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Hotrod Deluxe Reverb/switching supply problems

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  • #1

    Hotrod Deluxe Reverb/switching supply problems

    Not super familiar with SS devices but im learning. I cant tell if the TL072 (u2) is bad or not. The amp was also channel switching by itself. i replaced the two 470ohm resistors and raised off the board. switching problem fixed.
    Reverb input stage is faulty. U2a TL072. pin 1 output is bad. pin 3 input looks good on the scope. pin 2 is grounded (0.6ohms) R33 is measuring 170ohm (4.7K) and C14 is measuring 46ohms. The power supply rails are measuring +16.5V/-17.15V . To me thats the real problem. The scope shows both rails are clean from filtering. the zeners CR13/14 are 16v 5W zeners. My guess is those will need to be replaced to balance things out. am i close to being correct?
    I am going to replace R33 and C14 for good measure also.



    http://support.fender.com/schematics..._schematic.pdf
    Tags: None
  • #2
    C14 must be bad if it measures 46 ohms, there is nothing in parallel with it.
    Were you measuring R33 with the tank connected? That would give a reading around 170 ohms.
    Did you resolder the zeners? If the one for the negative side was not connecting, you might get that little bit extra voltage.
    Originally posted by Enzo
    I have a sign in my shop that says, "Never think up reasons not to check something."


    ​

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    • #3
      Originally posted by g1 View Post
      C14 must be bad if it measures 46 ohms, there is nothing in parallel with it.
      Were you measuring R33 with the tank connected? That would give a reading around 170 ohms.
      Did you resolder the zeners? If the one for the negative side was not connecting, you might get that little bit extra voltage.
      HAHA woops, yup the tank was connected.
      Resistors measure good now, C14 is measuring 14K. goona replace that when i pull the board. Will pull and test the zeners and resolder. how do opamps usually fail?
      im getting output on it but its ugly and low level. could it be simply the unbalanced supply voltages?

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      • #4
        Originally posted by ThisLifeILead View Post
        how do opamps usually fail?
        im getting output on it but its ugly and low level. could it be simply the unbalanced supply voltages?
        The difference between your supplies is less than a volt. I would look for something else.

        Most op amp failures will show as a dc voltage on the output. Read the voltage to ground at pins 1 and 7 and see what you have.

        Of course this will not be true of the opamps in the channel switching sections as they are supposed to have dc voltages there.

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        • #5
          Originally posted by 52 Bill View Post
          The difference between your supplies is less than a volt. I would look for something else.

          Most op amp failures will show as a dc voltage on the output. Read the voltage to ground at pins 1 and 7 and see what you have.

          Of course this will not be true of the opamps in the channel switching sections as they are supposed to have dc voltages there.
          Im getting 1.24v on Pin 1. pin 7 zero. This opamp is in the reverb section. I suspected it was faulty.I pulled and tested the 16v zeners (C13/14). I must have mixed them up because now the positive supply is +17.05v and the negative supply is reading normal. (-16.5v) one zener must be failing.
          I will replace the zeners and the opamp.
          Thnx guys!

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          • #6
            replaced zeners and opamp. better but still slightly distorted reverb. Swapped pans with an old used unit. Works great now!
            I learned stuff today :-)
            Thnx everyone.

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