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Ampeg SVT-ii Pro No output w/EQ on

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  • Ampeg SVT-ii Pro No output w/EQ on

    Output is normal until engaging graphic eq button, then no output and hum. Active/Peak light stays on at power on, standby off or on and EQ button off or on.

  • #2
    Look at your overall circuit. The whole amp is tube circuits, and they are working for you. The graphic section however is solid state using op amps, and they have their own low voltage power supplies. Also, the LEDs also run on those low voltage supplies.

    SO if it were mine, I'd first check the low voltage supplies, both +15v and -15v. One supply could be missing, or may have lost filter cap and so is real hummy.
    Education is what you're left with after you have forgotten what you have learned.

    Comment


    • #3
      Originally posted by Enzo View Post
      Look at your overall circuit. The whole amp is tube circuits, and they are working for you. The graphic section however is solid state using op amps, and they have their own low voltage power supplies. Also, the LEDs also run on those low voltage supplies.

      SO if it were mine, I'd first check the low voltage supplies, both +15v and -15v. One supply could be missing, or may have lost filter cap and so is real hummy.
      Hum only increases with the master volume. Gain and volume do not increase hum. EQ was fine until I installed a new shuangang 6550B. Other 5 are GE 6550A's. Coincidence or otherwise?

      Comment


      • #4
        Originally posted by ca7922303 View Post

        Hum only increases with the master volume. Gain and volume do not increase hum. EQ was fine until I installed a new shuangang 6550B. Other 5 are GE 6550A's. Coincidence or otherwise?
        Bias lights are both green. Fault light does not come on. EQ light stays on.

        Comment


        • #5
          Originally posted by Enzo View Post
          Look at your overall circuit. The whole amp is tube circuits, and they are working for you. The graphic section however is solid state using op amps, and they have their own low voltage power supplies. Also, the LEDs also run on those low voltage supplies.

          SO if it were mine, I'd first check the low voltage supplies, both +15v and -15v. One supply could be missing, or may have lost filter cap and so is real hummy.
          I've got the EQ board out, can you point me in the right direction to test? Thanks.

          Comment


          • #6
            Originally posted by Enzo View Post
            Look at your overall circuit. The whole amp is tube circuits, and they are working for you. The graphic section however is solid state using op amps, and they have their own low voltage power supplies. Also, the LEDs also run on those low voltage supplies.

            SO if it were mine, I'd first check the low voltage supplies, both +15v and -15v. One supply could be missing, or may have lost filter cap and so is real hummy.
            I see on schematic that test points 9 & 11 are the +15/-15 points. I'm not getting either one with EQ engaged, balanced out engaged and all pots in middle. I'm measuring at R44/R45 or should I be at the diodes?

            Comment


            • #7
              Start right at the power supply. If the 15v supplies are not making 15v, then it can't reach the EQ board.
              Education is what you're left with after you have forgotten what you have learned.

              Comment


              • #8
                Originally posted by Enzo View Post
                Start right at the power supply. If the 15v supplies are not making 15v, then it can't reach the EQ board.
                With EQ board upside down, measured 14 pins connected to ribbon cable from left to right.

                Comment


                • #9
                  Are you using the 2 Pro schematic? See attached. I don't see R44/R45. I see R60/R61 on pg.4
                  Attached Files
                  "Everything is better with a tube. I have a customer with an all-tube pacemaker. His heartbeat is steady, reassuring and dependable, not like a modern heartbeat. And if it goes wrong he can fix it himself. You can't do that with SMD." - Mick Bailey

                  Comment


                  • #10
                    Originally posted by g1 View Post
                    Are you using the 2 Pro schematic? See attached. I don't see R44/R45. I see R60/R61 on pg.4
                    Looks like I was using SVT-ii schematic. Is there a SVT-ii Pro schematic or do I need to use the svt-2 pro schematic?

                    Comment


                    • #11
                      Originally posted by g1 View Post
                      Are you using the 2 Pro schematic? See attached. I don't see R44/R45. I see R60/R61 on pg.4
                      This ii pro does not have a r60/61. Looks like it is missing an ic6 that's on the ii non-pro schematic.

                      Comment


                      • #12
                        The power supply is not on the EQ board. Go to the power supply itself, as g1 also suggested. Find where the 15v supplies actually comes from. Offhand in your chart I do not see -15v. It could be as simple as a failed 15v zener, but you have to look in the power supply.
                        Education is what you're left with after you have forgotten what you have learned.

                        Comment


                        • #13
                          Originally posted by Enzo View Post
                          The power supply is not on the EQ board. Go to the power supply itself, as g1 also suggested. Find where the 15v supplies actually comes from. Offhand in your chart I do not see -15v. It could be as simple as a failed 15v zener, but you have to look in the power supply.
                          D6 & D11 are both reading (0.25/0.25) diode test.

                          Comment


                          • #14
                            Here is the layout, does this look like your main board? R60/61 are near the top just left of center near the 5=pin connector that says "to preamp".
                            Attached Files
                            Education is what you're left with after you have forgotten what you have learned.

                            Comment


                            • #15
                              Originally posted by Enzo View Post
                              Here is the layout, does this look like your main board? R60/61 are near the top just left of center near the 5=pin connector that says "to preamp".
                              I'll Have to dig into it this afternoon when I get off work. Thanks for your help.

                              Comment

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