Fatboy 12" Unipivot Install

Fatboy 12" Unipivot Install

Postby YoungDave » Mon Dec 24, 2018 7:45 pm

Just received my Fatboy unipivot and I have some trouble already...

1. The arm is decidedly unbalanced in the azimuth plane - it is heavily weight-biased to the inside, so that the inner side azimuth weight always drops all the way down to the arm base, even with the dual pivot (and the additional weight it provides on the platter-side of the bell) removed. It certainly seems like the arm should be approximately neutral in the azimuth plane, else it will drag too heavily on the dual-pivot plate. The weight bias is so large that even unscrewing the opposite side weight all the way to the end of its travel will not balance the arm - it still tilts solidly to the inside, right down until the side weight contacts the arm base. I should add that I am certain the spike is in the center of its conical receptacle.

2. In the previous version of the unipivot JMW 12 3d, the offset counterweight or the Counterintuitive accessory could be rotated to provide a stable azimuth reference. This is no longer possible because the fatboy uses a concentric counterweight (a very nice mechanism, I might add).

3. The spike mounting cone in the bell housing must have been screwed in too far - the arm sat so high that I had to go to the lower limit of my VTA tower to get the cartridge mount head level. I used an allen wrench to back this out a bit but it surprised me that I had to do it.

Has anybody any insights to these problems? I fear that I will have to return the arm, and I really want to make it work. I frankly suspect a manufacturing flaw, and I might add that the lack of any kind of documentation is very disappointing - I thought VPI had improved that aspect of their business, and I was wrong.
2004 HRX, heavyweight platter, Gimbal Fatboy arm, Phoenix Eagle/Roadrunner, Dynavector XV-1s, Manley Steelhead, DEQX
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Re: Fatboy 12" Unipivot Install

Postby 10ovr » Mon Dec 24, 2018 8:51 pm

Easy fix,,Trade it for a Gimball
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Re: Fatboy 12" Unipivot Install

Postby teenage diplomat » Mon Dec 24, 2018 9:15 pm

The only thing I can think of off-hand is the inside weight on the azimuth ring may be larger, and therefore, heavier, than its outside partner. Unscrew both of them and compare. Switch them just to be sure.

One more thing to check: I found with my Fatboy unipivot that getting it set exactly on the pivot spike is really finicky. Play with that some more just in case.

If no joy, VPI will make it right. Just get in touch with Marc, who took great care of me a few months ago when my Fatboy started "crying" (short version of the story: arm was replaced).
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Re: Fatboy 12" Unipivot Install

Postby Mr_Putty » Mon Dec 24, 2018 10:42 pm

YoungDave,
I still struggle with correct arm spike mount position, after several remounts. I even mark the arm position with tape before removing the arm. What you are describing is exactly what the arm does when it is not really on the spike, but close to it. When you get it correct you will easily notice how freely it moves side to side (azimuth). Carefully lift the arm with both hands and predict where the spike would be to correct the balance, and slowly feel you way to it. It does get easier with practice. I screw both weights all the way in before finding the spike.
Happy Holiday!
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Re: Fatboy 12" Unipivot Install

Postby Stringreen » Tue Dec 25, 2018 12:02 am

Sure sounds like the arm is not properly on its spike....
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Re: Fatboy 12" Unipivot Install

Postby tony22 » Tue Dec 25, 2018 10:14 am

Stringreen wrote:Sure sounds like the arm is not properly on its spike....


+1, but I wouldn't discount teenage diplomat's thought on the weights. Easy enough to check if YoungDave has a light weight scale.
Pass XP-20 & XP-15, AVA 600R, Shahinian speakers, PS Audio DMP & DS, Avenger w/mods, 12" Fatboy gimbal + 2nd mount, Benz LPS, VAS & SS carts, Ortofon Quintet Mono, Logitech Touch, RSA Haley and PSA conditioning, PAD and AZ cables.
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Re: Fatboy 12" Unipivot Install

Postby YoungDave » Tue Dec 25, 2018 4:40 pm

Thanks, all, for your replies. I have used your suggestions in my further study of the installation.
1. The outrigger weights are the same, left and right, at (iirc) 26.2g each. Just out of curiosity I also weighed the dual pivot - it weighs 6.65 g.
2. The best suggestion, I thought, was that I somehow had missed the spike mounting and the resulting offset caused the bias. This indeed proved to be the case (in spite of 15 years of experience with the unipivot, and thinking I of course could reliably place the arm on the spike!

It turns out that the spike cup inside the arm bell is rather roughly machined (where one would want as mirror-smooth as possible), and my spike hung up on one of the ridges in the cup - in fact, using a loupe and some extra light, I think I see an actual dimple in the cup, 1/8 of an inch or so from the apex. It may be that as I gently moved the arm around the spike, searching for the apex of the cup, that I located it in that dimple. That is enough offset to wreck the natural left-to-right balance of the arm.

When VPI gets back from their midwinter break, I will contact them and get a replacement cup.

The important thing is, it now appears that all is not lost. Yes, I would have preferred a bearing cup with a better finish, but this is easily fixed.

I also would have liked, and indeed expected, comprehensive documentation of the arm, its features, assembly, adjustment, etc. I certainly hope that VPI compiles such documentation for this and all their products in future.
2004 HRX, heavyweight platter, Gimbal Fatboy arm, Phoenix Eagle/Roadrunner, Dynavector XV-1s, Manley Steelhead, DEQX
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Re: Fatboy 12" Unipivot Install

Postby YoungDave » Tue Dec 25, 2018 7:42 pm

I had an idea to compare the fatboy arm's spike cup with the spike cup off my old trade-in arm, a 1st-generation 3d arm.

The spike cup is simply an allen setscrew (albeit a rather large one) with a cup machined into the end that would normally have a spike on it.

This cup, on my new arm, was about a 45 degree Vee-shape, right down to its apex, and was very rough - it even had a sharp ridge of "flash" material around the edge, and at least one dimple on the wall of the vee that was big enough to engage the spike.

The cup on my old arm also has a 45-degree slope, but at the apex it has a further cup machined into it, with about 75-degree sides and a much narrower contour, and maybe 1/8 inch deep. This narrow cup provides foolproof capture of the spike. Also, the whole piece is much smoother than the new one. Not polished by any means, but machined smoother. No wonder I had never had any trouble setting my arm on the spike for all those years...

Any of you trading in an old arm for a fatboy would be well advised to inspect the spike cup carefully, with magnification and plenty of light.

I'll be returning the "new" cup, from the new fatboy, in my trade-in arm, and using the old cup in my new arm. I will so inform Bob, the retailer. By the way, Bob, of Bob's Devices, has been very helpful and informative all along the way and has been a pleasure to deal with.
2004 HRX, heavyweight platter, Gimbal Fatboy arm, Phoenix Eagle/Roadrunner, Dynavector XV-1s, Manley Steelhead, DEQX
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Re: Fatboy 12" Unipivot Install

Postby Shep » Thu Dec 27, 2018 10:23 am

YoungDave, I certainly appreciate your disappointment that VPI fails to include ANY documentation with their top of the line tonearm. Also, no instructions or guidance on their website that I could find...really poor support for a product that's been out more than a few months. Thankfully, I was able to get assistance here that allowed me to complete the Fatboy gimbal install.
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