What arm is this

[Lawrence001]

I bought a Pioneer PL-12D in immaculate condition. The original tonearm is in a box and there is a different one fitted. Does anyone recognise it?

The guy who sold it reckons his dad made his own tonearm in which case it might be a known DIY design. Or maybe he got confused about him fitting one vs actually making one.

I'm also a bit confused about what to do with the anti skate thing with string, there doesn't seem to be a weight or anything to attach the looped string to.

https://uploads.tapatalk-cdn.com/20191025/3546a6ff3204b73502d5bbff729ea9d1.jpghttps://uploads.tapatalk-cdn.com/20191025/585e91f6453c777f966f0887440ebb32.jpghttps://uploads.tapatalk-cdn.com/20191025/f4bbd02ec4f92182abbe127827cc5b76.jpghttps://uploads.tapatalk-cdn.com/20191025/de5510f912a029ae5fe56c64b74420a9.jpghttps://uploads.tapatalk-cdn.com/20191025/6ee831f100cc554d0640f80f9835c6b5.jpg

Sent from my POT-LX1 using Tapatalk

[ianlenco]

I'm guessing that is a unipivot arm and the contraption to the side is to keep the arm level rather than act as a anti-skate.

[Lawrence001]

I'm guessing that is a unipivot arm and the contraption to the side is to keep the arm level rather than act as a anti-skate.Thanks Ian are unipivot arms quite common in the DIY community? It lifts off a point in the base so I'm guessing that's the "pivot"?

Any idea where I would put the loop of string hanging off the end of the contraption?

Sent from my POT-LX1 using Tapatalk

[Beobloke]

It does indeed look home-made. I particularly like the gentle undulations in the arm tube...

Frankly, I’d get the original arm back on there pronto!

[Lawrence001]

It does indeed look home-made. I particularly like the gentle undulations in the arm tube...

Frankly, I’d get the original arm back on there pronto!There is a straight arm tube in the box as well. I wonder if he tried both and preferred this one. I wouldn't know where to start with this, looks like a great project for someone on here... [emoji38]

Sent from my POT-LX1 using Tapatalk

[YNWaN]

I’m not so sure it is home made - if it is then it is very professionally finished. The mechanism on the side is the bias and works in a way I’ve not seen before - very clever in fact. The hanging thread would attach to the rear of the upper bearing assembly - near where the rear counterweight stub is (perhaps even around it). As the arm tracks the record it would cause the hanging weight construction to twist and therefore said weight will try to return to its resting position creating the required side force.. as the record is played the hanging weight will twist further and so gradually increase the amount of force applied.

Looking again at the hanging thread it may actually attach (loop over it) to the end of the counterweight stub.

The headshell looks very Decca like.

[Lawrence001]

I’m not so sure it is home made - if it is then it is very professionally finished. The mechanism on the side is the bias and works in a way I’ve not seen before - very clever in fact. The hanging thread would attach to the rear of the upper bearing assembly - near where the rear counterweight stub is (perhaps even around it). As the arm tracks the record it would cause the hanging weight construction to twist and therefore said weight will try to return to its resting position creating the required side force.. as the record is played the hanging weight will twist further and so gradually increase the amount of force applied.

Looking again at the hanging thread it may actually attach (loop over it) to the end of the counterweight stub.

The headshell looks very Decca like.Thanks Mark that makes complete sense in terms of the physical setup (see photo) but the arm swings very strongly back to the start position when you lower it so something's not quite right. I did untwist the string when it arrived as it was tangled but I think I did it right. I can't see an obvious way of changing the force easily.

The gentleman who made it (now long retired) was a precision engineer in the aerospace industry at Smith's Industries and his son mentioned he used the workshop to make it (assuming it is not actually a commercial model), so it is quite nicely made yes. [emoji3]https://uploads.tapatalk-cdn.com/20191025/1c7031fabebea70e636bfc426e3523e1.jpg

Sent from my POT-LX1 using Tapatalk

[YNWaN]

Given his skill set and access to equipment it possibly is home made then :). The colour of the bearing assembly suggests stainless steel - perhaps even a titanium alloy. You might have to give the thread a negative twist before it is fitted to the stub. Looking at the trapeze arrangement it would suggest the small weight can be moved along the bar - presumably for some adjustment, though precisely how that would occur escapes me at present (will think on it :)).

[scotty38]

Can you not unravel the thread along the uppermost bar so that the weight and assembly hangs lower? The lower it is (and longer the threads) the lower the twisting motion will be if so.

[YNWaN]

Oh, I think that if you move the weight along the bit of wire it hangs from it will alter the moment of leverage and so alter bias - I wouldn’t have thought it would be by much though.

At the start of playing a record the threads should be almost completely untwisted.

[Lawrence001]

Oh, I think that if you move the weight along the bit of wire it hangs from it will alter the moment of leverage and so alter bias - I wouldn’t have thought it would be by much though.

At the start of playing a record the threads should be almost completely untwisted.I didn't think it did but giving a bit of a push it does, but you're right relative to the overall force it's not a great deal.

