Thorens TD125 mk2 Resto

Last week I picked up a second Thorens TD125 mk2 from ebay. My existing TD125 is still in very fine fettle but I reasoned that it would be worth getting a second for spares before prices go up.

In my opinion the TD125 is a much underrated deck and tends to be be forgotten a little in the shadow of the TD124. Prices for the TD124 have gone sky high and I think that it's likely that this trend will start to drag up the prices of the other top end vintage Thorens decks such as the TD125 and TD126.

In the ebay listing it was described as "Spares or repair, no return" (you know the sort of thing). However, it was super cheap with a 'Buy it now' of just £65!

At this point I must say a quick thank you to the seller Significant Sound who packaged the deck extremely carefully and even refunded the difference when the postage came to less than the quoted £25. Superb service!

I was expecting a complete wreck but when the TD125 arrived I was pleasantly surprised. As described in the advert it came without a plinth or tonearm but it was basically all there and even had the little 45rpm adapter widget.

The outer edge of the platter had been lacquered at some point and was badly corroded underneath the old lacquer and the other bad news is that the speed control circuit board was cracked. The power cord had been cut and little metal leg which allows you to stand the deck on a flat surface while out of the plinth was missing.







All things considered a bargain for just £65 (plus postage).

 

I'm pleased with the repair which I managed to make to the circuit board.

Firstly I used a strong elastic band to 'clamp' the PCB together and squeeze the crack tight. I then applied bead of araldite to the top surface to make the area structurally sound again.



Once I'd done that I carefully scraped away the enamel either side of the crack on the PCB copper traces and then with the help of a flux pen soldered a bridge across the join.



After doing this the turntable ran! Here I'm setting the voltages for the first and second motor phases together with making adjustments to the fine speed control. I do this maybe once a year on my other my other TD125 and it can make quite a difference to the smooth running of the motor.

 

The speed control circuit board has two large 1000uF axial electrolytic caps for the power supply. These tested well within spec so I have left them in for now.

I managed to get the deck running at 33rpm and 45rpm but with 16rpm selected the motor would just vibrate back and forth or will just about rotate with a very jerky motion but won't turn smoothly.

I though that maybe the the contacts on one of the potentiometers used to adjust the motor phase voltages looking loose, as if it wasn't in firm contact with the horseshoe shaped pad which it sits on. Foolishly I tried to bend the contacts down with a pair of tweezers. That turned out to be a stupid idea as the metal was incredibly brittle and the contact snapped clean off. I don't know if maybe it was already cracked?

So now I need to find a replacement for at least one of these big old potentiometers. If I can find a suitable more reliable modern replacement I may replace them all for good measure.

Can anyone help point me in the right direction for a suitable replacement part? All of the modern potentiometers seem to be much much smaller. The are 500 ohm BTW.



Pound coin for scale.

 

Never mind, I've found some suitable replacement potentiometers and I've just ordered them.

What do you think of those tube shaped 200pF capacitors (the ones next to the IC's)? I haven't got a clue what they are but they look pretty grotty, would it be worthwhile swapping them for a more modern component, maybe a polystyrene axial?

 

Nice find!

I didn't realise these were so complex internally.

Those caps are definitely going to be film types at the pf level - very likely polystyrene. Just be careful with the soldering iron tip as this type of cap doesn't cope well with heat on the leads. Should last forever and not degrade, and since we've no signal path consideration here I probably leave them.

 

Hi Rob,

When I re-read my thread I realised that it wasn't very clear. Just for the record it's these caps which I was questioning the condition of (indicated by the biro tip).



Weird looking things.

Did you realise it was these ones which I was talking about?

By the way, I'm currently listening to a Leak Stereo 70 amp from 1972 and it sounds rather good to me. I think I'm in danger of joining the glowy dial appreciation society*.

*Yes I realise that the stereo 70 doesn't actually have any glowy dials.

 

Ah, I was looking at the component to the right, which in the other pic looks like a polystyrene cap.

Never seen a cap like that before, but I doubt it contains anything prone to drift.

 

The good news is that the new potentiometers fit perfectly and look much smarter than the old originals. The bad news is that they haven't fixed it.



The turntable will run at 45rpm and 33rpm but will not run at 16rpm. When 16rpm is selected the motor just vibrates back and forth but does not rotate.

Here are the instructions in the service manual:



I can carry out the voltage adjustments for phase 1 but I cannot get any sensible readings when trying to adjust phase 2. Turning the trim pots when trying to adjust phase 2 doesn't seem to do anything.

I'm going to need some help with this one!

Here is the circuit:



Any suggestions?

I have another TD125 mk2 in perfect working order so I can take readings from the good one if it's helpful.

Thanks in advance.

 

Woohoo! I've managed to fix it all my own little lonesome

Transistors T1 and T2 turned out to be defective. Also the speed selector switch needed a really good clean.

I've been able to set the phase 1 and phase 2 motor voltages and the speed control voltages and it's now running well at all three speeds with the strobe markings nice and static.

To test the circuit I just some generic plastic cased (TO92?) NPN and PNP transistors at locations T1 and T2, just what I had in my spare parts bin. The originals have a metal can type construction (TO39?) and get quite hot. I'm not sure if the small plastic ones which I have just fitted can cope with the heat so I'm going to swap them out for original specification metal capped devices before leaving it switched on for any length of time.

The original component at T1 is a 2N2218A and T2 is a 2N2904 PNP.

Farnell still stock the 2N2218A. They don't have the 2N2904 but the website suggests 2N2905A as a suitable replacement.

I'll order the parts next time I'm getting a minimum carriage paid order together.

