A quick visual guide to externalising PSU from your Technics SL 1200

[Frankyc2003]

Good afternoon everyone,

After doing my home work and getting delivery of an upgraded Inspire Hifi platter, it had become essential to externalise the PSU as the plastic cover was well in the way.

Sonddek from PFM did a great post here http://www.pinkfishmedia.net/forum/s...d.php?t=101356
on doing so. A great walkthrough and very efficient at getting the job done.

So I am not wanting to redo what he did, just merely doing a longer step by step for people less DIY minded (like me!).

Usual precautions should be taken, and as Sonddek said himself SAFETY FIRST! Electricity isn't a TOY!

''I advise anyone who is not very clued up about electrics NOT TO DO WHAT I'VE DONE HERE, as there is plenty of scope for killing yourself or burning your house down when everyone has gone to bed.''

Onto our first picture, our shopping list from MAPLIN.

All in all, it costed 26 GBP! In time of recession, you won't get any cheaper for an external PSU!
Tools required are your box standard set of screw drivers, pliers, cable cutter and stripper, a 30W soldering iron, some good quality solder, a bit solder wick or a solder sucker! A hammer, some blu tak, a fair bit of patience and more essentially a clean and well lit worktop!

The main challenge when opening up your TT and revealing the motor PCB and its satellites, nominally the Traffo, the Regulator, the PSU PCB, is to understand where the power comes from and where does it go to... So basically the IN points in each PCB...

So, first we need to isolate the PSU and Traffo from the mains.

In this picture, we clearly see the black mains cable being soldered to the 2 AC posts. We just need to desolder them and clean them to remove any old bits of wire. This is the IN point for our PSU PCB.

Then onto the IN point for our Motor PCB.

These 2 posts are basically where the current enters the Motor Board. Just unwrap the cables carefully, there is usually no solder there. So an easy one.

Next is just a little bit of tidying up.

We need to unsolder that thin blue cable, because further down the line we shall be soldering our very own cable at that point of the PSU PCB. You can either keep it, or discard, but we won't use it...

OK, before we can completely remove the Traffo and the PSU PCB, we need to disconnect the ON/OFF switch (the on located on top of the Strobe turret).

Here just isolate the 3 thicker cables coloured Blue, Brown and Red. Make sure you don't confuse them with the smaller gauge ones which are feeding the speed switches, the light turret, the strobe turret and the pitch slider.
Just cut those 3, tidy them away to avoid any contact with moving parts.

At that point we can safely remove the screws holding the Traffo and PSU PCB. And lift the 2 units out of the Turntable.

2 things need to be done at that point, first we need to connect the remaining bits of the Brown cable to the remaining bit of the Blue cable. The reason for this is since we disconnected the ON/OFF switch we have left an open loop on the PSU PCB, by soldering these 2 together we are basically resolving this problem. To prevent any shortening, cover the soldered end with a neat bit of heat shrink.
Secondly we need to remove the loose end of Red cable. Just unsolder it. It's good practice, but has no electrical impact on the modification.

Moving on swiftly to our next set of mods...
So, now we need to attach the newly released parts to our new casing.

Just roughly position both parts onto the base, and use a very small phillips screwdriver, position it through each intended hole and tap with a hammer to imprint the correct position for drilling later on. Just to be clear, the Traffo needs 3 holes, the PSU PCB needs 2. The PSU PCB has got a little screw printed next to the correct holes (as seen in my first 2 pix). DON'T attach screws to any other holes as you might connect to a live part of the PCB!

Onto a little bit of drilling,

We don't need tremendous power and speed to drill through the plastic case. A normal drill bit for wood will do, and just make sure not to drill at too fast a speed, slowly does it! Secure the pieces of plastic with blu tak on your work top. And off you go! We are using M3 screws to secure Traffo and PSU PCB, so a 3mm drill bit will do a beautiful job.
Don't forget to drill also the lid with one hole at each end for Power cables to enter the casing...

To secure the PSU PCB you will need to add a couple of bolts and discs. (All included in the Maplin pack listed at the beginning).

Excuse the really crude drawing, just to illustrate the idea that the PSU PCB shouldn't rest directly on the floor of the new casing.


Now is time to start reconnecting things together.

In this picture we first reconnect the mains black cable to the 2 posts we desoldered earlier on. See ref picture. Then we need to feed the Blue cable, (from the new black cable connecting our case to the TT), to the PSU PCB. Here you might have to drill the entry point a bit as it could be a little tight. Use a 2mm drill bit at very slow speed, making sure the board is well secure.
The other strand form that cable connecting the PSU to the TT should be Brown and we need to tie it to the Traffo yellow cable left loose earlier on.
A bit of heat shrink, and cable zip tie to leave things real tidy here...

Then onto our final bit of reconnecting, our newly created link between PSU and TT needs to be attached to our 2 poles left open earlier on.

Make sure you use just enough cable here, as you wouldn't want a loom of unwanted power cable lying beneath the spinning platter!

Here is a wider pix.

I have added a little ferrite ring here for ultimate RFI rejection... I am not sure it will do anything, but it kind of helps attaching the cable at the bottom of the TT.

And That's It really, after that all you have left to do is correctly dress cables with zip ties, making sure nothing is loose, or could get potentially loose...


Well, I hope this will help a few more people getting a really cheap and effective modification to this beautiful turntable that is the Technics SL 1200!!!

Enjoy!

[Wakefield Turntables]

Excellent. I like the idea of getting some small ferrite coils under the hood. Why didnt I think of that ???

[OneyedK]

I don't know if the pictures are from your deck.
But if so, while you're at it, replace C1 with 1000µF/50V.
Have a look at the underside of the pcb, as you will notice, solder pads and holes are there to accommodate an axial cap, as the radial version will be too tall.
A lot of decks are equipped with a 470µF/50V, while the schematic still states 1000µF (the correct value).
No wonder that a lot of people claim impressive results with aftermarket psu's.

[Frankyc2003]

Ah! Wonderful idea...
It is my deck indeed, I did look at the service manual for values, but this one clearly was hidden right under my nose! I can see now the axial holes...
Thanks for the tip!

Don't you love a well engineered piece of equipment!

[OneyedK]

Don't you love a well engineered piece of equipment!

Oh yes I do.
But I think Technics missed an opportunity to redesign the pcb after the mkII.
Some (important) traces on the pcb are very long and the whole damn thing is not properly shielded.
Imagine what they (the engineers) could've done with a double sided and properly designed pcb. In the 70's, it would have increased production cost, but in the 90's and later, it would have cost them next to nothing extra.

My board, still waiting for PH regulators to arrive and still with the "old" motor driver chip:

[joniakaidiot]

Great tutorial, Francois!
Thanks for sharing this information!

[Frankyc2003]

Morning all, just a quick update pic to show the new uprated cap in place as suggested earlier. One thing to note, I had to tweak the brake on the PCB to restore a nimble Start/ Stop response. Otherwise all good.
Oh and also just make sure you have good clearance between platter and new capacitor. Its a chunky one! Hence the tight fit!

[Wakefield Turntables]

Francois,

A man after my own heart, I had my pcb reapped as well. Just out of interest what differences are you expecting to see with the bigger cap?

[sq225917]

Ferrite half way down the cable means half a radiating cable, put it as close to the point where it enters the chassis as poss.

[Gromit]

Fantastic information Francois - thanks for sharing. Still not sure I'd tackle it myself (I just about know how to solder an RCA plug...and even then it's a bit hit & miss) but this half-whetted my appetite to give it a go on my 1210 which is saying something.