NAD 5120 or Tesla NC470

Posted by : S

Sabtu, 29 Maret 2014

Introduction

I got back into vinyl records via my son, who started to collect LPs and i decided he should get a turntable as a surprise Christmas present back in 2011. I was an avid audiophile in the late 70s and my father had a great system,including a Thorens deck. With the arrival of CD, I whole heartedly embraced that format and hadn't owned a deck since around 1981. I decided that a decent but cheap second-hand deck would be ideal for my son, it wasnt a main present, more a novelty to see if he liked the format, so i didnt want to get a more expensive Thorens or Rega, brands i remembered from my past. Why my son had started buying vinyl without a deck I'm not sure, I think he liked the aesthetic and tactile element of LPs, which i completely understand.

Looking on ebay, many of the decks i remembered were going for high prices second hand, and in the end i bid for and won a humble NAD 5120 . I looked for decks close to where i lived as i reasoned that without the original packing, delicate turntables wouldn't travel well. The price was good, around �20 i recall, and the seller was quite close to me, so i collected it. I had a vague memory of the NAD having some strange flat arm, and looking unlike other decks. I also recalled visiting Brighton around 1980 , and popping into a Hi Fi shop ( when such things were common), which stocked the NAD brand. The salesman was talking to a customer about the new wonder-amp, the NAD 3020, which was getting rave reviews and being coupled with very high end turntables including the Linn Sondek LP12. The salesman, as i recall, lowered his voice and said that NAD were going to bring out a Linn-killer deck for �100, and it was being built "behind the Iron Curtain", hence cheaper production costs, and the customer should delay any purchase til it came out.

It was a long time ago, but it stuck with me as I remember thinking "good" - Linn were the darlings of the magazines and their dealers were zealots who rubbished anything which wasn't of that brand. I hated them.

Fast forward to December 2011 and I had a NAD 5120 , purchased secretly as a surprise for my son. I needed to test that it worked. I had an A&R Cambridge A60, which had a dedicated DIN phono input, and a pair of RAM 200 speakers, both of which belonged to my father and I helped choose with him. the speakers were made around 1980 by RAM, a long gone but fondly remembered British speaker manufacturer from Norfolk

The deck came with an unidentifiable cartridge so i ordered a budget Audio Technica AT95 on-line.

I'd heard the deck play when i collected it and it seemed OK though there was a little hum.

Its Czech , and its a Tesla !

I was immediately struck by how different this deck was in design. Substantially

plastic, though its substantial plastic, with a very strange flat arm featuring a balance weight on a spring with an oil damping container on the arm; a lid with no end pieces, just high side "cheeks" on the plinth, and a strange platter that seemed to also be a mat. All finished in the NAD dark olive green . This truly was unusual, like a Citroen 2CV is an odd car.

I did some research on the web, mainly the wonderful vinyl engine web site and its associated forum, which has many friendly and helpful members.

I discovered that the deck was in fact a Czech design and in fact was simply a NAD re-badged version of a turntable widely sold on mainland Europe as the Tesla NC470, built in the Tesla Litovel factory in what was then Czechoslovakia. The designer was Jir� Janda , about whom I could find very little, but some of his patents are here

He developed a number of audio products under the noses of the authorities in then Soviet-era Czechoslovakia through Audio clubs where members effectively made amps and equipment as part of the club. It also appeared that many of his ideas went into the deck, built by Tesla, who perhaps were a distant relative of Pro-ject, one of the few remaining turntable manufacturers in Europe. It was intriguing and part of the purpose of this blog was to try and find out some more history of the development of this deck. Perhaps someone out there knew Jiri or worked on the development of this deck, if so I would love to hear from you. I did find out that Jiri died in 1993, a short obituary can be found here , and a few pictures of him exist on the web and he looks a very kind man.

So i guess that the old saying that, necessity is the mother of invention, was true with this deck. I have come to appreciate just how clever the design of this deck is, and I guess that , where a material,technology or method was unavailable , a clever alternative was figured out. That flat arm, with its anti-resonance balance weight being a case in point.

I also discovered that while NAD actively promoted the deck as having a floppy arm, they later appeared to loose faith and reverted to a more conventional round arm for the deck.

