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Fender Hot Rod Deluxe III BIAS Rising HELP

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  • Fender Hot Rod Deluxe III BIAS Rising HELP

    Hello All! I’m working on a Fender Hot Rod Deluxe III. From 2009 Revision B. The bias keeps rising and I’m not sure why. With the bias adjustment pot all the way counter clockwise, and power up, the bias continues to climb. I turn off the amp at 88 mvDC. No red plating.... There’s also a buzzing noise around C39 area. Heres my voltage readings on the bias circuit:

    Test Point 41 is -107 vdc
    Test point 42 is +48.9 vdc
    Test point 43 is -49.3 vdc

    As you can see, these voltages are good, comparing to the schematic.

    Here’s a list of components that I replaced, with no change.

    C37, C43, C40, C41, C31, R97, R95. And tested tubes. Tried different tubes!



    I’ve checked about everything. What am I missing guys? Any help will be greatly appreciated! Thanks
    Coop

  • #2
    Seems you're speaking of idle current (bias typically refers to grid to cathode voltage). How did you measure?

    Please post schematic.
    - Own Opinions Only -

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    • #3
      Schematic https://www.thetubestore.com/lib/the...atic-Rev-B.pdf



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      • #4
        Bias voltage is taken with meter set to DC volts, negative meter lead to chassis ground, positive lead to Test Point 30. As stated on the schematic.

        Comment


        • #5
          Ok, thanks.

          Here's a list of common reasons for bias drift (not specific to this amp):
          - bad (leaky/gassy) tube(s)
          - bad tube socket contacts
          - leaky (carbonized) tube socket
          - leaky coupling cap(s) C26/27
          - leaky bias supply caps.

          Does the bias voltage at "C -" change during warm-up? What DCV do you measure?
          Last edited by Helmholtz; 09-16-2020, 11:49 PM.
          - Own Opinions Only -

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          • #6
            Your starting Bias voltage at TP41 is WAY HIGH, as are those at TP42 & TP43. Either your line voltage is high, or the primary of the power xfmr, if Universal primary, is strapped for 100VAC (assuming 120VAC AC mains). What are you reading for heater voltage? I'd be expecting way more than 6.3VAC, based on your TP voltage readings.
            Logic is an organized way of going wrong with confidence

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            • #7
              Disconnect C26 & C27.
              Does the voltage on the junction stay stable C-
              Check the DC voltage on Tp 30 whilst adjusting R82. If it all settles down, replace both C26 & C27 as one/both is leaky.
              Ensure C37 & C43 are the correct value.
              Support for Fender, Marshall, Mesa, VOX and many more. https://jonsnell.co.uk

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              • #8
                Originally posted by nevetslab View Post
                Your starting Bias voltage at TP41 is WAY HIGH, as are those at TP42 & TP43. Either your line voltage is high, or the primary of the power xfmr, if Universal primary, is strapped for 100VAC (assuming 120VAC AC mains). What are you reading for heater voltage? I'd be expecting way more than 6.3VAC, based on your TP voltage readings.
                There are 2 diffent power supplies in the manual. You probably looked at the one for the "Deville".
                - Own Opinions Only -

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                • #9
                  Originally posted by Helmholtz View Post

                  There are 2 diffent power supplies in the manual. You probably looked at the one for the "Deville".
                  Ah......I missed that! I WAS looking at the DeVille schematic. OPPS!
                  Logic is an organized way of going wrong with confidence

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                  • #10
                    I went ahead and changed out C26 and C27. I double checked all capacitors are the correct value. Still no change. Not sure where to go from here. I’m so fed up with this amp! You wouldn’t believe the hours I’ve spent troubleshooting, research, etc.

                    I’m in the USA. AC line voltage is 120

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                    • #11
                      I'd pull the output tubes to protect them. Fire up the amp and monitor bias supply voltage as the amp warms up. Make sure it is holding steady.
                      "Yeah, well, you know, that's just, like, your opinion, man."

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                      • #12
                        Originally posted by CoopDaKill View Post
                        Not sure where to go from here.
                        Did you check the other possibilties I listed in post #5? And please answer the question about bias voltage.
                        BTW, "correct" capacitance value doesn't mean a cap is good.
                        - Own Opinions Only -

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                        • #13
                          I went ahead and changed out C26 and C27. I double checked all capacitors are the correct value. Still no change. Not sure where to go from here. I’m so fed up with this amp! You wouldn’t believe the hours I’ve spent troubleshooting, research, etc.

                          Comment


                          • #14
                            I checked the new caps before installing. I tried all of your suggestions. Thanks for all your guy’s advice. I have to put this amp on the back burner for now. It’s literally exhausted me! I don’t usually give up, but this one has me beat. lol

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                            • #15
                              Originally posted by CoopDaKill View Post
                              I checked the new caps before installing. I tried all of your suggestions. Thanks for all your guy’s advice. I have to put this amp on the back burner for now. It’s literally exhausted me! I don’t usually give up, but this one has me beat. lol
                              So you put in new bias supply ecaps?
                              - Own Opinions Only -

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