A unipivot made from scrap

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cressy
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#76 Re: A unipivot made from scrap

Post by cressy »

This is where we are at, now photobucket has decided to play ball

Image

If it does what i want it to then I might finish it abit better than the mk1

The bearing housing is exactly the same dimensions at the mk 1 housing to keep as much continuity as poss. The arm tube is about 3/4" shorter as the headshell is longer so's the cart and pins clear the walnut mass block, and that block is smaller than the mk 1. So overall its probably a couple of g lighter. However the extra couple of grams in the alternative counterweight probably means the mass of the complete assembly is about the same
Last edited by cressy on Thu May 04, 2017 10:14 pm, edited 1 time in total.
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Paul Barker
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#77 Re: A unipivot made from scrap

Post by Paul Barker »

Looking good.
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#78 Re: A unipivot made from scrap

Post by steve s »

cressy wrote: Thu May 04, 2017 7:34 pm I had one of those a few years ago too dave, twas a great arm, wish I still had it.

I have thought about making something of that ilk, but my metal lathe wot I got off our very own Mr sheils is still sans a chuck, and wants a more powerful motor before it it can turn steel.

Wish I could afford this but it'll go for really good money
http://www.ebay.co.uk/itm/162495099649
It'd be perfect for me :(
They look a very nice lightweight machine Ant
This week I have been packing a full machine shop and tooling for someone i know who is moving house, he is struggling as he has cancer. Once completed I'm buying his milling machine and surface grinder and bringing them home in the next few weeks
My large garden shed has to be lifted up and moved with a more solid concrete floor and path to it, power installed etc
It's really keeping me busy...

Good work too on the tone arm
The tube manual is quite like a telephone book. The number of it perfect. It is useful to make it possible to speak with a girl. But we can't see her beautiful face from the telephone number
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#79 Re: A unipivot made from scrap

Post by cressy »

Got the outrigger on

Image

Drilled and tapped an m4 hole in the side of the housing at 90 degrees to the wand, then simply screwed a short piece of m4 stud in.

The temporary weight on it I had to make by drilling out the hole in a lenco arm rider weight to 4mm, then cutting the weight in half. The lenco rider weight is 18g which was too heavy, this bit is 9 g so is about right. I expect the thumb nuts I ordered to be lighter so will see how that goes. With a cart on it the azimuth weight is likely to end up further out so will clear the plug on the wiring loom better.

I have changed the stub design a little too. The mk 1 used some more 8mm copper tube with a length of 6mm studding inside it for weight. Instead I have used some m8 stud, and sleeved it with some heatshrink as the alternative counterweight hole is 10mm rather than 8 for the lenco one.

It static balanced in 2 seconds flat. Much much easier than the mk 1

On to the bias.....
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#80 Re: A unipivot made from scrap

Post by cressy »

Ok so setting the bias weight may be a little tricksy. So, im working off a complete lenco arm I have spare to work out how to do it.

The bias arm on the lenco runs along the centre line of the arm, and the graduations cut into it give differing bias force. So measuring from the centre point of the arm where the tube is crossed by the knife edge, to where the grads are on the bias arm should give me the distances
From the pivot point I need to give a set amount of bias on the mk 2, that is reasonably close to the bias amount the grads would give on the lenco arm. Obviously this is a longer arm so it wouldn't be exactly the same. Should be close enough as it is a spare lenco bias weight i'll be using.

Actually, I might use the lenco bias outrigger, should go on the back of the housing with some persuasion
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#81 Re: A unipivot made from scrap

Post by cressy »

Wonder if a digital stylus guage stood vertically would give a useful bias reading if the arm is allowed to push against it.
Anyone know?
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#82 Re: A unipivot made from scrap

Post by cressy »

I read an article by James kogen called ' the skating force phenomenon'. relatively hard going, but quite in depth, interesting and useful.
There is a pdf on the audiomods site of it.
I also found someone on a linn forum had done something similar to what I was thinking, as is usual.

Set up a heath Robinson sperryment with a stock rega rb 250 I keep around to see what reading appeared on the guage.

Image

The bias on the arm was set to 2g, and the reading on the guage corresponds very well to what kogens graphs say it should be. ( i was looking at fig 8 as the carts I use have elliptical stylii)

So with his research and graphs, you can interpret a reading from a vertical sfg.

This is handy to know, and will make it easier to set up the bias on the cx mk2 arm
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#83 Re: A unipivot made from scrap

Post by cressy »

Last bit of structural work done to the arm. My back is killing me today from hoiking the lathe about over the last few days, man portable it is not. so I thought id do something that doesnt involve lifting anything. Must be getting old.....


