Treble Control Dead...!!

Discussion in 'General Electronics Chat' started by wolf1419, Nov 3, 2008.

  1. wolf1419

    Thread Starter Active Member

    Nov 1, 2008
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    It's a Fender Princeton Chorus PR82 where everything works fine except the treble control, which has been cleaned, but still has no affect on the sound(0-10), just a slight scratchiness, but no affect on tone.........is it the pot, cap, or resistor...? The bass control is quite boomy when turned up also, so generally the amp sounds dull. Any ideas.......:confused:
     
    Last edited: Nov 3, 2008
  2. bertus

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  3. wolf1419

    Thread Starter Active Member

    Nov 1, 2008
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    I'm sorry, I just edited my post - it's a Princeton Chorus PR82(solid state), and I don't see it there....:eek:
     
  4. SgtWookie

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    Jul 17, 2007
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  5. bertus

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    Hello,

    Yes, I see it also. There are only tube related schematics over there and no solid state.
    I do not want to send you away but perhaps the guys from the fender forum know the issue you mention.
    http://www.thefenderforum.com/forum/index.php?
    They have a tech & repair section over there.

    Greetings,
    Bertus
     
  6. bertus

    Administrator

    Apr 5, 2008
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    Hello,

    When I look at the tone-part of the schematic SgtWookie provided, I think there is something wrong with C20 or C21.
    If there is something wrong with R43 or C23, you would probably have no sound.

    Greetings,
    Bertus
     
  7. AG3Y

    New Member

    Nov 4, 2008
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    Not C21 , that would effect Midrange. C20 most likely !
     
  8. wolf1419

    Thread Starter Active Member

    Nov 1, 2008
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    If I have a clue it looks like C21 for bass, C22 for midrange.....is that correct......I'm in the diaper stage right now..learning to crawl.
    Is it true that bad resistors would mean no sound...?
     
  9. bertus

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    Apr 5, 2008
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    hello,

    You say you hear a scratching sound when you move the treble pot.
    This sound is probably a weak contact of the wiper of the pot, most likely made by dust inside the pot.

    Greetings,
    Bertus
     
  10. wolf1419

    Thread Starter Active Member

    Nov 1, 2008
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    It was very scratchy at first, and knowing it was dirty(i've cleaned many a pot in guitars & amps), so I cleaned it with contact cleaner as usual, and it got much better, but still no change in tone/treble. The "slight scratchiness I mention" is just to say that the pot does seem to have some very slight affect in the circuit. I included that to differentiate it from a pot being completely dead(disconnected, etc...) with no sounds at all, suggesting that it probably is still "in" the circuit, which is why I was thinking cap C20.........will a bad cap give us this result(no tone)...(open or short)...??
    I will be getting into the circuit board at some point, but thought I'd get into some potential causes, schematic reading, learning what components actually contribute to these circuits and gain some basics knowledge.
    Thanks for helping me through the thought process.
     
  11. bertus

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    Apr 5, 2008
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    Hello,

    As SgtWookie already has said, the schematic can be or not of your chorus.
    If the C20 is broken, it can make a short or does not do anything (full open contact).
    If the C20 has a short, it will influence the other controls also.
    If C20 is open , R40 will act as variable resistor and have no influence in treble at all.

    Greetings,
    Bertus
     
  12. wolf1419

    Thread Starter Active Member

    Nov 1, 2008
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    My schematic that came with the amp does match what you have here online, so this is the correct one.
    (If C20 has a short) I'm not noticing anything wrong in the midrange, but the bass seems overly boomy.
    (If C20 is open) R40 would act as a variable resistor - would that act like a volume control in that state?(I get no changes)
     
  13. bertus

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    Hello,

    Can you make a scan of the schematic part of the tone control of the one you have?
    You can place it as attachment on the forum.

    Greetings,
    Bertus
     
  14. wolf1419

    Thread Starter Active Member

    Nov 1, 2008
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    I'm sorry, but I don't have the ability to do a scan, but I checked my schematic and the one you have is "exactly the same"(it is directly from the original, right down to the printing, all parts, values, etc....)Hope that helps.
    It's great to have a mate from Amsterdam. My last recording console came from D&R Electronics/Holland and was quite awesome(56 input, 8 floating buss, fader automation, inline - excellent features/sound. Thanks for helping with the instruction.
     
