Adding a PSU to a turntable

[cpvh]

Hi All,

As some of you are aware, I have an old Sony Midi system complete with turntables.

In recent years, the only thing I?ve used the main boxy bit for, is to provide 12V to drive the turntable. I?d like to feed it from something a bit more compact. But as usual, have zero budget.

I?ve taken the bottom of the turntable and discovered It has a 9 point Something volt motor (it also has an auto Arm liftty and Putty away mechanism).

I checked the power out from the big box, and it?s a little over 12V. I had a dig in my PSU box and the nearest I?ve got claims to be 13.1 V.

Does the team think that?s near enough? Or do I need to get I?d down? If so, could I get away with adding a diode (and resistor (value?)?) in series?

If not, could I add a 12V regulator? Or would I need more than 13V to feed the regulator?

TIA

Chris

[tabbypurr]

A diode would do it.

NT

[Chris Bartram]

I'd *expect* it to be OK, but would be tempted to try to measure the voltage from the original PSU on-load, and see if it's 12v then, and get a PSU as close to that as possible, or use a higher PSU and a regulator. Apparrently you'll need at least 14.5v to drive a typical 12v regulator and *guarantee* 12v:

I'd expect the motor drive to be regulated in the turntable, so I'd expect the main problem is not burning that up :-)

[Brian Gaff]

The latter sounds feasible but you need to know the current drain when its running under max load and of course you need to measure the voltage supplied under max load too, it might not be a regulated supply. If you opt for an external psu, put it as far away as you can to prevent any inductive pick up, on the other hand by a turntable and donate the old working system to somebody else.

Brian

[Martin Brown]

One silicon diode and one Shottky diode in series will drop a shade under a volt which should be fine. It is unlikely that something that is old and designed for a crude bridge rectifier PSU would be particularly voltage sensitive. The PSU voltage of the original is quite likely to be load dependant and higher when not spinning up the turntable.

The main thing you need to know is how much current does the turntable draw at 12v.

Although low dropout regulators are available the most common ones need a couple of volts headroom.

[cpvh]

Hi All,

Thanks for the reply?s. I will try and get more info.

Someone spoke of it probably being regulated in the turntable.

There is no evidence of this.

The wires carrying 12.1 volts come in, go to a tiny PCB with no components on either side, and then goes off to Power the motor (small round can with 9V written on it) and also the arm twidly mechanism.

I do have an 8V PSU kicking about, maybe I should check what that appears to be putting out, and if it?s around the 9V mark, see what the turntable does with that.

Otherwise, I might go back to driving it with the big box.

Best regards

Chris

[Dave Plowman (News)]

Have you looked for a schematic of the amp online? Should show you the PS, and perhaps test voltages, etc.

[whisky-dave]

You'd need to check those claims, is this AC or DC 13.1 seems an odd DC voltage to claim.

Well a diode can take it down by about 0.7V per diode (silicon) that should be OK, but if it lowered too much the motor might slow down.

Standard regulators need about 2 to 2.5V over the 12V and cost about 25P depending on supllier but the LDO regulators can work at about 0.5V over the 12V but they are normally over a £1 each

[tabbypurr]

Silicon diodes drop 0.65v at very low current, 1-2v at full whack. A single si diode will be fine.

Ideally you want 9v, but it's been ok on 12.

an amp at most, but more when starting. A 3A diode should be ok.

Of course the 13.1v psu might deliver much more, if it's heavy especially. That would complicate things. Measure it.

NT

[tabbypurr]

That probably would work.

NT

[Vir Campestris]

Is the motor AC or DC?

It's quite possibly AC, so the turntable speed is synchronised to the mains.

Andy

[cpvh]

Hi All,

I measured the original supply voltage on load, and it was a shade under 12V (it?s a very lightweight turntable).

I had another dig in my spare PSU box and found a couple of 12V SMPUs. One of which measured 12.2V off load.

I decided to go for it.

The turntable played fine 11::^*>>)).

I don?t think I checked the on load voltage, as I assumed it would be the same with such a negligible load.

I listened to about a track and a half of an LP, and none of the magic black smoke escaped.

Thanks for all the advice.

[tabbypurr]

Good job. If you find a 9v wallwart change over to that, it'll avoid cooking the motor regulator.

NT

[tabbypurr]

Mains TT motors use the mains frequency for control, low v ones are dc motors with a negative impedance speed regulator.

NT

[cpvh]

Hi,

The only 9V Wallwart I Have is DC. I do have an 8V, but I imagine that would be too low.

What would the motor regulator consist of? (Thinking about it, there is a small screw accessible from underneath to adjust each of the two speeds (which may or may not be something to do with a motor regulator, or maybe an alternative method??)

[tabbypurr]

why not use the 9v one? 8v would probably work too.

pass transistor, preset pot & a few other bits

they usually have a bit of slotted rubber covering a hole. Poke a jeweller's screwdriver in to adjust the preset pot, but don't short it to the case. A plastic tweaker is better. They seldom need adjustment.

NT

[cpvh]

Let?s try what I meant to say!....

The only 9V I have is AC.

I will give the 8V one a measure, and give it a try.

Thanks again

Chris

[tabbypurr]

right. Even if rectified & smoothed it would then be the wrong voltage.

[cpvh]

I seem to have invented perpetual motion!

The 8V wallwart (rated at 230 milliamp) showed about 0.5V off load. I cut its plug of and tried again on the bare wires. 15V!

I thought, what the heck, it?ll probably drop when it?s on load.

Turned the turntable on, and it span at what looked to be about 33 1/3.

Checked the V on load 19!

Switched off quick, and will wire the 12V beast back in tomorrow.

Ta muchly for all the advice.

Best regards

Chris

[tabbypurr]

they're not normally that bad :/. A 12v heavy type wart will often give 19v off load, dropping to 12 at max load.

NT