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Thread: L100 Pots

  1. #1
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    L100 Pots

    I am repeating one question I buried in another thread and a new one. Please bear with me:

    L100(A) late model

    After removing them from the board, total immersion wash in isopropyl (twice) and some DeOxit they are close to 100% These only had a little noise in them (but enough to be annoying) and no severe dropouts so I'm sticking with them
    EVERY new L-pad I have ever purchased was less than perfect and had a little noise in it right out of the gate and had to be hosed down with something (and I'm not alone going by Parts Express reviews), so...........that's another reason why, it is also a "keep everything as original as possible" job

    Scrubbed them up so deeply as they were out for the first time in 40 years for the re-cap

    My problem/concern now is they are TOO clean (I can feel the friction factor, DeOxit is brutal in that regard). I don't want them to become compromised by excessive friction wear (they're L100s so they will get used!) or have to pull this job back apart
    What should I use to put a film on the wire and wiper for the long term? Light film of dielectric grease maybe? Any recommended aerosol solutions (that I can source in California, U.S.A.)? I'll go either way with a definitive suggestion/advice as I have no aversion to opening these up
    I did have to open one up to blow out some bits of debris that were interfering with things; these old pots are practical to work on if need be

    Also:

    What is the L-pad's resister value for "FLAT"?

    The seemingly obvious 4 ohms? Or my pots' actual measured value divided by 2?

    Would like to set my knob's index mark in relationship to the scale on the foilcal as accurately as possible staying faithful to the original design

    The theoretical "FLAT" or "0" setting in real terms, not the lines and dots on the front of the box (as real as we can get measuring resistance)

    I ask because all 4 of my pots in rehab measure the same and match well, but all 4 are just a hair under 7 ohms

    Is there a "spec" for what should be the flat setting according to JBL?

    After all of this time (insanity) invested in a cap stuffing level restoration of the two cap wonder network, I'd like to have the "FLAT" resistance value where JBL intended as an accurate baseline/reference electrically

    I'd like to know that what I am hearing is what they considered to be "FLAT" on paper

    Thank you for your patience and the fact that I do not know any math

  2. #2
    Senior Member 4343's Avatar
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    Cool

    De-Oxit Fader Lube. F5. I never use the raw stuff.

    Got it at Guitar Center.
    Mike Scott in SJ, CA
    Drive 'em to the Xmax!

  3. #3
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    Quote Originally Posted by 4343 View Post
    De-Oxit Fader Lube. F5. I never use the raw stuff.

    Got it at Guitar Center.
    I do use Fadrelube on membrane type sliders and plastic controls..............touch up on carbon pots occasionally
    Won't do a whole lot for metal to metal (been my experience anyway)
    The film is too thin

    Might be an improvement if this were something I could access easily in the future for periodic treatments
    Thinking of/looking for something longer term/lasting

    Thanks for the suggestion

  4. #4
    Senior Member grumpy's Avatar
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    "Thinking of/looking for something longer term/lasting"

    HD rotary switch and resistors (i.e., precision, limited-range stepped attenuator version of an L-pad).
    Still have to deal with switch contacts, but they can be reliable and are normally serviceable.
    A number of JBL designs have done this (but using shorting bars vs. a rotary switch, but the switch
    could fit the l-pad form factor).

    Best to switch when -not- under extreme load.

  5. #5
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    Quote Originally Posted by grumpy View Post
    "Thinking of/looking for something longer term/lasting"

    HD rotary switch and resistors (i.e., precision, limited-range stepped attenuator version of an L-pad).
    Still have to deal with switch contacts, but they can be reliable and are normally serviceable.
    A number of JBL designs have done this (but using shorting bars vs. a rotary switch, but the switch
    could fit the l-pad form factor).

    Best to switch when -not- under extreme load.
    Thank you
    Yeah, I'm not a fan of the continually adjustable crap; I like it like the "N1200" style
    I was thinking in terms of longer lasting contact conditioner/lubricant (but it's "OK" now, I quit over analyzing things and just got on with it)
    The one I did open was in good shape and by design is "self cleaning" (the Japanese pot is very well made by the way)
    Plenty of the hard black corrosion after 40 years on the solder lugs but the wirewound sections looked good
    It's not as if it's a control that's in constant use; it's just such a bitch to get to I was thinking of something that might prolong the time between silent and scratchy

    I have come to one conclusion; you will never get an adequate spray of anything on these pots when they are still mounted inside the box, ergo it makes sense, the guys who just replace them period if they have to pull them out for whatever the reason (recap job etc)

    Got one reassembled today and the other tomorrow hopefully ("crossover") There may not be a whole lot to an L100 "network" but I'll tell you one thing; restoring one nicely and keeping it all original is not an easy task (I am reusing every bit, including wire)

    Building the little board from scratch would have been easier JBL's use of the fairly heavy stranded wire all tied to that little terminal strip (and hardwired) makes for some challenging soldering and wire dress, especially so that your connections don't fracture and break from flexing That, and 4 decades of oxidation and ancient flux to clean makes each and every joint extra special They cut the input to network wires at exactly the needed length but then give you an extra 1/2 a foot on everything else (where you don't need it); that's some fun too, more so because I am changing out the terminals

    Came out very nice though with the wires neatly dressed and proper soldering; the factory sure didn't bull shit around throwing these things together, and in!

