Any one done the Tim C vibro-king mod?..need help

Discussion in 'Amps/Cabs Tech Corner: Amplifier, Cab & Speakers' started by pfrischmann, Oct 4, 2004.


  1. pfrischmann

    pfrischmann Member

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    Hi guys. I decided to do a mod Tim Cowles came up with for the vibro-king but I do not understand part of the directions. Any of you techs want to lend a hand?


    The schematics and mod can be gotten @ blue guitar schematics

    http://blueguitar.org/schems.htm#Fender

    Step # 4 is where I'm having trouble. I believe Tim is refering to R22 on the schematic. Can anyone confirm?

    Thanks,
    Paul

    Here's the mod.....

    (1) At the plate of V4A are two coupling caps in series, a .22mfd. and a .0068mfd. Remove the .22 cap and replace it with a straight wire. It is
    identified as C5 on the circuit board. After each of these caps is a 470kresistor to ground. Unsolder and remove both resistors from the circuit. They are R21 and R15 on the board.

    2) At the cathode of V4B is a 1.5k resistor and a .47mfd. cap. Remove the.47 cap identified as C10 from the circuit.

    3) Change the treble tone cap in the tone stack from 150pf to 390pf. It is C7 on the circuit board.

    4) From the wiper of the treble cap to the phase inverter is a 100k resistor. Remove it and replace it with an 820 ohm 1/2 watt resistor.

    5) Change the output coupling caps, C13 and C14 from .047mfd to
    .033mfd.

    6) Change the cathode bypass cap at V4A from 22mfd to .68 or 1mfd. It is
    C4 on the board.
     
  2. pfrischmann

    pfrischmann Member

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  3. fullerplast

    fullerplast Senior Member

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    This should read from the wiper of the treble pot , not cap. He is referring to R58, I believe.
     
  4. pfrischmann

    pfrischmann Member

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    Thanks fullerplast!

    I missed the tone designation on the schematic. I should have traced from there.

    Now.....I have to figure out why I get a crackling noise whenever I hit a low C.....

    I wonder if the added bass from the mod is causing the mojo blue frame speakers to bottom out?

    This is getting complicated....
     
  5. fullerplast

    fullerplast Senior Member

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    Here are some things to try:

    Check your solder joints carefully. Reheat if questionable.

    Try connecting the amp to an external cabinet to make sure the problem is vibration sensitive. (Isolate the amp from the speaker)

    *Carefully* probe around with a wooden chopstick to see if you can locate a sensitive joint while the amp is on.

    I'm assuming that since you are doing this mod you know the high voltage dangers and are being safe....
     
  6. pfrischmann

    pfrischmann Member

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    Thanks again fullerplast,
    I tried the amp trough a marshall 4X12 and do not hear the same fizz etc.. It is a bit difficult to tell though as many things in the room rattle.
    The chopstick thing was my next shot.
    I am being safe and following standard safety procedures.
     
  7. fullerplast

    fullerplast Senior Member

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    One more thing-

    Make sure you check all the tubes before you go crazy tracking down a suspected solder joint! That has gotten me before....Swap them out, one or two at a time.
     
  8. pfrischmann

    pfrischmann Member

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    Thanks Fullerplast!!

    It looks like it was a bad solder joint. Boy am I sloppy...I got a lot to learn.

    Thanks again.
    Paul
     
  9. phil123

    phil123 Member

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    Paul,

    What were the results of the Vibro-King Cowles mod? Did you like what you got out of it? Just curious to know the results.

    -Phil
     

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