( Actually the arm is not floppy, its made of a ruler shaped piece of circuit board, and while it can be flexed , this is not normal, indeed the adverts NAD had showed it being bent over. do not do this as it will do no good to the copper tracks along the flat arm).

So you can see examples on ebay with both a flat arm and a round tubular arm. You can even swap arms as the arm simply plugs into a 4 hole socket which also forms the vertical bearing. Over time I have had a number of these decks pass through my hands in addition to the original flat arm example, as i have acquired spares and repairs decks to try and get them working.

Variations on a theme

Im not sure what the start and end dates of the production of these decks was, id guess from the early 80s to maybe the late 80s. Variations i have encountered are :

Original flat arm model with orange bulb at front. Inside a DIN socket plug is used as a solder junction between the fine wires from the arm bearing to the signal cables. Sub chassis can be levelled using plastic washers as shims above the primary suspension springs and their rubber bushes.
As 1. But with a round tubular plastic arm
As 2. But the orange light bulb at the front is replaced with a Red LED and one less wire is needed to provide power to the front light.
As 3. But additions include no longer using a DIN socket as a junction, but has moulded turret tags at rear right of deck. Also suspension springs can be adjusted via screws from below. Improved grounding arrangements in arm pillar reduce hum using a staple shaped metal insert to ground the arm pillar through its horizontal bearing.

In addition to these , i have found some minor differences in the arm and spindle bearing arrangements. Some examples sit the bearing on a tiny blue metal disc, size a hole-punched chad and housed inside the base of the brass and black plastic bearing sleeve assembly, cleverly common to both arm and spindle . Others used a nylon disk, and others no disk at all. Blue metal disks in both bearings is best for ground continuity.

One example I had had a staple-like metal link inside the arm pillar to improve the grounding from the external arm pillar metal case, via the arm horizontal bearing to the sub-chasis.

As far as im aware the only difference between the NAD and Tesla models was the badge and maybe the colour of the paint used ? not sure if Tesla sold the deck under their brand before NAD got involved or after, I do know that if you attempt to clean the plastic upper chassis of a NAD unit with alcohol, the olive green paint comes off revealing a more metallic grey underneath. I wonder is this was the original Tesla colour, and NAD just resprayed them ?

There is another variant which was sold as the Lenco L800, which looks to be a flat arm unit except that the upper half of the plinth was made from wood rather than plastic. I've only seen pictures of this, never seen one, but would be tempted to get one.

Generally the decks hold their value well on ebay, perhaps partly due to the NAD brand, with the tubular arm models occurring more often than the flat arm. In the UK you can get an example for around �40-70 depending on condition. Indeed honest sellers often advertise them as non working due a number of common problems :

Speed selector not working. The selector doesn't cleanly nudge the belt to the correct part of the motor pulley
Automatic function doesn't work. The Deck has a simple auto raise and turn off feature at the end of a side, this seems to work on some decks and not on others. Its nicely done and doesn't appear to interfere with the functioning of the arm either way
Motor randomly turns either way. This is sometimes due to a bad phase capacitor. If the deck always turns the wrong way or has trouble holding speed, it could be a bad motor winding.

What is inside ?

First disconnect from the power, and i mean disconnect at the wall, don't rely on the power switch at the front, as the motor is always partly live.

So getting back to the run up to Christmas 2011, when i finally fitted the cartridge, aligned it, and tested it in secret, I had music through one channel and loud hum through the other. If this was to be a success , I needed to figure this out.

Despite the decks largely plastic construction,its well made, the plinth is in two sections , held by 3 Philips screws. I have taken my decks apart in excess of 20 times and not stripped any threads. I would say the plinth is less resonant than other plastic decks from Dual and various cheaper Japanese Direct Drive models.

So to open the deck up you first remove the lid, which slides out of the two rear hinges. The hinges pull out of their slots. Next you can remove the whole arm, it pulls away from the pillar at the point where it pivots. This requires some care , and if you have a stylus plastic guard or cover, its worth applying that first.