    You know the resistance value I need to use in order to properly index my knob to the "FLAT" setting?

  6. #6
    Senior Member grumpy's Avatar
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    I'd have to pull out a woofer and measure, ... that's assuming 4311WX-A is the same part and orientation/marking for "0"... turns out the marking on mine are just linear 0-10 (5 is 1/2 way)
    on "presence" and "brilliance" :P Brilliant. Sorry.

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    Quote Originally Posted by grumpy View Post
    I'd have to pull out a woofer and measure, ... that's assuming 4311WX-A is the same part and orientation/marking for "0"... turns out the marking on mine are just linear 0-10 (5 is 1/2 way)
    on "presence" and "brilliance" :P Brilliant. Sorry.
    Don't do that!
    Maybe I am over asking; there has to be a formula (or arithmetic) to know the value
    I just divided in half and turned the pot to the middle and they are like they were before I took them apart with the dots pointing to what is "0" on the foilcal
    So, I'll probably have them better matched than JBL did (I could see that they didn't waste a lot of time with a precise alignment )
    You can stick the knob on any way you want
    I was just shooting for a little more precision (I'd rather have the theoretical ideal spot on zero than the dots lining up perfectly from -3 to +3)

  8. #8
    Senior Member grumpy's Avatar
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    If the crossover in questions operates from +3 to -3 dB or more (relative to "laboratory standard" as labelled),
    then the L-pad is really running from 0 to -6dB or more. An 8ohm L-pad at midpoint
    that had that range (e.g. a 3dB pad) would be 2.34 in series and 19.4 in parallel
    (2.4 and 18.5 is also close at 3.1dB).

    http://www.sengpielaudio.com/calculator-Lpad.htm

    Might see if that sort of matches your measurements (or if the printed scale
    vs actual attenuation has some other offset besides the 3db guess above).

  9. #9
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    Quote Originally Posted by grumpy View Post
    If the crossover in questions operates from +3 to -3 dB or more (relative to "laboratory standard" as labelled),
    then the L-pad is really running from 0 to -6dB or more. An 8ohm L-pad at midpoint
    that had that range (e.g. a 3dB pad) would be 2.34 in series and 19.4 in parallel
    (2.4 and 18.5 is also close at 3.1dB).

    http://www.sengpielaudio.com/calculator-Lpad.htm

    Might see if that sort of matches your measurements (or if the printed scale
    vs actual attenuation has some other offset besides the 3db guess above).
    Those series and parallel suggestions would be an ohms measurement, yes?
    That is what I am looking for, a simple "assembly spec" that I can set, as measured in ohms (and be as close as I possibly can to the theoretically "laboratory standard" or 3dB
    The range of the pots, as measured across 1&3 is 6.9 ohms (-3dB) to 34 ohms (+3dB)
    Across 2&3 6.9 ohms (-3dB) to 0 ohms (+3dB)
    Clearly JBL just divided by 2 and then all the little dots and lines line up
    But, is that really "flat" or as flat as an L100 is going to get?

    Or does "laboratory standard" just mean "dead center" on the control?

    There is a small +- tolerance difference between the 4 pots but it is very nominal (I will work from the smallest value of the 4 obviously)

  10. #10
    Senior Member grumpy's Avatar
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    yes, ohms. Should have said 19.5 ohms for the larger (standard/available value of 39 ohms /2).

    L100 flat... there's a good one.

    I'd assume that meant "as intended" such that deviations from there are for
    environment or user preference adjustments.

  11. #11
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    Quote Originally Posted by grumpy View Post
    yes, ohms. Should have said 19.5 ohms for the larger (standard/available value of 39 ohms /2).

    L100 flat... there's a good one.

    I'd assume that meant "as intended" such that deviations from there are for
    environment or user preference adjustments.
    Yep, that's what this has all been about; finding the number that JBL considered "flat" for the L100 as deemed by the moniker the "laboratory standard"
    You know, if you take a trip down memory lane and read their hype and ad copy, they insist it to be a most accurate, honest and flat system...................God Bless their hearts!

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