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Thread: Jelco SA-750D fitting questions

  1. #1
    blackstar Guest

    Default Jelco SA-750D fitting questions

    I've just bought a Jelco SA-750D from petrat of this parish, as a replacement for a RB300 on an NAS Interspace Jr. If anyone can help me out on a few minor points, I'd sho appreciate it;

    1. Arm position: as the arm rest has such a long 'rest' I've had to position the arm so that it is not in the 12 to 6 o'clock position, and the counterweight is slightly to the 11 o'clock position instead. Is this an issue because I'd like a nice gap between the arm rest and the platter.

    2. I've set the pivot to spindle at 214mm as per the arm specs, and set the VTF to approx 2.7g to have a little listen. Safari so goody, but I need to set the alignment and stylus position; can anyone recommend a good printable protractor?

    I'm sure there are more questions but I'll start off with those two.

    Cheers,

    M.

  2. #2
    Join Date: Jan 2009

    Location: Essex

    Posts: 31,853
    I'm openingabottleofwine.

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    [1] As long as you can still lift/lower the arm at the beginning and at the end of the record side, then no, it won't matter.

    [2] Jelco, like many Japanese arms use their own geometry, which doesn't correspond with either Loefgren 'A' or 'B' or with Stevenson. Given the published arm geometry, the two null-radii are at 59.1mm and at 112.5mm. Most arm alignement protractors follow the Loefgren 'A' (or Baerwald) geometry with null points at 60.1mm and 121mm. You could use such an alignement protractor if the cartridge fixing slots of the headshell allow you to mount the cartridge askew in the shell.
    Barry

  3. #3
    blackstar Guest

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    Cheers Barry, there is sufficient movement from side to side in the headshell so I'll go for that suggestion. The Conrad Hoffman arc generator seems to be a good option.
    As for the position of the arm, I have no problem with the lifting/lowering, just that if the counterweight pointed to 12 then the arm rest was pretty much in play until the cartridge was over the start of the record.

    Thanks,

    M.

  4. #4
    Join Date: Jan 2009

    Location: Norwich

    Posts: 2,814
    I'm Hugo.

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    Quote Originally Posted by Barry View Post
    [1] As long as you can still lift/lower the arm at the beginning and at the end of the record side, then no, it won't matter.

    [2] Jelco, like many Japanese arms use their own geometry, which doesn't correspond with either Loefgren 'A' or 'B' or with Stevenson. Given the published arm geometry, the two null-radii are at 59.1mm and at 112.5mm. Most arm alignement protractors follow the Loefgren 'A' (or Baerwald) geometry with null points at 60.1mm and 121mm. You could use such an alignement protractor if the cartridge fixing slots of the headshell allow you to mount the cartridge askew in the shell.
    Sorry Barry but you are wrong. Jelco use Stevenson geometry, optimised for DIN inner and outer groove radii. Run the figures through Conrad Hoffman's arc template generator and you'll see that overhang and offset agree with Jelco's specifications, and the inner and outer groove null radii are 57.5mm and 115.5mm respectively.

    As for the arm being rotated in its collar, this will have an impact on the bias compensation force because the spring is more extended than intended when the arm leaves its rest. Not the end of the world, because bias is best set by ear, but something to bear in mind.

    Quote Originally Posted by blackstar View Post
    I've just bought a Jelco SA-750D from petrat of this parish, as a replacement for a RB300 on an NAS Interspace Jr. If anyone can help me out on a few minor points, I'd sho appreciate it;

    1. Arm position: as the arm rest has such a long 'rest' I've had to position the arm so that it is not in the 12 to 6 o'clock position, and the counterweight is slightly to the 11 o'clock position instead. Is this an issue because I'd like a nice gap between the arm rest and the platter.

    2. I've set the pivot to spindle at 214mm as per the arm specs, and set the VTF to approx 2.7g to have a little listen. Safari so goody, but I need to set the alignment and stylus position; can anyone recommend a good printable protractor?

    I'm sure there are more questions but I'll start off with those two.

