Problem with setting up Jelco SA-750L

[luke1369]

I'm having trouble setting up a recently-purchased Jelco 750L on a Thorens 520 turntable. I've set up a couple of different tonearms, but I'm seeing that my understanding of tonearm set-up is not very good, and no amount of web searching is answering my questions... so I'm hoping someone will be able to help.

I'm mounting a Denon 103 and though I've been fairly meticulous (I cut and drilled the armboard myself) it is obviously mistracking, especially on certain records. The Jelco came with an Expressimo Audio aftermarket weight (the seller was not able to find the stock counterweight, which is fine--it was priced to sell). I find that I have to move the counterweight very far back on the stem in order to get proper VTF. And even then, it is mistracking. The weight is 120 grams, which is rather light... but my understanding is that I would need a heavier counterweight only for heavier cartridges (the Denon is on the lighter side). Am I mistaken? Wouldn't a heavier counterweight, closer in to the tonearm itself, necessarily result in too much VTF--and thus wouldn't I need a lighter counterweight rather than a heavier one?

[Barry]

Your Denon 103 weighs 8.5g, which is within the 4 - 12g counterbalance range of the Jelco SA750L with the lighter of the two counterbalance weights.

Clearly there is a problem with the counterweight fitted. Is the Exspressimo Audio counterweight the correct one for your arm?

[luke1369]

Hi Barry. Yes, I checked all the ranges and found that the 103 should be within the acceptable limits... one thing I haven't done is weigh the stem itself. But it is possible that this is a custom weight for very light cartridges (?). One other data point, Expressimo still sells counterweights, and lists the standard weight as 160 grams and the heavyweight as 200 grams. But the thing that throws me off is, I *AM* able to get the correct VTF (2.5 grams), with the caveat that the weight is well back on the stem. Is that enough to cause some tracking problems? I know from my years with the Ittok that one tries to get the counterweight as close to the arm itself as possible. I suppose one next step might be to email Expressimo and see what they say.

[mbic]

I’ll ask a stupid question, “is the cartridge correctly aligned?”

[Barry]

The heavier counterweight for the the Jelco SA750L arm weighs 216g. Your CW weighs 120g, so the heavier one is about double yours, This means it will be closer to the arm pivot, at roughly half the distance to counterbalance the cartridge. Such an arrangement is likely to be more stable, as now the heavier counterweight is much closer to the pivot point and in contact over its whole length with the stub, instead of extending out beyond the end of the stub, as you have found.

If you look at the Lenco Heaven forum https://www.lencoheaven.net/forum/index.php?topic=978.75, a poster using a Koetsu Black cartridge (mass = 10g) found that using the heavier CW improved matters, for the reasons described above. Interestingly although the CW has roughly double the mass, the distance to the pivot is roughly halved, so the effective mass of the arm will be largely unchanged, thus the resonant frequency of the arm cartridge combination will be unchanged.

Your Denon DL103 has a mass of 8.5g, so the results will be similar.

But you talk of the cartridge mistracking, so maybe Micheal has a point - is the cartridge correctly aligned in the headshell? Furthermore is the arm itself correctly mounted on the deck, i.e. the centre of the arm base is 290mm from the turntable spindle.

And what alignment prescription have you followed: Löfgren or Stevenson? Jelco arms, like all Japanese arms are designed to be set up according to the Stevenson prescription. What have you used to minimised the arm/cartridge tracking error - the protractor supplied by Jelco or one by a third party?

[DiveDeepDog]

VE quotes spindle to pivot @ 290 with cartridge overhang of 15mm?
What tools are you using for setup?

The 103 should be fairly immune to small errors in setup,,the conical tip is forgiving.

[Barry]

I realise that we have perhaps overloaded you with information and questions. So to recap:

[1] use of a heavier counterweight is a good idea as it will be securely attached to the rear arm stub.

[2] regarding mistracking, it is important to be assured the tonearm is correctly positioned and mounted on the turntable. If the turntable spigot to arm pillar distance is incorrect, the stylus overhang distance may be difficult to achieve. (Was a mounting template supplied with the arm?)

[3] the headshell is slotted to allow the position of the cartridge to be adjusted to achieve the correct overhang. But owing to the required clearance of the slot width and the diameter of the cartridge fixings, it is possible that the cartridge can 'slew' in the headshell, thus deviating from the correct offset angle.

[4] Adherence to the correct prescription for tracking error is important to minimise tracking error distortion, it is unlikely to result in audible mistracking.

[shane]

Interestingly although the CW has roughly double the mass, the distance to the pivot is roughly halved, so the effective mass of the arm will be largely unchanged, thus the resonant frequency of the arm cartridge combination will be unchanged.


Not quite, and this is where it gets really counter-intuitive. The term “effective mass” is really a way of expressing the rotational inertia of the arm about its pivot as seen by the cartridge, and you need to remember that the mass of the arm behind the pivot has to balance the mass in front of the pivot. Now in terms of static balance, if you double the mass of the back half of the arm, but at the same time halve the distance of the CofG of that part of the arm to the pivot, the arm will stay balanced, but remember, effective mass is a measure of inertia, not of actual mass, and as the link you provided states,“Moment of inertia is the mass multiplied by the square of the distance between the mass and the pivot point.”
It may sound like a trivial difference but what it means is that of you double the mass of the counterweight but halve it’s distance from the pivot, halving the distance has twice as much effect as doubling the weight so you actually halve the effective mass of the counterweight.

