Dual 621 Weight Issue

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Tinkaroo
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Re: Dual 621 Weight Issue

Post by Tinkaroo » 06 Sep 2014 15:07

dualcan wrote::shock: == Nuns!
:old-git: Reverse!!
:drunk:

[-o< k
I see you remember Father Jack pretty well. I've got the box set of the series since it is much more fun to watch than Canadian politics.

Now comes the time to play devils advocate! :twisted:

I've read some fixes for the spring of the resonator weight if it is too badly damaged, but my thinking would be that it would never be the same. In that case there would need to be alternative solutions such as finding a weight that has similar anti-resonant design even if isn't quite as good as the original.

It is difficult to find a used 604/621, 701,704 or 721 weight.

What about a 502 weight since they are more readily available?

Any other suggestions?

dualcan
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Re: Dual 621 Weight Issue

Post by dualcan » 06 Sep 2014 17:45

There are two factors which govern tuned (anti-resonance) counterweights.
Primarily, when a weight is suspended (spring leaf) from the (counterweight) shaft, it will vibrate out of phase to the rest of the arm thus cancelling most of the vibrations.
Secondly, everything has a resonance frequency (think of opera singer and wineglass). The self-resonance frequency of a tonearm with cartridge can be further diminished if we tune the suspension system for the actual weight. In the case of the early anti-resonator weights (701 721) they were set for one or two predefined frequencies. The obvious problem here is that we use different carts with different weights, thus the dead-weight changes and therefore the calculation for the resonance frequency is off.
The later units (741 and a few others) had extra client adjustable anti-resonators which allowed for ULM or standard 1/2" (different weight) carts to be brought into the proper resonance/anti-resonance range by means of calculating the deadweight of the cartridge with hardware against compliance.
This is the most effective way to go, and if you can find one of those you hit the jackpot.
A bit of background.
Nothing could make an engineering team developing a tonearm more happy than specifying a single weight/style/contour cartridge for the arm. With that, the complete arm can be designed from the get-go to work withing the resonance envelope, minimize the counter-"weight" /travel/ adjustments, remove head-shell and simplify anti-skate force adjustments. This is perfectly seen from the earliest B&O arm ST1 also used by a very special PE 3210 in the 1950ies:
http://old-fidelity.de/thread-2442.html (look at entry 11).
Later on the Concorde design Ortofon line and today the still practiced P mount system goes in that direction.
Dual tried to go the same way with the TKS and ULM designs, but needed to cater to all the other Cart manufacturers as well, hence the technical overburden required in the arm design.
Coming back to the defective weight in question, once a tuned resonator is opened and "adjusted", it may perhaps no longer droop, which is by itself an improvement over having the actual weight touching the frame/shaft), the resonance tuning will be lost. The tuning process is out of the scope of DIY unfortunately!

OT:
--and I have the set as well---
Regards,
k

Tinkaroo
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Re: Dual 621 Weight Issue

Post by Tinkaroo » 07 Sep 2014 12:00

Thanks for the explanation Klaus.

What if somebody has a 604 or 621 which is suffering from more than a case of brewers droop. Suppose that it is completely Fubahr! :cry:

If someone can't find a good condition original 604/621 weight then what other models might provide close enough anti-resonance for that particular tonearm as the original weight would.

The link I provided from the German Dual forum seems to indicate something in the last paragraph.Here is a rough translation:

Hello, I have an AT 440ML at 604 with different weights, a resonance test using the Hifi News test plate, made ​​times. For use came an unknown weight of the 500 series, one from 704 and one from 1249 with the 1249, I came to a Tonarmfrequenz of 8 Hz horizontally, vertically 8 Hz, the 704-weight to 8 Hz horizontally, vertically 6 Hz and that of the unknown weight of a 5 series on 10 Hz horizontally and 8 Hz vertical. There are only slight differences in the Tonarmfrequenzen. With all the weights you move in quite deep Tonarmfrequenzen. The simple of 5XX seems there still are most suitable. greeting, Jörg



Thanks!

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Re: Dual 621 Weight Issue

Post by dualcan » 07 Sep 2014 16:54

The general gist of the translation tends to come to this point:
A family of (Dual)tonearms with the same head-shell system (TK14/24) and the same size platter (12") meaning effectively the same tonearm weight, can accept the c weights from each other whereby the more sophisticated the system (anti resonance)especially the tunable ones, the better. The only caveat is the i.d. of the alu tube and perhaps its sleeve mounting. In short, if is from the same group, and it slips in, use it.
Re your translation of Joerg's findings, his resonance frequencies dovetail nicely into the prescribed safety window calculated by Orofon engineers:

28336

in all instances. Keep in mind Joerg made the test with one cart. The tunable ones can compensate for different weight carts.
I can't see any other problem, however, caveat emptor!
...And I'm off to the Races today!
Naturally, that would be 1229 Dearing rings and such Races!
Regards,
k

Tinkaroo
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Re: Dual 621 Weight Issue

Post by Tinkaroo » 08 Sep 2014 11:54

Carpe Diem as Robin Williams said in one of his better movies.

