"Tuning" Analog Delays Trim Pots Without An Oscilloscope

Discussion in 'Effects, Pedals, Strings & Things' started by critter74, Feb 25, 2012.

  1. critter74

    critter74 Supporting Member

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    So I shave a DOD-FX90 Analog Delay pedal. Anybody who has one of these knows they are pretty awesome. I've had it for well over 20 years...

    OK long story short, I let someone borrow it and I think they tweaked with the internal trimmers to get a longer delay time. No big deal, it's not a regular pedal for me. However I do want to get it "tuned" properly again. I know that usually to get pedals that use analog delay chips (delays, flangers, chorus, etc) an oscilloscope is used.

    Is there a method or rough procedure to doing it by ear? I know I can "just try it and see" but with 4 internal trim-pots, I was hoping for a little methodology and insight into how the trim-pots work and effects the chips...
     
  2. AXXA

    AXXA Supporting Member

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    I've looked into this for my DM-2's. They sound great, but I think they may have been tweaked a bit, and I am interested in trying to get them as 'stock' as possible, but I'm worried I may actually screw them up. I've tweaked a few trim pots, but never on a delay.

    Anyway, I found some instructions on a Boss forum, I believe it may have been part of the Boss Area website.... I think it described how to use an oscilloscope for the DM-2, but it did give me some insight on the trim pots. It may be helpful if you can find it. Have you tried searching for the FX90's calibration method?

    BTW, don't shave your pedals, tone is in the whiskers.....
     
  3. critter74

    critter74 Supporting Member

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    D'oh! Im'a gonna leave that typo in. Makes it all better that way...
     
  4. Jack DeVille

    Jack DeVille Member

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    The short answer is: No.

    Calibrating/aligning a BBD circuit properly and accurately requires a signal generator and an oscilloscope, at a minimum.

    There are a few sections to the process:

    -Clock trim
    -Bias
    -Clock balance
    -Regeneration (if applicable)

    You will need a schematic and all appropriate reference materials, if applicable.

    First on the block is to set the proper clock frequencies, if specified. Calibration sheets will specify this if required.

    Next up is to bias the BBD. Generally, you want symmetrical clipping of the waveform injected via your signal generator.

    Clock balance will help/eliminate clock noise/whine. Adjust the clock balance trim (if provided) so your output waveform is "free" from tics.

    Regeneration is probably the only portion than could be performed "by ear." Max out your regen control and adjust to the specified/desired quantity.

    This isn't to say you can't just dink around and get the circuit "close," but close is really only good in horseshoes and atomic bombs.

    There is an ever-present double edged sword within BBD circuits. To maximize performance, trims are required, but where a trimmer is placed, a screwdriver is soon to follow.

    :BluesBros
     
  5. critter74

    critter74 Supporting Member

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    Oh is that all. :roll

    In all seriousness, thanks,. Thats pretty much what I had thought and was discovering from some reading I had been doing.

    Thing is, its all out of whack and though I love it (it really is a cool old analog delay) I'm not going to spend the money to ship it to someone to have it properly calibrated. It would probably cost more for shipping+labor than the pedal itself is worth.

    Another weird thing- before I let this person borrow, it was doing some king of weird/cool things. Mostly at certain delay times, the repeats would be like an octave above or below the initial input signal. Not every repeat but sometimes like the 3rd repeat would be like an octave down, then another random pitch and then normal. It would do it pretty consistently at certain (lower) delay settings.

    Do you know what would cause that?

    Thanks :aok.
     
  6. abfackeln

    abfackeln Supporting Member

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    I've done this rudimentary-style of delay tuning on all of my delay builds (primarily DM-2/Aqua Puss clones), with great results. Is an o-scope better? Absolutely. But this method has served me well in over 20 builds without any complaints. Again, as Jack stated, a schematic is a must. And this method is geared towards a DM-2 based circuit with only 3 trimmers (for bias, cancel and clock). YMMV.....

    (paraphrased from my own post on another forum):
    1) Set all 3 trimmers to their "noon" position (halfway through their rotation)
    2) Set the Delay and Feedback controls about halfway up, Mix knob all the way up
    3) Start playing short repeating notes (with your guitar), while adjusting the bias trimmer. There's only a small range where you'll actually get repeats - you (typically) won't get any repeats at either end of the rotation. Once you find the "sweet spot", adjust the trimmer slowly in either direction until you get the least amount of distortion/best sound in the repeats.
    4) Adjust the cancel trimmer. This is best done with at least a signal tester/audio probe. If you're using a signal tester, attach the neg to the ground of the input, output to the amp, and touch the pos to the middle leg of the cancel trimmer. You'll hear a bunch of white noise, with a high pitched squeal (the squeal is the clock noise). Adjust the trimmer until you get rid of the high pitched squeal. It's generally somewhere around noon. If you don't have a signal tester, just set the trimmer around noon.
    5) Adjust the clock trimmer. This sets the max delay time - longer delay will add more noise (clock noise), while shorter delay will be quieter. Somewhere just short of noon should give approx. 300ms (more or less) with the Delay knob maxed.
     
  7. critter74

    critter74 Supporting Member

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    Awesome. I'll give this method a try. This is pretty much what I was looking for. As I said, it's not a regularly used delay, so I'm not looking for exact calibration. Just approximation to get it usable again. I'm sure I can find a schematic somewhere without much fanfare...

    Thanks again.
     
  8. bean

    bean Supporting Member

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    Pretty much this, all the way. I do it this way most of the time.
     

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