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Thread: Boss limiter LM2 no level control

  1. #1
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    Boss limiter LM2 no level control

    Hi,
    This unit does not have a working level control. It doesn't have much happening when switched on except a volume increase.
    So when I hit the stomp switch volume goes up and not much else. The tone control does work. The threshold, maybe a little bit of an effect but it's hard to tell much of a difference.
    So far I have tried tracing a signal and I get a nice clean sinewave up to the gate of Q2 but on the source it is irregular. Tried another Jfet in, a J201, with the pin out adjustment and no change
    I have pulled the pot and it measures the 100k that it should. The connections all appear to be happening up to IC 2.
    IC1 has a very small clipped wave on pin8.
    Voltages at the supply start -9v but are lower at ~-6v on pin 2 and 6 of IC1.
    Are one these IC's bad? Is there any other ways of checking them?
    Thanks for any help
    Attached Thumbnails Attached Thumbnails boss_lm-2.png  

  2. #2
    Senior Member Enzo's Avatar
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    The level pot may be 100k, but that isn't the test that matters, what does is whether the wiper makes contact through its range.

    IC1-8 feds directly to IC2-6. Op amp inputs are at "virtual ground" and you can't often read signals there, so what does the output of that amp look like at IC2-7?

    Is this drawing from Boss? The switching circuits are missing.

    IC1-3 is the control voltage pin - the IC is a VCA. What is going on ther? Does it vary with signal level?
    Education is what you're left with after you have forgotten what you have learned.

  3. #3
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    Checked the pot again,,my meter is showing continuous contact on the wiper.
    Pin7 of IC2 is the same is it is on pin 6 of IC2. But I'm not using vitrual ground. Should I use the input ground?

    As far as I know this a schematic is from Boss, but heck I just got it off the internet so who knows.

    Checking signal using input ground-- pin 7 IC2 is much larger but not a clean sine wave.
    pin 3 IC1 - signal does not vary there with level control(I think that's what you asked for)
    Last edited by pontiacpete; 08-08-2017 at 03:56 PM.

  4. #4
    Senior Member Enzo's Avatar
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    Not quite. Look up "virtual ground" as regards op amps. It isn't a point in the circuit, it is a condition of op amp balanced inputs. Just means it can be difficult to measure inputs at op amp input pins, so we mainly look at outputs.

    On that control pin, it isn't the level control that should make it vary. I mean the level of the input signal. Note the input signal branches down through the lower circuits to create that control so a hot signal versus a small signal or no signal SHOULD make a difference in that control voltage.

    The op amps are common, if you suspect it, then I'd just replace it. I don't know if the upc1252 is readily available or not, but same really applies.
    Education is what you're left with after you have forgotten what you have learned.

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