It was worth the wait for info on the weight, and Klaus as always is worth his weight in golden Dual's in terms of his knowledge about them.Now he is chasing the chases or possibly in search of the lost chord. #-o

I have my friend Christine keeping her eye out for a weight for me, but it may be a bit of a wait. If anyone out there has one for a 604,621,701,704 or 721 in good shape at a fair price then please send me a PM. It needs to have the bouncy,bouncy, springy thingy rather than the fallen and can't get up aspect to it.

Flapjack_MC
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Re: Dual 621 Weight Issue

Post by Flapjack_MC » 08 Sep 2014 23:52

Not good. It obviously got crunched some point in its life. I've always kept the plastic cover on and treated it very carefully, so I am still mystified. :cry:

http://i399.photobucket.com/albums/pp72 ... 5fa9ca.jpg

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Re: Dual 621 Weight Issue

Post by dualcan » 09 Sep 2014 23:48

Hi,
Since the spring plate is not broken off the chrome shaft, you can reshape it with slim flat nosed pliers. Take a look at how the actual weight is mounted onto the spring plate (either straight or at an angle) and depending on this, reshape the end. The foam is in the right place, just scrunched a bit... Just don't bent the end too sharp if a bend is needed to avoid breaking it off.
Regards,
k

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Re: Dual 621 Weight Issue

Post by Flapjack_MC » 10 Sep 2014 00:12

Got it. Thanks Klaus. I will carefully give it a go.

- Mark

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Re: Dual 621 Weight Issue

Post by Flapjack_MC » 11 Sep 2014 23:29

I don't know if I can get it any better than shown below. I have it straight, but it isn't centered in the housing. At least it isn't touching the casing and floats free. Some observations:

- The spring is indeed very fragile and bends very easily.
- I can not see how this ever was right with the black attaching screw being in the 6:00 position. Seeing how this all works now, it most definitely should be in the 12:00 position as lini said in his post. Changing it to 12:00 was super easy, so that's what I did.
- Dual turntables are very well made. The fit and finish and engineering is excellent, though many of you probably already know that!
- How will this fix affect the sound quality? I guess I'll just have to try it out and see what I think.

Thank you everyone for your help.

http://i399.photobucket.com/albums/pp72 ... 4c53e3.jpg
http://i399.photobucket.com/albums/pp72 ... f4a18f.jpg

Tinkaroo
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Re: Dual 621 Weight Issue

Post by Tinkaroo » 17 Sep 2014 21:52

I found a replacement weight for my Dual 604. :D

The replacement comes from a Dual 704 and looks to be identical to the type used on my 721.

I think the bar on the middle part of the back end is a better idea than the open end on the 604-621 weight.
Dual 704 Weight.JPG
(140.72 KiB) Downloaded 1421 times

ummagumma
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Re: Dual 621 Weight Issue

Post by ummagumma » 19 Dec 2014 07:16

this is interesting, I have a 621 as well with that piece dangling out the back of the arm

it works fine, I assumed it was supposed to be like that?

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Re: Dual 621 Weight Issue

Post by Gelid » 20 Dec 2015 06:42

Pardon my butting in, but I also have a sagging AR weight and was wondering about it.

Alright... am I correct in assuming that the brass setscrew in the center acts to balance the AR weight? (Presumably, so it floats freely on the spring)

I just followed the first step of the instructions provided by Dualcan a few posts ago, turning this "central grub screw" setscrew in one turn at a time, for a full 9 turns. The AR weight hung suspended, not touching anything. When I went one more turn for good measure, the setscrew came out the other side and the weight was floppy as ever. Pushing the inner weight to one side, was able to gently shake the screw out and re-install it from the outside.

I did not disassemble anything, as the weight was orientated correctly. All I did was remove the whole counterbalance from the tracking arm, to see what I was doing, and turned the screw back in until the AR weight was suspended freely.

Is it possible that the resonance this device is absorbing slowly turns this brass setscrew out of position, resulting in a very gradual drop of the AR weight? Vibrations can move anything, if left unchecked, and our Duals are very old.