Have you ever looked at a Dynavector DV505 and wondered why it has such a small counterweight so far from the vertical pivot? It’s to increase the vertical effective mass to the same sort of value as a conventional arm whilst increasing the overall mass as little as possible.

[luke1369]

Thanks to everyone for the very detailed and informative responses--it's one of the reasons I'm glad that I joined this forum.

The cartridge is aligned and overhang is correct. I have several protractors, and used the MoFi Geodisc for this one, double checking with a couple of others. I cut and drilled the armboard myself, and tried to be very meticulous about the work--I won't say an error is impossible, or that it's perfect, but I'm pretty confident in the work, having double and triple checked it.

"Mistracking" might not be the right word to use, but on one record I played, the stylus didn't seem to be riding in the groove right... almost as if it was skimming over the record, rather than reading the groove... this was a relatively thin lp from the 80s, the other records I played were modern heavyweight jazz reissues, so thicker vinyl. I played very few tracks once I had set up the tonearm, maybe only an album side in total, because I could see/hear that something wasn't right, so I took down the turntable to re-check everything and gather more data.

I contacted Brian from Expressimo Audio to get his input, and he tells me I need a heavier counterweight, one that will allow proper VTF and be closer in to the arm, about 3/8" he says. This is the crux of my confusion, I think--I would expect a heavier counterweight mounted closer in to the arm would result in too much tracking force--and here I have a light counterweight mounted near the end of the stub, and I am struggling to get 2.5 grams of tracking force. This could be a result of my childish understanding of cartridge compliance.

[shane]

How are you measuring the tracking force?

A heavier weight will not in itself give you an increase in tracking force. The tracking force is set by moving the counterweight closer to or further away from the pivot. A heavier counterweight will need to be set closer to the pivot to give the same tracking force as a lighter one.

Are you getting confused between tracking force and effective mass? Tracking force is the downward force on the stylus set by balancing the arm. Effective mass is a way of expressing how much effort the stylus has to exert on the cartridge to make the arm move around. The effective mass of a particular arm/cartridge setup is a fixed quantity determined Bt the design of the arm and cartridge, whereas the tracking force is set by adjustment.

Are you able to set the tracking force to zero so that the cartridge floats above the disc?

[Barry]

Not quite, and this is where it gets really counter-intuitive. The term “effective mass” is really a way of expressing the rotational inertia of the arm about its pivot as seen by the cartridge, and you need to remember that the mass of the arm behind the pivot has to balance the mass in front of the pivot. Now in terms of static balance, if you double the mass of the back half of the arm, but at the same time halve the distance of the CofG of that part of the arm to the pivot, the arm will stay balanced, but remember, effective mass is a measure of inertia, not of actual mass, and as the link you provided states,“Moment of inertia is the mass multiplied by the square of the distance between the mass and the pivot point.”
It may sound like a trivial difference but what it means is that of you double the mass of the counterweight but halve it’s distance from the pivot, halving the distance has twice as much effect as doubling the weight so you actually halve the effective mass of the counterweight.

Have you ever looked at a Dynavector DV505 and wondered why it has such a small counterweight so far from the vertical pivot? It’s to increase the vertical effective mass to the same sort of value as a conventional arm whilst increasing the overall mass as little as possible.

We have both made mistakes: me in my sloppy lack of distinction between mass and effective mass, and you in your statement that the double mass at half the distance has its effective mass halved compared to that of the lighter mass.

The double mass has half the moment of inertia (MoI), but because the distance has halved the effective mass being MoI divided by the square of half the distance leaves the effective mass unchanged at double that of the lighter mass.

[luke1369]

How are you measuring the tracking force?


I used a digital stylus gauge; I also have a Shure one somewhere...

Perhaps naive but... I thought if I used a medium compliance arm with a medium compliance cartridge, and it was a combo that others found to work well (this one is important), and I was able to get the proper VTF, I might be all set. The aftermarket counterweight turned out to be a wild card, I think--the seller could have mentioned it was for lighter cartridges, but anyway, it was well priced and no ill will on my part.

Was I able to zero the tracking weight so the cartridge floated parallel above the disk? That's a big NO--the arm was tipped back and not at all steady. This was covered in the Jelco set up documents, I did try to do it, but I thought with the above parameters set, I'd be okay. I really appreciate Shane and others taking the time to walk me through this; I am getting a better understanding of the subject for sure.

[Barry]

If you can't get the arm to 'float', you need to move the counterweight forward. Then move the weight a little further towards the pivot and using your stylus gauge you can then home in to achieve the desired VTF.

Is the difficulty in achieving the desired VTF what you mean by mistracking?

[luke1369]

I think I have what I need to take some next steps. Thanks again all, I will post again if problems arise.