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Muff Pedal Distortion Repair

Discussion in 'General Electronics Discussion' started by amonarch71, Sep 3, 2018.

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  1. amonarch71

    amonarch71

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    Aug 27, 2018
    I am fixing a Muff Pedal and I accidently blew the red LED out while testing it with a 9 volt battery directly to the LED. Kits_V1_Big_Muff_67_No_1_schematic.jpg
    I have a schematic of the circuit, but I do not have a schematic that shows the LED in the schematic for the
    Muff Pedal.

    I have tested LEDs quite often with a 9 volt battery, and I have never blown one up... large pop noise, smell but no smoke.
    I was wondering if the diode was damaged or maybe there is another part of the circuit that was damaged to cause this
    malfunction?

    Thanks
     
  2. Harald Kapp

    Harald Kapp Moderator Moderator

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    Nov 17, 2011
    You've been just lucky then. LEDs need current limiting.
     
  3. AnalogKid

    AnalogKid

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    Jun 10, 2015
    During normal operation, is the LED on continuously, does it flicker with the music content, etc?

    ak
     
    darren adcock likes this.
  4. amonarch71

    amonarch71

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    Aug 27, 2018
    What value should I use in series with a 9 volt battery?
     
  5. amonarch71

    amonarch71

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    Aug 27, 2018

    I have not seen it in normal operation. It was given to me from a friend who didn't want to fix it.±
     
  6. Alec_t

    Alec_t

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    Jul 7, 2015
    If the LED is intended simply to confirm the pedal is active, then to conserve battery power you could connect between the power rails a high-efficiency LED in series with, say, a 2.2k resistor. That would give a LED current of about 3mA.
     
  7. kellys_eye

    kellys_eye

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    Jun 25, 2010
    Why were you testing the LED? They are usually very reliable. If the circuit wan't working properly it is unlikely to be the LED that caused the fault.....
     
    Cannonball likes this.
  8. AnalogKid

    AnalogKid

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    Jun 10, 2015
    Another thought is to put the LED in series between the battery and the circuit. The opamp static current limits the LED current.

    ak
     
  9. WHONOES

    WHONOES

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    May 20, 2017
    There isn't an opamp in the circuit diagram!
     
  10. AnalogKid

    AnalogKid

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    Jun 10, 2015
    ok, oops (but there is an opamp version of this product). The static current of the circuit is the LED current.

    ak
     
  11. WHONOES

    WHONOES

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    May 20, 2017
    I think the only opamps available in 67 was the μA702 (from 1962) and the much improved μA709 (from 1965) but I could be wrong.
     
  12. 73's de Edd

    73's de Edd

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    Aug 21, 2015
    Sir amonarch71 . . . . .


    That LED must have been an add on . . .probably for power indicator . . . as it certainly isn't being shown on the shematic.
    Can you track down what two points it is / was wired into on the board ?

    As far as the basic circuity design proper, it almost looks bulletproof.
    No high current / power consumption . . . . .Fairchild silicon planar transistors . . . .no electrolytics . . . .using carbon composition resistors . . . .so . . . .might check the higher value 390K-100K-470K units to see that they haven't gone astray in value.
    Also check the pots, in case certain ones had normally got a LOTS of use.

    73's de Edd
    .....
     
    Richard9025 likes this.
  13. Cannonball

    Cannonball

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    May 6, 2017
    Install a 1K resistor in series with the LED and the on/off switch and ground.
     
  14. AnalogKid

    AnalogKid

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    Jun 10, 2015
    There is no on/off switch. Plugging in the guitar completes the battery connection to GND through the input jack. This is very common in instrument pedals. When you plug a mono 1/4" plug into a stereo jack, the ring contact is connected to GND through the plug sleeve.

    ak
     
  15. Cannonball

    Cannonball

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    May 6, 2017
    Connect the resistor the + side of the battery, the other side of the resistor to the anode of the LED and the cathode of the LED to the switched ground the that that is made when the guitar is plugged in.
     
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