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Thread: Fender Princeton Chorus - Taming the drive channel

  1. #1
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    Fender Princeton Chorus - Taming the drive channel

    I know 90's Fender solid state drive channels are an unpopular topic.
    I know easiest solution is a different amp or using a stand alone preamp.
    I have other amps but I love this amp, it was the first amp I ever owned, the only thing I don't like is the distortion on drive channel.

    Theres some discussion here about the limiter circuit:Princeton Chorus Limiter circuit
    but what I'd like to do is smooth out the distortion. If you could point out the key components I willing to try different values, I have a range of resistors and caps to play with. I also have some OPA2134pa op amps I can sub in too.

    here is the area in question
    Attached Thumbnails Attached Thumbnails princeton-chorus-drive.jpg  

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    Member R ski's Avatar
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    The chorus limiter portion looks weird, opto isolator kind of biasing the clipping diode bridge. What are the characteristics of the distortion that sound awful? Is it shear high frequency sounding or low fuzz that muddles the distortion? My first guess would be to unsolder the two coupling capacitors C11 and C12, isolating the op amp bridge portion. Try two red leds opposing between the capacitor lifted, they will soften the clipping characteristics, that's a start.

    Changing the op amp will define some overall tonal change. I copied an older MXR distortion pedal from the 80's and found an NE55xx (single op amp) really improved compared to a TL071 or the stock 741 .. that circuitry used germanium diodes for clipping that worked well with that configuration.

    Trial and error will eventually get what your hoping for, two considerations, firstly, are electronic components available where you live? Secondly, is the circuit board durable enough to desolder components, a 25 watt soldering iron should work best.
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    Quote Originally Posted by R ski View Post
    ......Try two red leds opposing between the capacitor lifted, they will soften the clipping characteristics, that's a start......
    I'm not hacking on your suggestions. They are very good. I just got a chuckle out of the fact that you specified red led's.
    “Yeah, well, you know, that’s just, like, your opinion, man.”

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    Quote Originally Posted by The Dude View Post
    I'm not hacking on your suggestions. They are very good. I just got a chuckle out of the fact that you specified red led's.
    Well you know red's an angry color needed for that extra grit. Thinking about this what about UV led's or infrared?

    nosaj

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    Supporting Member The Dude's Avatar
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    Quote Originally Posted by nosaj View Post
    ......what about......infrared?......
    Yeah! If you play the right notes, you might be able to change TV channels.
    “Yeah, well, you know, that’s just, like, your opinion, man.”

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    Lifetime Member Enzo's Avatar
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    Much as I appreciate a laugh, the different colors have different forward voltages. In some Fender switching circuits, that makes a difference between working or not.
    Education is what you're left with after you have forgotten what you have learned.

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    Yes, red is usually at the low end of the scale as far as voltage. However, IR would be even lower, so maybe a valid question as I don't think you would have interference with it being in the chassis.

    Certified Dotard

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    Supporting Member The Dude's Avatar
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    Yep. I believe, as a general rule, the higher the light frequency, the higher the voltage drop? So, in a clipping stage, I guess color might be enough to make a difference? I suppose it depends on the signal level at that stage as to what you want there. I haven't encountered the Fender phenomenon where a different color LED can make the difference on whether a switching circuit works or not, but it makes sense. You can also take a bag of LED's of the same color and find different voltage drops within the batch. I was mostly just being a smart aleck. I'll stop now.
    “Yeah, well, you know, that’s just, like, your opinion, man.”

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    Quote Originally Posted by R ski View Post
    The chorus limiter portion looks weird, opto isolator kind of biasing the clipping diode bridge. What are the characteristics of the distortion that sound awful? Is it shear high frequency sounding or low fuzz that muddles the distortion? My first guess would be to unsolder the two coupling capacitors C11 and C12, isolating the op amp bridge portion. Try two red leds opposing between the capacitor lifted, they will soften the clipping characteristics, that's a start.

    Changing the op amp will define some overall tonal change. I copied an older MXR distortion pedal from the 80's and found an NE55xx (single op amp) really improved compared to a TL071 or the stock 741 .. that circuitry used germanium diodes for clipping that worked well with that configuration.

    Trial and error will eventually get what your hoping for, two considerations, firstly, are electronic components available where you live? Secondly, is the circuit board durable enough to desolder components, a 25 watt soldering iron should work best.
    I have pleny of components and a good soldering iron. If there are some key components in the circuit I can socket them for testing.

    I'll give your suggestion a try. Sounds like I could put that on a switch. Do I keep C11 and C12 in the signal chain when bypassing the limiter circuit? Also i'm guessing the opposing leds would be in parallel not in series?

  10. #10
    Lifetime Member Enzo's Avatar
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    Dude, I once went nuts sorting out a Fender switching circuit. SOmeone had replaced an LED with another same color LED< but the thing had a different forward voltage. In the switching circuits, voltage is set in the control circuits by component drops. especially in those four or five button circuits. I learned my lesson that day.
    g1 and The Dude like this.
    Education is what you're left with after you have forgotten what you have learned.

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    Supporting Member The Dude's Avatar
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    That's good to know. Stored in my memory banks!
    “Yeah, well, you know, that’s just, like, your opinion, man.”

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