Anyway, back to this project, ive drilled and tapped another m4 hole in the rear of the housing for some more stud to screw into. Just above the counterweight stub This piece holds the bias weight, and is on the plane of the pivot point so it doesnt affect the azimuth, stright along the cenre line of the arm. I used the stud again rather than butchering another lenco part as the thread means I have much finer control over the amount of bias applied. I might put a longer stud in if it needs it.
Also gave the tube a lick of black paint. Doesnt look so Heath Robinson now.

Image

The bit of walnut attached to the headshell with the rubber band simulates a cart, it weighs 6.5g, which handily is almost the same as the shure m97xe which is 6.6g. So i can balance the arm accurately as if the cart was installed, and check the bias is working and the azimuth is unaffected by it.

Happily, this is the case
Last edited by cressy on Sat May 06, 2017 3:13 pm, edited 1 time in total.
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#84 Re: A unipivot made from scrap

Post by cressy »

View from the front, you can see the simple copper wire loop that the weight runs through to provide the bias.

Image

I had to remove the outrigger on the arm pillar that held the arm rest as there wasnt enough clearance with it on, i'll have to add another wire rest for it and screw it down onto the base in the same way as i did the bias weight loop.

In the pic you can just see the hole i'll, just below the azimuth weight just to the right of the tube
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#85 Re: A unipivot made from scrap

Post by cressy »

Arm rest done, weight added to the bias weight to get the figure up to where the graphs in the article say it should be, wiiring loom installed, cart on, sit back, listen.

Image

Does what I wanted it to
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#86 Re: A unipivot made from scrap

Post by Cressy Snr »

Been down and had a listen. Still the slammy bass, but the whole thing is a lot more stable, with a nice, even sound across the rest of the music spectrum.
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#87 Re: A unipivot made from scrap

Post by cressy »

Ok, now I am happy with it, what next.
First, the top of the bearing pin needs cutting off. The brass bit on the top is part of the cause of the earlier stability issues, as there is a lump of mass 40mm above the pivot point waggling about. Should get a subtle improvement in stability. I reckon there will be some vibration in this part that will be removed once it is cut off.

Second, replacing the temporary azimuth weight with a proper thumb nut when they arrive should be an improvement as the temp one kept moving about when being cack handed with cueing the arm. Will make setting the azimuth easier and much more accurate.

Third, i think ill add one of those tiny bubble levels to the top of the headshell above the mass block to make setting azimuth more accurate than relying on doing it by eye

Fourth, a finger lift. Forgot it, and cueing is abit brown trouser without one. Wouldnt want something expensive in the headshell put it that way.....

It meets the design goals I set for myself, I.e the pivot point is level with the record surface, and the centre of gravity is just below the pivot point. In this case, just 7mm below it.
Also that it has an accurate bias mechanism, and an unspoken one, that I could use the m97 with the brush up, tracking at 1.4g without having it skip, wobble all over the shop, or have issues with the low teacking weight. The rega has a few 'moments' when asked to do this.

The treble is much improved with this one, so I think the CX mk 2 is a successful redesign
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#88 Re: A unipivot made from scrap

Post by IslandPink »

A stirling effort there Ant, glad it's working out. All the upgrades sound logical, this could be a really fine arm. I remember now that Frank Schroeder reckons cartridges last longer on the 'Referenz' because of the wood - less of the HF ringing of the metal wands being fed back to the cart.
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#89 Re: A unipivot made from scrap

Post by cressy »

Personally I think that wood and plastics used in an arm is a good idea because of their damping properies. Used in specific areas where tolerance isnt that critical I reckon its better than the usual alloy/ steel ect. See the grace 714 and schroeders.

Same for plastics, the idea for the acrylic headshell came from the roksan nima, and that is a lovely arm. If i was in the market, thats what i'd be looking at.

Doing the 'flick test' on the mk 1 arm wand with the headshell, housing and counterweight stub attached, and theres bugger all ringing, not like the rega casting which is bell like

Selecting the materials for this arm was based on what crap was hanging around, and what I could accurately machine, so abit of an accident but it seems to work
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#90 Re: A unipivot made from scrap

Post by cressy »

Just put on the nastiest record i could think of, Peter gabriel 'so'

Trying to trip it up, didnt work. Just played it.

Imaging is very good, very wide, and things in that image are stable. This is an area where the rega based arm is quite lacking compared to this thing. Dont really know why, must just be a unipivot thing
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