DIY power supply for Technics SL1200/1210

[jonners]

The KAB external power supply for the Technics provides a regulated 34V DC that goes to the internal diode bridge. The deck's own regulator on the main circuit board drops that down to 21V. I decided to make a regulated 21V supply and bypass the internal reg completely.
I downloaded the circuit diagrams from the Technics forum at Vinyl Engine, and saw that it would be possible, by cutting a wire link, to attach the new supply without having to remove the circuit board. (Referring to the page showing the Drive Control PCB, I cut the vertical link near the top of the board, just left of the word 'Technics', and connected the 21V DC to the lower part of that wire. The earthing point is at the bottom corner of the board. The page from the manual shows the underside of the board, but of course when you remove the platter and the plastic cover you will be looking down at the board from above.
Connecting the new supply in this way disables the strobe light and the cueing light. I've never had much use for either of them, so that was fine by me. If you want to keep them, it's not too difficult to find a different way of connecting it up. (See below - Post no.10). My way of connecting also in effect eliminated a supply bypass cap C3, so I attached another one.
My first impressions are positive. Not a dramatic difference, but more spacious and with a quieter background.

John

[Marco]

Interesting, John. Can we see some pictures of your work?

Marco.

[jonners]

Interesting, John. Can we see some pictures of your work?

Marco.

Sorry, haven't got a working camera at present.

John

[Marco]

That's a shame, John. Perhaps you know someone who does? The D.I.Y threads tend to work best when there are accompanying pictures which show work completed or in progress.

Interesting that you thought the external PSU didn't make a dramatic difference. Adding the KAB one to a 1210 in my experience (and others) certainly does - it quite simply transforms the deck and moves its performance to another level

Marco.

[jonners]

Marco - My listening impressions are at an early stage - and I must say I find that sort of thing difficult unless I can do side by side comparisons. It would be possible to create of way of switching between the two supplies but I don't think I can be bothered - too busy listening.

Regarding the possibility of pictures, I think that it's really preferable anyway for people to understand what they are doing, rather than follow a 'soldering by numbers' guide. After all, this project involves mains voltages and I wouldn't like anyone to frazzle themselves as a result of attempting to copy what I did.

Anyway, the wire link that I snipped can just be seen on the first picture in this sticky:http://theartofsound.net/forum/showthread.php?t=1020 It's on the side of the board that is nearest the back of the deck, near a white connector. The earth point can be seen in the next picture, at the corner of the board, below AC terminals 1 and 2.
John

[jonners]

As the new PS settles down my impressions are that the sound is improving further. I'm using the standard, unmodified Technics arm, by the way, with a Sumiko headshell and Denon DL110. Standard rubber mat, no other mods.

John

[jonners]

A bit of further info that may help anyone who is thinking of trying this: I measured the current drawn from the 21V dc supply. On starting and stopping the current peaks briefly at 400mA (450mA at 45rpm), and settles to a steady 170mA when the table is rotating.

I used an 18V AC (two 9V windings joined in series), 75VA mains transformer and a L200 regulator on a heat sink. A 1.5amp LM317 reg should be OK too.

John

[leo]

Wish I had a TT, plenty to keep a tinkerer like me busy

Seriously though I'm surprised theres not a lot more of TT diy on the forums to say how popular they are

Sounds like your having fun John, keep us updated

Leo

[jonners]

Wish I had a TT, plenty to keep a tinkerer like me busy


Leo

Well better get one double quick! If you're like me you may start to wonder why you bought so many CDs.

[jonners]

A few pictures coming up.

First, though, I very much now agree with Marco that the PS mod lifts the deck to another level. As I listened to a wider range of music and the new supply bedded in I was more and more impressed. Significantly more dynamic contrasts, more solid and deeper bass, more focus in the mid and treble.

Prior to taking the pictures, I briefly reverted to the original supply. (Yes, this mod is easily reversible.) It was like a blanket damping everything down and sucking some of the life out of the music. I won't be going back there again!

By the way, if the loss of the strobe troubles you, and you think you may lie awake at night wondering if your table still rotates at exactly 33.3333...rpm, fear not. The original supply still connects to the driver for the strobe leds, so you can have the tt rotating via the new supply and simply plug in the original supply and switch on as before to get the strobe working. So you get a free strobe disabler thrown in!

The first picture shows the vertical wire link near C3, which has been snipped.
The -ve lead of the new decoupling cap is connected to one of the test points. It is marked G.
The new PS lead will enter near the original mains lead. There is a convenient hole in the metal plate, which I have enlarged.

The second pic shows the +ve supply wire and the +ve lead of the decoupling cap soldered to the lower part of the snipped wire link. You need to make this joint fairly quickly, since you don't want the wire link coming unsoldered from the board.

The last pic shows the -ve supply lead connecting to the earth point at the bottom left corner of the main board. Abrading the surface of the metal lug slightly will help the solder stick. Blurry pic, but you get the idea...

It's fairly simple to do, but first do please check with the circuit diags. so you understand what you are doing.


John