Fender 68 Custom Princeton Problem

Discussion in 'Amp Tech Center' started by ilgfo, Sep 20, 2019.

  1. ilgfo

    ilgfo TDPRI Member

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    First and foremost, hello everyone, been reading TDPRI for a few years since I started playing guitar. This forum is fantastic with a wealth of info. Thanks to everyone for helping me out along the way.

    So here is what happened: I was playing guitar and turned on a pedal and the second I stepped on the pedal my amp volume dropped very low, I then turned the pedal off and the volume got a bit louder again but it definitely didn't go back to as loud as it was. I then shut the amp off and panicked a little and dismantled my pedalboard. Turns out I had a bad cable that was pinched and it was shorting the tip to the sleeve.

    I kept the pedalboard out of the equation and turned the amp back on about 30 minutes later and it was definitely not right, I normally play with the volume at 2 and now 2 seemed to be at 4. Im pretty handy with electrical, electronics etc and I decided to take the amp apart and check the tubes and some TP voltages. For the sake of keeping this short I thought some voltages were definitely not right and wasn't sure about the rectifier tube so I ordered a new set of tubes.

    The tubes showed up today, I plugged them in and fired the amp up, but it still seems too quiet on 2. So then I searched for a schematic and could not find one, but I found one for a 65 Princeton Reverb Reissue (See Attached) It came from this site which has great pictures of the circuit board https://guitarampblog.wordpress.com/2017/01/26/fender-65-princeton-reverb-schematic-vs-68/

    I figured since the amps are very similar the voltages should be the same but I don't know, and can't find any info. I decided to check some test points and here are some readings:

    C28 TP30 445VDC - My reading = 416VDC
    C26 TP31 425VDC - My reading = 403VDC
    C29 TP32 250VDC - My reading = 255VDC
    TP33 & TP34 3.3VAC My reading= 2.87VAC
    3V5 TP15 440VDC - My reading 407VDC
    3V6 TP18 440VDC - My reading 405VDC

    These seem low to me but I don't know for sure if these amps are the same and I am far from an expert here.

    Does anyone know what's going on here? Can anyone help me troubleshoot this thing? I am starting to doubt myself and wonder if its just me but I swear its too quiet, its driving me crazy!

    Thank you very much

    cj
     

    Attached Files:

  2. corliss1

    corliss1 Friend of Leo's Platinum Supporter

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    It's hard to say what "too quiet" is - can you go to a store and try another to compare? Your voltages, while a tiny bit low, are well within spec.
     
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  3. ilgfo

    ilgfo TDPRI Member

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    Thanks Corliss1, I will take this thing up to a store tomorrow and check and not bother anyone here :) If its messed up I will just leave it for repair. A mod can actually nuke this post as it's a waste of time. Thank you!
     
  4. corliss1

    corliss1 Friend of Leo's Platinum Supporter

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    Never a waste of time - there's always valuable info here :D
     
  5. tarheelbob

    tarheelbob Tele-Meister Platinum Supporter

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    '68 Custom Princeton schematic
    upload_2019-9-20_20-45-43.png
     
  6. tarheelbob

    tarheelbob Tele-Meister Platinum Supporter

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    bias schematic....
    upload_2019-9-20_20-49-40.png
     
  7. ilgfo

    ilgfo TDPRI Member

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    Tarheel you just put the biggest smile on my face! Thank you!! Corliss was right, never a waste of time! I combed the internet and could not find this darn schematic. Awesome! I love this place
     
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  8. ilgfo

    ilgfo TDPRI Member

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    Any chance you have this in a higher resolution? I cant make out the majority of the writing on it
     
  9. corliss1

    corliss1 Friend of Leo's Platinum Supporter

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    Here you go
     

    Attached Files:

  10. ilgfo

    ilgfo TDPRI Member

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    Corliss1 THANK YOU SO MUCH!!! This is awesome! I had no clue these existed, made things much easier over here. Are they online and I'm just blind? I would love one for a 65 Princeton 15W 12" Tweed also.
     
  11. corliss1

    corliss1 Friend of Leo's Platinum Supporter

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    There aren't special schematics for different versions - the only change with that model is the speaker.
     
  12. ilgfo

    ilgfo TDPRI Member

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    Embarrassed to report back here but just wanted to let you guys know it was all me, took amp to the store and plugged in next to another one and everything is fine. Siigh... In the end it wasn't really an exercise in futility as I learned a few things along the way, picked up a service manual and finally registered on TDPRI :) Thanks again for your help.
     
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  13. corliss1

    corliss1 Friend of Leo's Platinum Supporter

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    Any time you get to learn something, it's never a waste :D
     
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