in until there is a slight change to the sound, a roughness to it, then back off the adjuster by 1/8 turn (45 deg).
Re-apply the lock nut, while holding the adjuster firmly in position with a screw driver.
I mounted the motor on the PTP plate and hung this in its location on the plinth, it now runs completely silent, I had to touch the spindle to check it was running as there was no audible or visible sign it was working. Completely noise/vibration free and ready to fit.




Platter

I cleaned the platter that came with the deck, but I was never comfortable with the amount of drilling which had been needed to balance it, which also looked poor.






I purchased a platter off "the Bay" which had only four drilling's.
Someone had lacquered it and it had yellowed over time.
I removed the old lacquer, gave it a clean and re lacquered it.

That's everything ready for assembly, see pictures of the finished deck here.