Next you can remove the mat which is rubber with an inner plastic insert which has one of those 45 adaptors for DJs in the middle. The mat is unusual in that its also substantially the platter. These mats often discolour becoming a snowy grey colour, but washing and cleaning with platanclene rubber restorer will bring them up like new. Beneath the rubber mat-platter is a simple pressed steel sup platter which has an upward facing lip which engages in a trough on the underside of the mat.

The painted steel sub-platter comes off revealing another sub-platter made of plastic which has the spindle passing through its centre. This looks similar to sub-platters found on Pro-ject and Rega decks. Between the two sub-platters sits a humble and unnoticed copper foil washer which helps to provide a ground between the steel plate and the spindle. You notice the paint is removed so the washer makes a good contact.

The plastic sub platter can be removed , the rubber belt fits around this. Pulling the sub platter out by the spindle reveals the main bearing, which is narrower than the spindle. Many decks these are the e same or larger diameter. As with everything you remove ,these pieces should be placed somewhere safe and ideally soft, as the main bearing should not be scratched or damaged.

Now you can remove the upper part of the plinth as you should be able to clearly see 3 Philips screws around the circular central recess. Once removed and safely stored,you have the problem of separating the upper half of the plinth from the lower. Gentle steady pressure with the fingers through the circular recess into the base i find works. Gently ease the top plinth away from the base. Eventually it should come free, and again place it somewhere safe.

At this point you should be looking at the black plastic base with all the wires , levers , springs and metal sub-chassis

At the back left you have the AC synchronous motor, held in place by two brackets which also act as guides for the speed selector contraption, lets call it a sled, as it slides. AC motors spin at a single speed, and the tiny aluminium spindle connected to the motor sets the platter speed. The belt is guided between two diameters of the pulley when changing speed, rather like the de-railer gears on a bike.

Centrally you have a metal sub-chassis which sits on 3 large springs via 3 rubber mounts. To the sub-chassis is attached all manner of bits, but the main things are the main bearing housing, that the spindle sits in, and the arm pillar, a fat circular metal tower on the right. It rotates with the arc of the arm, limited by a screw in a slot at its base. The sub-chassis normally 'floats' when the full platter system is present due to the compression of the springs. With the platter etc. Removed the springs force the sub-chassis up, but its limited by a locking wheel located beneath the deck. You can tighten this when shipping the deck to restrict the movement of the sub-chassis, loosen when its in its intended location.

There are two sets of wires in the deck. Power to the motor which is mains i.e dangerous , and signal wiring from the cartridge which are not dangerous. The mains cable enters at the rear of the deck and snakes around the partitioned wall to the motor, with further wires passing around the front of the deck to the switch at the front. Close to the motor is a silver phase reversing capacitor. Its function is to start the motor right direction as AC motors can turn either direction. A second suppression capacitor is located at the front by the mains switch and light bulb. The deck has no safety earth, but this is not uncommon and does not mean the deck is necessarily any more dangerous than any other household appliance, assuming its not been tampered with or modified.

The tiny thread-like wires emerge from the base of the arm pillar. These carry the signals from the cartridge via the arm. These wires are extremely delicate, as , by necessity, they are thick enough to carry the signal but thin enough to not impede the movement of the arm. These connect to the main signal wires either using a DIN socket as a junction, (no idea why ? Love to know, maybe for easy testing in the factory ?) or in later models via some solder tags moulded into the deck base. Conventional signal wires take up the audio story and snake around the back of the arm and snake around various partition mouldings in the outer wall of the base moulding.

A ground continuity wire also emerges at the same point from the deck, its anchored to one of the three screws at the base of the arm pillar. Its role is two fold, to ensure ground continuity from the deck to the preamp to avoid hum loops, and also to ensure electrostatic continuity from the spindle to a static point. Phono sections on amps usually have a binding post to attach this wire to.

There is a mechanical linkage from the front switch to the arm raiser and various springs and pulleys, some of which provide anti skate from the arm support arm, others provide the hit or miss auto raise function.

Now lets jump back to my sons deck. its now December and the big day is approaching and this deck is far from working. I have hum only on one channel . I decided that i must have an issue with the arm wiring, and gently tugging the coloured wires emerging from the base of the arm revealed that the blue one was not connected to anything.

I'm quite a practical person, and have some modest skill at soldering so I'm not too intimidated by these issues.

