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Thread: Fender M80 Chorus clipping on left channel

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    Fender M80 Chorus clipping on left channel

    I've been working on a Fender M80 chorus. I'm not sure about it's history, but long story short there were several damaged components. I've got everything working except for one mystery I can seem to pin down. On the left channel, the top half of the wave for is cut off. It seems as though it's happening on U13 MC1436 input. Any help is appreciated. R143-A.png is between R143 & C69 on the left channel which is okay. R144_R117.png is between R144 & R143 which is where the waveform gets clipped. Green is the left channel (clipped) and yellow is the right channel(OK).
    Attached Thumbnails Attached Thumbnails Click image for larger version. 

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    Schematic

    I attached a better quality schematic.Click image for larger version. 

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    Lifetime Member Enzo's Avatar
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    Take away any speaker load and run the signal through the amp with no load. Does it still do it or does the signal clean up?

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    The result is the same with no speaker load. I should mention that when testing the headphone output, it does not have the issue.

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    Quote Originally Posted by bbakehorn View Post
    The result is the same with no speaker load. I should mention that when testing the headphone output, it does not have the issue.
    This is sort of contradictory in that all the headphone jack does is disconnect the speaker. When you don't get it through the headphone out, are you checking with the scope or by sound?

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    Lifetime Member Enzo's Avatar
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    Agree^^^

    The phones jack has feed-through cutout contacts for the speaker, but the extra contacts - it is one of those 9-pin guys - do introduce some resistors and caps. But I can;t see how resistor voltage dividers could selectively clip the top but not the bottom of the waveform. But that only happens when you ARE using the phones. Normally it is just straight through.


    I find myself straining to come up with some scenario like the tiny e-caps on the phones jack are somehow having a polarity thing affecting the feedback loop and...and...and... I think something more basic is going on.

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    Okay, I did some more testing. Attached are 3 images from the scope. headphone-output.png is left (green) and right (yellow) from the headphone jack itself. R144_Headphone.png is headphone jack left(green) and R144 (yellow) with the headphone jack connected. R144_NoHeadphone.png is R144 (yellow) with headphone jack disconnected. So maybe the lower volume of the headphones and something in the headphone circuit is cleaning up the signal?

    Click image for larger version. 

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    Ok, I think the misunderstanding here is that we thought you were looking at the output.
    I think what you are seeing at R144 and R143 is being fed back from the output.
    Have you cleaned the headphone jack? The switches in it connect the speakers when the headphone jack is not in use.

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    I used LTSpice to model the left channel circuit. Through trial and error using the simulator, I was able to get the same scope results. I narrowed it down to resistor R165 (.47ohm 5W) being open. I replaced the resistor, and now the output is perfect. Thanks everyone for your help. I'm guessing the lower power of the headphones, was throwing me off track of what the real problem was.

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    Old Timer J M Fahey's Avatar
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    Thatīs why we are somewhat obsessive about *how* are things measured or tested.
    Damaged/defective elements may still behave "almost normal" under some very light loads (such as headphones or line out or plain driving output VU-meter) , yet fail miserably when asked to do the real job.
    Typical scope input impedance is 1M , meters are often 1M to 10M, easy peasy, that can throw you off track with fake "acceptable" results.

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    Juan Manuel Fahey

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