Sent from my POT-LX1 using Tapatalk

[Lawrence001]

Can you not unravel the thread along the uppermost bar so that the weight and assembly hangs lower? The lower it is (and longer the threads) the lower the twisting motion will be if so.It was higher up, I thought I'd "untangled" it but maybe I have undone the setting [emoji50] it wasn't obvious though as the loose bit wasn't connected to the arm then. I think one string was wound more than the other. You can only get one "wind round" it's not long enough for a second one. Maybe winding one round but not the other will have the desired affect but I tried and it didn't seem to help.

This reminds me of one of those mathematical puzzles involving knots or something [emoji38]

Sent from my POT-LX1 using Tapatalk

[scotty38]

Maybe just terminology but if this is the method of adjustment then it needs to be longer to lessen the twisting effect. Making it shorter will increase the force applied.

Obviously I'm talking about the "filament wound around in a spiral" bit versus the thread going to the back of the arm....

[YNWaN]

“Filament wound around in a spiral”?

[scotty38]

Just trying to describe the bit of thread I was on about to avoid confusion. I used quotes to try and show I wasn't being literal, hope that makes sense now...

[Lawrence001]

I tried it and it still plays nicely with the loose thread unconnected. Would be nice to understand the logic behind this though.

Ideally I would transfer this arm onto a better deck and replace the stock arm, but I wouldn't know where to start...

Sent from my POT-LX1 using Tapatalk

[Lawrence001]

Maybe just terminology but if this is the method of adjustment then it needs to be longer to lessen the twisting effect. Making it shorter will increase the force applied.

Obviously I'm talking about the "filament wound around in a spiral" bit versus the thread going to the back of the arm....I can't see any way of making them any longer and I'm assuming that it was set up correctly so won't need longer threads making. But I'll look again later, it's packed back in the box now.

Sent from my POT-LX1 using Tapatalk

[scotty38]

Nope I didn't mean make a longer thread, was just asking if it could be unwound from the horizontal arm that it "appears" to be wound around. If it's not wound and can't be unwound then just ignore me....

[YNWaN]

Just trying to describe the bit of thread I was on about to avoid confusion. I used quotes to try and show I wasn't being literal, hope that makes sense now...

I don’t think the shorter threads leading to the trapeze type element are wrapped around at all - it looks like they just go into some tubing (to create the required spacing).

Lawrence, did you try giving the mechanism a reverse turn first before fitting it, so that it unwinds as you move the stylus to the play position?

[scotty38]

Ah ok got it then, thanks

[Lawrence001]

It was already fitted and hanging but looked a bit tangled so I unwound it carefully to the position you can see in the photos. You're right that the thread is held by some sort of rubber tube at the top. I'm confident that the lengths of thread are as set up by the designer but not necessarily the way they are currently hanging.

Sent from my POT-LX1 using Tapatalk

[Barry]

I don’t think the shorter threads leading to the trapeze type element are wrapped around at all - it looks like they just go into some tubing (to create the required spacing).

Lawrence, did you try giving the mechanism a reverse turn first before fitting it, so that it unwinds as you move the stylus to the play position?

I agree regarding the 'tubing'. This type of trapeze arrangement is used on the 'Well Tempered Arm' to provide both vertical and horizontal support of the arm tube, as well as anti-skating bias.

As shown, the arrangement will provide a side force that increases as the arm moves towards the centre of the record, whereas most bias devices provide a force that decreases as the arm moves inwards. It is not clear to me which is correct, or if the bias ought to be constant and independent of arm position.

[Lawrence001]

I had an idea while I was looking at the arm. If I loosen a screw at the back the tube that holds the weighted contraption can be moved round to increase or reduce the turning force on the arm. The only thing is that to get it to a sensible amount, assuming the loop is tied to the end of the tonearm as suggested, it needs turning so far that the lid would not work. Apparently the lid was definitely used all its life and it hasn't been adjusted to get it in the box. So back to square one in think.https://uploads.tapatalk-cdn.com/20191027/e30324efc17864b51258c4ea3de3d39d.jpg

Sent from my POT-LX1 using Tapatalk
https://uploads.tapatalk-cdn.com/20191027/905c5a70dc580cc9eed77adcdbcfce14.jpghttps://uploads.tapatalk-cdn.com/20191027/57a3403613bb3a088b3b1fdbd70ae683.jpg

[scotty38]

Moving the weight around the off centre hole is to adjust the azimuth I believe....

[Lawrence001]

Moving the weight around the off centre hole is to adjust the azimuth I believe....I'm not sure which hole you mean?

Sent from my POT-LX1 using Tapatalk

[scotty38]

If you look at the large counterweight you'll see the arm doesn't pass through its centre so you can imagine as it's rotated its weight will shift sideways if you like... That will allow you to alter the azimuth of the arm given that it's a unipivot it will tilt to one side or the other. Hope that makes sense

[Lawrence001]

If you look at the large counterweight you'll see the arm doesn't pass through its centre so you can imagine as it's rotated its weight will shift sideways if you like... That will allow you to alter the azimuth of the arm given that it's a unipivot it will tilt to one side or the other. Hope that makes senseAh I see now very good point thanks. I just put it back to its natural position, it was off centre but it didn't seem right, probably no surprise with the thread falling off the end after being transported. I wonder if that would counter the pull of the thread and net it off but with some bias as the tonearm sweeps, I'll have another look tonight.

Sent from my POT-LX1 using Tapatalk