  15. bertus

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    Hello,

    When C20 is "open" then the volume change will be minimum, the total value changes from 220 K to 270 K.
    The volume will change from 2.1 to 1.7 times (gain combined with R44).
    A gain change of factor 2 ( 3 dB ) will be heard.

    Greetings,
    Bertus
     
  16. wolf1419

    Thread Starter Active Member

    Nov 1, 2008
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    Well, I did open the amp, got to the board(quite a small feat getting under that board w/o removing a bunch of stuff), checked the pot on the board and it appears to function, R value changes distinctly pin 1-2, and 2-3(from 0-infinity) similar to the bass and mid pots. C20 appears to be "open", since I can't get an ohm reading on any range, but I'm not certain with the value of that cap if it would allow a reading, or not. I tried all my ranges and it appears "open". I put it all back together for the practice and to be sure I didn't ruin or affect anything.
    The treble pot still does not appear to affect volume in any way, at least not noticeable, but I haven't played it loud either. Something much more noticeable and a possible clue is that the midrange and bass controls are greatly affecting volume. They actually shut the volume completely off when counter-clockwise to 0. I don't think any amps do that.......is that a clear clue, and does that still point to C20 as the culprit??
     
    Last edited: Nov 9, 2008
  17. bertus

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    Hello,

    A capacitor can not be measured with a normal ohm meter.
    A capacitor consists of a couple of plates with an isolator (dielectricum) in between.
    A capacitor can be measured with a capacity meter.
    So you will always measure "open" with an ohm meter.
    With the "open" I mean is that the leg of the capacitor does not make contact to the plates anymore.
    With a "short" I mean that the isolatin between the plates is broken and you can measure 0 ohms with an ohmmeter.
    More information on capacitors can be found here.
    http://www.educypedia.be/electronics/capacitors.htm
    On the resistor page of this site you can find information on potentiometers.
    http://www.educypedia.be/electronics/resistors.htm
    This is a site with links for mainly educational purposes.

    Greetings,
    Bertus
     
    Last edited: Nov 10, 2008
  18. wolf1419

    Thread Starter Active Member

    Nov 1, 2008
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    Thanks for the links. I've been searching extensively(studying, but not truly understanding the big picture) in other sites similar to this for basics and methods of testing.
    I seem to have a pot(variable resistor) that is functional, C20(tone cap) which is not shorted, and possibly open, but do not have the proper tools to measure it(my meters don't do that test).
    Based on the interaction of the mids and bass each taking my volume level to 0 when fully counterclockwise, does that give us a clue to the offending part......?
    I have a clue when it comes to the individual isolated(series) parts of the circuit, variable resistor R40 resistance effectively blocking low frequencies passing through C20, but cannot "picture", or understand how the 3 tone circuits affect each other.......parallel(confused with interaction, and why the tone controls(mid & bass) should be able to cut my total signal completely off, while still acting like tone controls at the same time).
     
  19. bertus

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    Hello,

    When you look at the schematic, R41 (bass) and R42 (mid), when these are turned completely counterclockwise as you say, the right point of C21 is at ground level.
    When this happens and C20 is "open", no sound is heard at all, the bottom of R41 is grounded and no signal is coming on the top of R41.
    When C20 was OK you would hear a very sharp sound.
    When C20 was shorted you would hear a "flat" sound (no effect on the tone).
    So I think that you will have to replace C20.

    Greetings,
    Bertus
     
  20. wolf1419

    Thread Starter Active Member

    Nov 1, 2008
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    O.k., so turning a pot counterclockwise to "0" would be down(graphically) in the schematic and toward ground level. If all the tone caps are good, only the associated tone(t,b,m) would be reduced, but if a cap is open it puts the entire frequency/signal to ground, causing "no level", acting as a volume controls in this case....(correct?)
    Now, as the treble pot is turned counterclockwise to "0", it has no effect due to the open C20(no frequency change) and all mid & bass signals pass as long as they are not fully counterclockwise(grounded). Looking at the scematic, does this signal actually pass through the treble pot, or just bypassing the actual pot(through outer pins closed)..?

    One more question regarding tone caps/pots. Are they "cut only" in actuality, or are they boosting and cutting(t,b,m) from center=5. I think guitars are considered "flat" at 10, and treble is cut only. Is it different in these amplifiers...?

    I know this must be simple stuff for you and I really appreciate your patience in helping my understanding. I also have a limitaion called CFS, which affects my cognitive functions, so sometimes things just don't sink in and process as well as they might otherwise, so thanks!!
     
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