Help Ressurecting an Odyssey - geometry and effective mass?

by **AndrewL** » 14 Jul 2008 15:58

Seb wrote:The only critical assumption is the 9 mm (FE distance).

That diagram looks about right, I got something close to this but couldn't get the headshell to draw correctly.

I will measure the distance "FE" tonight on the arm.

Do you think a straight arm will be possible? It looks to me from my sketches that making the arm 300mm long, but retaining the 60 deg triangle increases the angle between 'AC' and 'DC'. However, we want to reduce this angle to something like 18 deg for angular offset.

To make this work the angle at the small dome would need to be much greater than 60 deg (approx 100 deg) or the 20mm between the two domes much longer something like 74mm.

-- Andrew

by **odaey** » 14 Jul 2008 16:36

Hi to all,

Did some rough measurements a while ago regarding maintaining the vertical alignment/azimuth of the cartridge in the vertical axis.
Since the sides of the cart i was using was flat. I used a right triangle to check if the arm maintained the azimuth of its cart in its vertical motion.

I placed the long side of the triangle directly perpendicular to the side of the cart and the other side of the triangle on the surface of the record. Quite surprising as it is, the sides of the cart maintained its vertical alignment up to about 8 cms from record surface. Could it be the weird angular offset compensating for it? (dumb question from a dumb person)

This was just a rough check, if there's a better way of doing it I would gladly do it if needed.

Funny thing is now after all this measurement, I kinda see the this arm like an S shaped arm ( or a Z).

by **AndrewL** » 14 Jul 2008 20:18

odaey wrote:Funny thing is now after all this measurement, I kinda see the this arm like an S shaped arm ( or a Z).

Yes, not a bad description, S, in reverse...

-- Andrew

by **Seb** » 14 Jul 2008 21:53

AndrewL wrote:
Seb wrote:The only critical assumption is the 9 mm (FE distance).

That diagram looks about right, I got something close to this but couldn't get the headshell to draw correctly.

-- Andrew

this is because I put my draw on a A4 format page...

send me your email, I'll send you in a letter format.

now, this draw is may be not exactly like your tonearm is. It will if the effective length is exactly 230mm and if the angular offset is exactly 23.5°.

best regards

Seb

(and kids are slipping now ;-)

)

by **AndrewL** » 15 Jul 2008 12:00

I would have said "FE" looks more like 8mm on my arm, but then the screw hole is definitely inline with the axis.

Assuming our investigation so far is correct, then the angular offset and null points specified by the manual are not quite how it was actually made/built.

So, it would appear the implementation/manufacture of the arm does not quite exactly match the designer's intent.

C'est la vie....n'es pas?

-- Andrew

by **Seb** » 15 Jul 2008 12:25

AndrewL wrote:I would have said "FE" looks more like 8mm on my arm, but then the screw hole is definitely inline with the axis.

Assuming our investigation so far is correct, then the angular offset and null points specified by the manual are not quite how it was actually made/built.

So, it would appear the implementation/manufacture of the arm does not quite exactly match the designer's intent.

C'est la vie....n'es pas?

-- Andrew

I think the designer rounded certain values. that's life !

BTW, what geometry do you want ? Bearwald ? Lofgren ?
Lofgren is said to be better on longer tonearm (don't ask why, I don't know?)

inmates, your opinion about this ?

best regards

Seb

by **AndrewL** » 15 Jul 2008 12:30

I think the designer rounded certain values. that's life !

Or, perhaps the folks that made them, I expect we'll never kmow the truth here.

BTW, what geometry do you want ? Bearwald ? Lofgren ?
Lofgren is said to be better on longer tonearm (don't ask why, I don't know?)

I vote for Lofgren.

inmates, your opinion about this ?

Yes, opinions please.

-- Andrew

by **Seb** » 18 Jul 2008 17:50

update and shame on me...

a straight arm is not possible (unless you accept and enormous headshell) the perpendicular distance between the stylus point and a straigth tonearm tube should be 47 mm - the distance between dot E and stylus point is 93.7 mm. now, imagine the headshell....

J shaped I'm afraid

best regards

Seb

by **AndrewL** » 18 Jul 2008 19:24

Well, I had suspected as much...the best drawing I could come up with was also curved, otherwise the distance between the two domes has to be ~70mm to make the angular offset ~18 deg and that wouldn't work.

So its good in some ways to have confirmation from both of us that straight is not possible - TBH I thought I was going mad.

Still some of the greatest arms are J-shape - SME for example.

Also, it now makes sense why the image posted by odaey was a J-shape....so another confirmation.

Now, this makes me wonder....would a 10.5" arm be a better proposition for a J-Shape? Better stability....?

J7 can you do a J-shaped arm?

-- Andrew

by **audioorigami** » 18 Jul 2008 20:05

it may be best to stick to a straight wand ...

whats the longest you can get with the straight wand?

by **AndrewL** » 18 Jul 2008 20:18

If I understand it correctly, then the effective length is tied to the distance between the two rear offset pillars. So we either go J-shaped or stick with the existing effective length

Do you have a pipe bending tool? The curve in the arm tube we need is 30 deg, I think, and is 30 deg whatever the eventual effective length we select. The longer the tube after the bend the longer effective length can is/will be.

cheers,

-- Andrew

by **audioorigami** » 18 Jul 2008 20:26

hi m8
i dont have a bending tool...could be a bit hit and miss getting the angle correct...and there is the possiblity that the alloy tube may crush

it would need the sort of bending tool that had a spring inside the arm tube to stop it crushing

would need to check if thats available

bw
j7

by **AndrewL** » 18 Jul 2008 20:29

Hi j7,

Well the good news is that the headshell is a doddle once the bend is correct.

As far as I can tell the headshell is just a square block, perhaps with slots, for cart adjustment.

Importantly, on a J-shape, again as far as I can tell, the tube enters the block straight on, which I know was an issue we discussed, this is much simpler than the angle it enters on the existing arm - so no drilling at an awkward angle required.

-- Andrew

by **AndrewL** » 18 Jul 2008 20:40

What about a wooden jig to bend against? And one of those bending springs? Inside to stop it crushing....

There's a couple of sites around, on one I found a plumbers bending spring for 10mm pipe a couple of £.

Edit : found one for an 8mm bore http://www.toolbank.com/ and stick "pipe bending" into the search field. Would that be about right for a 10mm outer diameter?

-- Andrew