    Cheers,

    M.
    Assuming that you use a Windows PC, the best free option is Conrad Hoffman's Custom Arc Template Generator which you can download at http://conradhoffman.com/chsw.htm . Just type in the spindle-pivot distance (214mm), select 'DIN', and whichever alignment geometry you want (Stevenson is standard for the Jelco), then print off the template. Arc templates are very fiddly to use, but are totally accurate in terms of getting the overhang spot-on, so the tip follows the correct arc across the record. The grid alignment markings are nice and clear too.

  5. #5
    blackstar Guest

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    Very interesting about the bias compensation and the rotation of the arm in the collar. I do set the bias by ear which is handy, but I'm a bit dismayed that the rest gets in the way, forcing me to rotate the arm ....if you know what I mean.

    Thanks very much for the information.
    Last edited by blackstar; 05-03-2017 at 09:21.

  6. #6
    Join Date: Jan 2009

    Location: Essex

    Posts: 31,853
    I'm openingabottleofwine.

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    Hugo, the figures I quoted were taken from vinylengine.com.

    However if we compare the arm geometry as quoted by Jelco:

    Effective arm length: 229mm
    Pivot to TT spindle distance: 214mm
    Overhang: 15mm
    Offset angle: 22 degree,

    then I agree it is very close to the arm geometry for an arm following Stevenson's method:

    Effective arm length: 229.01mm
    Pivot to TT spindle distance: 214mm
    Overhang: 17.41mm
    Offset angle 22.2 degree.

    If we use the DIN suggested values for record side radii of 146.3mm (outer) and 57.5mm (inner), then the calculated null points for the Jelco arm are 59.1mm and 112.5mm.

    The null radii if Stevenson's method was followed would be 57.5mm and 115.7mm.

    Not a lot in it, but the two are not the same.
    Barry

  7. #7
    Join Date: Jan 2009

    Location: Norwich

    Posts: 2,814
    I'm Hugo.

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    Well, I've just run the figures through the VE calculator and they agree entirely with Jelco's published specs and with Conrad's little program, using 214mm spindle-pivot distance and assuming that you are using DIN inner and outer groove radii. So, I stand by my assumption that Jelco's chosen alignment is to Stevenson/DIN.

    All of which is of debatable relevance, because a Jelco arm is really quite flexible in terms of alignment options, and Stevenson may not be the one that works best for all users, particularly those with old LPs with different inner and outer groove radii. Most of my LPs fall very much into the DIN era.

  8. #8
    Join Date: Jan 2009

    Location: Essex

    Posts: 31,853
    I'm openingabottleofwine.

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    My mistake was in assuming the ‘Stevenson’ geometry conformed to Stevenson’s original method, which chose record inner and outer radii of 54.821mm and 145.325mm. These values were based on listening tests conducted by Stevenson on both 7” and 12” diameter microgroove records.

    Give these record radii, the null radii are at 60.325mm and at 117mm.

    However there is in principle no reason why an alternative choice of record inner and outer radii could not be chosen. So if the DIN recommendation of 57.5mm and 146.05mm is used, then I agree the null-radii become 57.5mm and 115.7mm.

    And this presumably is what Jelco and other Japanese arm manufacturers have done.


    Curiously though, vinylengine.com quote null radii of 59.1mm and 112.5mm (https://www.vinylengine.com/tonearm_...&amhi=&cw=&mp=).

    If one back calculates, these null-radii are those that correspond to a Baerwald geometry using the IEC recommended record radii of 60.325mm and 145.05mm.


    So thanks for pointing out my mistake – I will have to update the article I wrote for The Knowledge: http://theartofsound.net/forum/showt...earm-alignment

    All this has probably confused Blackstar further, for which I apologise. But if his Jelco arm is missing an alignment protractor he can always use the ‘SmartTractor’ alignment gauge you market.
    Barry

  9. #9
    blackstar Guest

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    Not at all Barry and Hugo, most informative and I have already used Conrad Hoffman's protractor to set up.....a little tweaking and i will be there.

    Thank you both for the help.

  10. #10
    Join Date: Jan 2017

    Location: Sweden

    Posts: 6
    I'm Ake.

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    I to have a 750D and I´ve downloaded the software but the default is 57,5 and 146,05mm was that what you used?

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