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Thread: Mu-tron Flanger

  1. #1
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    Mu-tron Flanger

    http://music-electronics-forum.com/a...ronflanger.pdf

    This powers on but no effect. Does pass signal although it's weak. I replaced the main 470uf 35v filter which didn't have much affect. Although I'm sure it doesn't hurt.

    I'm a bit unclear on this power supply. It seems as though both ends of the primary are used for full wave rectification. And the CT is common concerning DC voltages. I guess it makes sense now that I write this. However the note next to it specifies the supply node arrows as +/15v. So I'm confused as to where the bipolar DC supplies meet...as in where is 0v?

    Finally. I have +/-7.5v on the opamps'pin 4/8. Referenced to audio ground at the output jack.

    I'd like to confirm the power supply is working correctly before proceeding. That may fix it.

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    Also there is 25vDC across the main filter.

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    Lifetime Member Enzo's Avatar
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    You mean both ends of the secondary.

    This is a simple 15v DC supply, it makes your 25vDC, and then the regulator makes it into 15v. You have the + and - ends of a single 15v supply, call it V. Now look immediately left of that. See the 5602701 IC? I think it is U18? From your parts list it is just a 4558 anyway. Note the output pin goes to ground, and it has power from V+ and V-. Look at the input of that IC, input pin 5 is fed by a pair of 22k resistors between V+ and V-. The junction of those two resistors is at the half voltage point, or 7.5v away from either end of V.

    That means the input of that IC is at the mid point, and since it is just wired for unity, so it the output pin. And that output pin connects to ground. That means that V+ and V- are now each 7.5v away from ground. We forced an artificial ground there. If you understand flying rail power amp, it is a very similar concept.

    The note is confusing at first. It means the IC is powered from the + and - ends of 15v, it does not mean there are separate +15 and -15v supplies to ground.

    Sounds like the power supply is fine. Bad filter cap would make it hummy, not weak, I'd say.


    It passes signal but is weak even in bypass mode? Great, that narrows the problem down to the input stage or the output stage. Put it in bypass mode (effect off) and troubleshoot the weak signal. It goes into the input jack through some small parts, through an IC, then up and over to the output jack through cap and a switch.

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    Thanks!

    Ok yeah I thought it was odd that that opamp output was grounded. Interesting. So voltages are good then.

    So my mistake. The bypass level is fine. The pedal... after playing arpund with the pots... Does work but the effect is very weak and signal level is low too. There are 5 trim pots in here. Do you have calibration instructions for this?

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    Lifetime Member Enzo's Avatar
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    I have zero information on this, other than what you provided in post #1.

    But circuits are circuits.

    If your effects work, then we can assume the bucket brigade and clocking are working. The signal may be weak, but it still flanges, right? SO that is the bottom half of the schematic, plus the two squares in the middle of the effect channel.

    You have an input stage to buffer the signal, it splits three ways. Top branch sends it up and over for the bypass path, which is "fine". The low branch is through an op amp that controls Q1. I suspect that is a gate, but I haven't thought about it. The center branch feeds the effect. Now that center branch goes to another op amp which itself splits the signal into two paths. One path up and over to the output IC, and the other path through the delay circuits., the output of which mixes with the dry at that output IC.

    The flange effect is simply a delayed signal (that varies the amount of delay in a sweep) mixed with the dry signal. They fall in and out of phase as it sweeps.

    Your effect circuit starts and ends with part of the 571 "compander" chip. The two squares between are stages in the bucket brigade delay chip. My thinking is that since your effect happens and it is ALL weak, then I look for something common to dry and wet paths. And my first suspects are the op amps at the ends. SO the second op amp from the input jack or the op amp just left of the output jack. The op amp on the left by R4 Z1a maybe? It feeds both wet and dry paths. And the final op amp on the right near the output jack mixes them back together.

    SO one thing I always do is look for unwanted DC on op amp outputs. But after that, apply a steady signal and trace it through the signal path, where does it lose its level?

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