A few years ago when I purchased my other TD125 mk2 second hand I remember that I didn't even own a soldering iron and I had to pay someone to repair the speed control circuit for me. I'm still only a newbie beginner but it's extremely satisfying to be able to fix this one myself. Pretty much sums up what D.I.Y is all about for me



Time to go tonearm hunting......

 

Excellent work.

Mission 774.
Has it all - low inertia, low friction, underslung CW, play fee bearings, rigid structures, fluid damping, full adjustability, robust and will take just about any cartridge without a captive thread.

 

Funnily enough I was actually looking at those.

To be honest I don't really need a tonearm as this deck is only supposed to be a spare and my other TD125 has an SME3009 (type 2 imp fixed headshell) in superb condition.

I don't have enough space to have both decks hooked up at the same time. I'll keep an eye out for an arm and if I find a bargain I'll snap it up.

I could always install the wider arm-board from a TD125 LB and fit an SME 3012.........no, mustn't start thinking about that.......must resist.....

 

How's the main bearing? The mk2's I worked on all had some platter rock missing in the mk1. A good few drops of trusty EP80 (more viscous than the turbine oil Thorens used - part fill as Linn do) seems to help here.

If the 774 has good bearings, I'd not use the over-stiff exit leads the later ones offered, but maybe change these for some ubiquitous Van Damme Pro-Patch two core mic cable, which works a treat (connect the outer shield at the phono's and use the two cores as hot and return, as Mission did on the original exit cables).

 

The main bearing is fortunately in great condition with no detectable play.

I wouldn't advise using EP gear oil in the TD125. Gear oils often contain sulphur based EP additives which can be harmful to the sintered bronze bearings.

I use fully synthetic air compressor oil in mine which contains no potentially damaging additives.

Thanks for the info on the Mission tonearms.

 

I cannot dispute your suggestion, only to say I've used EP80 with no issues so far (more than ten years) in turntable bearings, including Thorens. Even Linn used it in the early LP12's before the black oil became available (engine oil - Castrol GTX when I was there plus Molyslip...).

 

Yes the original cable is far too stiff for suspended deck, and it also puts too much stress on the connector. Some nice flexy low cap mic cable works a treat. I use a short 0.75m length on mine for a total of 70pf for the arm loom, inc internal wiring.

 

Older gear oils often contained Sulphur or Phosphorous based extreme pressure (EP) additives which are well known to be corrosive to copper containing 'yellow metals' such as bronze.

Modern gear oils of the type used in manual car transmissions usually contain a less aggressive form of Sulphur EP additive which is not corrosive to yellow metals at low temperatures. This is why you can realistically get away with using a modern GL4 gear oils in a bronze TT bearing without any noticeable problems in the short term.
However that doesn't mean that they can be considered safe to use.

The EP additives bond to the hard steel gears creating a sacrificial coating which prevents metal to metal wear of the gears. Problems can arise when a hard steel surface in in contact with a soft yellow metal bearing. What happens is that the EP additives form a coating on the yellow metal bearing. As this coating is worn off it takes with it a microscopic layer of bearing and so causes erosion of the bearing surface.

Gear oils with a higher number rating usually contain higher concentrations of EP additives so a GL5 oil has more of these additives than a GL4 oil.

Engine oil would be a safer bet for use in a bronze TT bearing. Car engines have bronze bearing surfaces (e.g. valve guides) so obviously engine oils need to be safe to use with yellow metals. However engine oils also contain other additive packages such as detergents which are at best of no use to a TT bearing and at worst may actually be undesirable. However given that most of us have easy access to engine oil it would be my default recommendation for the average owner.

An even better choice would be a high quality oil of an appropriate viscosity manufactured from a high quality base stock and containing no additives which may be considered harmful to a bronze TT bearing. Good quality air compressor oil would fit the bill perfectly.

If your TT bearing is a little worn it may be beneficial to use a higher viscosity oil than originally specified by the manufacturer. You need to aware though that comparing oil viscosity ratings can be a little confusing though. For example an 75w gear oil is actually about the same viscosity as a 20w engine oil.

Here is a useful comparison chart:



Previously in my own TD125 I used to use a very high quality 20w50 engine oil designed for racing classic minis however more recently I have swapped over to a fully synthetic ISO 46 compressor oil which I believe is pretty much ideal. If you feel a thicker oil may be better then an ISO 68 compressor oil may be a good choice.

I hope I win some sort of award for the most tedious post ever for this!

 

Wow, that's interesting! Yes really

How do you know so much about oil?

 

I'm just a bit pedantic about such things!

I've been maintaining/repairing/restoring mechanical things like bicycles and cars for years which has taught me to really appreciate how critical correct lubrication is. That combined with my sightly OCD tendencies means that I'm the kind of person who will read about things like oil, greases or ball bearing grades purely out of curiosity.

In researching the best oil for my Thorens deck I spoke to the tech departments at several specialist oil suppliers (Millers, Morris, Pennine). A friend of an ex-colleague of mine works at Pennine oils and that's how I managed to acquire a litre of fully syn ISO grade 46 oil. Often such oils are only available to industry in big drums costing £100's.

I should probably buy a load of tiny vials and sell them on ebay as super duper special turntable oil with a massive profit margin.

 

You should!

Great tech info on oil choices for t/t bearings; that sort of thing should be stuck up in the reference area.

I was aware of the risk of sulphur content vs stripping the zinc out of brasses, but not the finer detail. Thanks.

 

Definitely.

Mike, please copy and paste that post into the reference area - 'articles' is probably the best sub room. I'd rather not move the post since it will interrupt the thread, and if you copy it there yourself it remains under your name as author.
Reference threads are locked so I can lock after it's posted.

Cheers,

PS: I've had a ltr bottle of EP80W-90 Hypoid Light gear oil for years and that goes into everything - which is probably wrong!