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Thread: Dunlop Univibe no chorus

  1. #1
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    Dunlop Univibe no chorus

    This pedal came to me with no signal passing with or without the effect. I found the voltage regulator was not working. Replaced and now it passes dry and effect signal but thereís no signal when pressing the chorus/vibe switch.
    I was able to get the chorus to work and red led to come up after probing around u3 pins 2 and 3. But havenít b en able to repeat this.
    Is there anyone here that knows these pedals that would be willing to help me?the schematic is here

    https://freestompboxes.org/members/l...ic%20(2-3).pdf

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  2. #2
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    Pins 2 and 3 of U2 should be sitting at about half of the supply rail, so around 7.5VDC ?
    Look in the power supply section of the schematic where Vbias is created. What is DC at U2 pin 7 ?

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    U2. Pin 7 has 7.4. Pin 2 has 7.4 also Pin 3 6.7

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    So is now working for vib but not for chorus?

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    "Everything is better with a tube. I have a customer with an all-tube pacemaker. His heartbeat is steady, reassuring and dependable, not like a modern heartbeat. And if it goes wrong he can fix it himself. You can't do that with SMD." - Mick Bailey

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    Yes vibe only. Chorus no sound

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    How about voltage at point A+ shown in power supply area? (check also C+)

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    "Everything is better with a tube. I have a customer with an all-tube pacemaker. His heartbeat is steady, reassuring and dependable, not like a modern heartbeat. And if it goes wrong he can fix it himself. You can't do that with SMD." - Mick Bailey

  7. #7
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    A+ voltage looks good 24vdc on all U 3-7,10
    C+ voltage is good also 13.2vdc on U2,8,11,and 12(although it’s on pin 7 instead, perhaps an error in the drawing)

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    Sounds to be as if U6 may be at fault.
    Is the signal path 9/13, 11/12 good & does it change state with the switch?

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  9. #9
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    I'm not sure I"m doing this right. With a signal at the input there's not much of anything to see on the scope on pins 9/13 and 11/12.
    But when probing with the DMM some weird things happen. First there's 9.5v on pin 9 and 19v on pin 13. When pressing the chorus/vibe switch pin 9 voltage jumps up for a moment and then back down to 9.5v, pin 13 nothing changes. The weird thing is that I loose all sound on the output and I can get it back when toggling the effect/bypass switch.
    Pin 11 has 19v on it and when switched it goes up to 20v, pin 12 has 0v the whole time

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