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Need help understanding this circuit (Mixing board meter bridge)

Discussion in 'General Electronics Discussion' started by soul73, Jan 17, 2019.

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  1. soul73


    Oct 27, 2016
    Hey guys,

    This is a circuit of my mixing board´s meter bridge.

    You can change the source (i.e. what gets displayed) by pressing the switch SW1. You can either display the tape inputs or the channel outputs.
    The current state is that tape is selected when I turn the console on. Pressing SW1 changes the source to channel. Pressing it again changes it to tape and so on.

    What I want to achieve is that channel is the default source when turning the console on.

    The current switch seems to be Off-Mom. Am I right that I can accomplish the aforementioned behavior by changing the switch to an On-Mom type?

    Eno a_1.PNG a_2.PNG
    Last edited: Jan 17, 2019
  2. soul73


    Oct 27, 2016
    Note to self: That probably won´t work!
  3. Alec_t


    Jul 7, 2015
    Can't you just swap the E2 and E3 connections?
    That seems an overly-complex circuit for just performing a toggle function with complementary outputs.
    soul73 likes this.
  4. AnalogKid


    Jun 10, 2015
    U1A and U1B form a set-reset flipflop. It is forced into one state at power on by R2-C1. To change which state that is, you can change either the inputs or outputs. Let's Go with the outputs. The two outputs are pins 3 and 4.

    Disconnect the left side of R12 from pin4/R4/C3. Leave pin4/R4/C3 connected.
    Disconnect pin8/pin9 from pin5/pin6/R3. Leave pin5/pin6/R3 connected.
    Connect R12 to pin3/pin5/pin6/R3.
    Connect pin8/pin9 to pin4/C3/R4

    Last edited: Jan 24, 2019
    soul73 likes this.
  5. soul73


    Oct 27, 2016
    I could, but I really want to do it properly. Also the LEDs wouldn´t show the correct setting anymore.

    Thank you so much, AnalogKid. You are an absolute legend!

    Would this be the correct modification then? -> [​IMG]
    Last edited: Jan 24, 2019
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