I realised that i needed to remove the arm pillar, and its held captive by a bolt in the curved slot at its base. Once this is removed you can carefully slide the whole arm pillar off its bearing. This reveals the same brass and black plastic bearing sleeve as the platter spindle uses. Neat, never seen this approach taken anywhere else. This is fiddly work as the heavy, substantially metal arm pillar is now only captive by 4 cotton thread-like wires, or in my case by only 3.

The wires solder to the base of the plastic socket the arm plugs into. The socket has two spikes which emerge from either side and sit in two cupped grub screws accessible from the outside of the arm pillar, forming the vertical needle bearings. Loosening the grub screws, only a little and the same amount on each side, eventually lets the socket piece loose of the top of the arm pillar and you can get at the solder tags on the base. However there is very little slack and these wires are hard to work with. I couldn't resolder the blue wire and ended up making things worse.

In the end i decided to run 4 new wires from the socket base to new RCA sockets which id fit to the back of the turntable base. Trouble was which wires to use. In the end i used computer mouse wiring which is very thin but not too hard to work with. This worked but was still a bit stiff for my liking , causing i felt some tracking issues with the arm, so in the end i got some fine Litz wire from ebay especially for arm rewiring, which is finer. This is all very fiddly and i have to say that unless you have a specific issue with the arm wiring , id leave well alone as :

The wires are fine and the soldering is hard. If you overheat the tags on the base of the arm socket you could deform it.
Loosening and tightening the side grub screws for the needle bearings is hard to judge. Too tight and the arm doesn't move freely, to loose and there is arm play.

The only other thing i have done on some decks is to lubricate the motor bearing. Again i emphasise that these decks carry live voltage, so only attempt this if the deck is disconnected and you are sure you can safely work with such devices.

On two decks i had noisy motors. The motor is clamped in place but what appears to be a black plastic thrust bearing protrudes from the base of the motor and through a hole in the base of the deck. Several of the decks I've seen have had the black bearing cap mangled. I assume owners have either :

Tried to remove the plastic cap with pliers. Don't , its doesn't come off from the bottom, you have to open the motor to get at it
Forced the motor stator down to try and reduce speed changer noise and unwittingly popped the tiny thrust ball bearing out through a tear in the base.

The correct way to clean and re-lubricate the motor is to remove the motor from the base, by removing first removing the motor spindle which is held in place by a tiny diagonal grub screw from the top, then remove the speed selector frame and its guides. They are held in place by two screws which pass through tags on the sides of the motor. The wiring from the motor has some opaque silicone sleeves to protect it .

If you remove the motor, beneath is a small circle of yellow foam rubber which provides some modest decoupling of the motor from the base.mi have tried replacing this with sorbothane rubber and got much more noise, so stick with the original foam, but it might benefit from a wash in warm water And a gentle dry in the sun.

Once you have the motor out, still captive by its wiring you can obtain more slack by removing the clip-on black plastic box section which protects the motor wiring. With now adequate slack, if you examine the seam around the edge of the motor there are 3 small slots into which you can get the blade of a screw driver into. By gentle and progressive twisting action you can slowly prise the two clam shell halves apart. The trick is gently and do a small amount of twisting in each slot. Too much in one slot and you'll warp the motor shell. Gently and slowly is the trick. After a while the two parts separate and you can access the central stator, and pull it clear of the two halves.

The lower spindle engages with a lower thrust bearing. That black plastic thing is a tiny well with a ball bearing inside.. You don't need to remove the plastic piece from the motor, but i found i could twiddle a wooden tooth pick around inside to remove dirt, then soft rag twisted in. Finally a small grain of rice sized blob of silicone grease. The upper bearing is a simple collar bearing and again a little silicone grease is fine, but make sure the rest of the stator is clean. In all the motors I've seen the motor is very clean inside.

Putting it back together her is a case of reversing the process. The two sides will only go one way with the wires together. Once in position gently squeeze the two halved together with grips, gently and again, a little rotate, a little rotate, a bit more, till the two halves are tight together again. This motor self centres, so you should find it spins freely after this process.

Much of the above is explained in a video I made of the repair of a noisy NAD motor, including the disassemble of the deck