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Ampeg SVT-7 Pro No Power On

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  • Ampeg SVT-7 Pro No Power On

    Brought up voltage slowly with Variac. No fan or lights coming on at any point. Was able to get some DC voltage readings on main board of about (125vdc) on right side of main board facing front of amp. Left side no DC voltage.

  • #2
    Do you have a schematic that is correct to your board? IMHO these things are dang near unserviceable (unless this is different than the one I had). I think most people just replace the main board if they can be purchased. There are cascaded SMPS power supplies, multiple protect modes that aren't really documented, and probing is dang near impossible because of the tight layout of the FETS, heatsinks, and caps.

    I guess you could check for shorted output devices
    Last edited by glebert; 05-25-2023, 05:12 AM.

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    • #3
      It is basically a Mackie 450Mk2 without the power factor corrector for more efficient use of power.
      If you are not experienced with Switch Mode and Class D, take it to someone more qualified or you may kill it completely.
      Support for Fender, Laney, Marshall, Mesa, VOX and many more. https://jonsnell.co.uk
      If you can't fix it, I probably can.

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      • #4
        Just a simple test, if the Mos Fet output transistors, Q10 - 13 are OK, check the secondary diodes on the power supply board. If faulty, replace ONLY with the original type and specification.
        Support for Fender, Laney, Marshall, Mesa, VOX and many more. https://jonsnell.co.uk
        If you can't fix it, I probably can.

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        • #5
          Originally posted by glebert View Post
          Do you have a schematic that is correct to your board? IMHO these things are dang near unserviceable (unless this is different than the one I had). I think most people just replace the main board if they can be purchased. There are cascaded SMPS power supplies, multiple protect modes that aren't really documented, and probing is dang near impossible because of the tight layout of the FETS, heatsinks, and caps.

          I guess you could check for shorted output devices
          I recently was able to resurrect a 6 pro. Is there alot of differences between those two?

          Comment


          • #6
            Originally posted by ca7922303 View Post

            I recently was able to resurrect a 6 pro. Is there alot of differences between those two?
            Taking a look at the 6 schematic and some other threads on it, they look completely different to me. The 6 looks like a "conventional" amp (although lots of preamp and driver tubes). The 7 is all about SMPS and class D amps, so way more "digital."

            Comment


            • #7
              Originally posted by Jon Snell View Post
              It is basically a Mackie 450Mk2 without the power factor corrector for more efficient use of power.
              If you are not experienced with Switch Mode and Class D, take it to someone more qualified or you may kill it completely.
              Jon, I can't find any info about Mackie 450Mk2, is that the name of a module from a powered speaker or something? I guess Mackie and Ampeg were both part of LOUD Audio back in the day?

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              • #8
                Originally posted by glebert View Post

                Jon, I can't find any info about Mackie 450Mk2, is that the name of a module from a powered speaker or something? I guess Mackie and Ampeg were both part of LOUD Audio back in the day?
                PSU attached and you will see they are very similar. Click image for larger version

Name:	Screenshot 2023-05-25 at 19.03.51.png
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                Support for Fender, Laney, Marshall, Mesa, VOX and many more. https://jonsnell.co.uk
                If you can't fix it, I probably can.

                Comment


                • #9
                  Here's the SMPS schematic (attached). There is a known issue with U2, but you'll have to troubleshoot to see if that is your problem.

                  0032543-xx.pdf
                  "I took a photo of my ohm meter... It didn't help." Enzo 8/20/22

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                  • #10
                    Originally posted by Jon Snell View Post
                    Just a simple test, if the Mos Fet output transistors, Q10 - 13 are OK, check the secondary diodes on the power supply board. If faulty, replace ONLY with the original type and specification.
                    Q10/11/12 are switching on and off with diode test, negative probe to source, positive probe to gate(.57v), then positive probe to drain(0.00v).
                    D39/40/53/54(0.005v) diode test
                    D26/29/43/44/61(0.48v) diode test
                    D14(0.012v) diode test
                    D15(0.2v) diode test D16/17/23(0.4v) diode test
                    D12/13(0.5v) diode test
                    Z1/3/4/6(0.59v) diode test
                    D1/60(0.4v) diode test
                    D7/18/19/20(0.55v) diode test
                    D2/63/64(0.5v) diode test Q13 according to schematic is a 3904 and I'm unable to find it on board to test yet.
                    Last edited by ca7922303; 05-27-2023, 04:50 AM.

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                    • #11
                      Originally posted by The Dude View Post
                      Here's the SMPS schematic (attached). There is a known issue with U2, but you'll have to troubleshoot to see if that is your problem.

                      [ATTACH]n982936[/ATTACH]
                      This is a Revision E board with a 2a265 in a socket w/heat sink. D9 is reading (0.5v) diode test

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                      • #12
                        The issue with U2 will not cause one rail to fail.
                        It is caused by either a damaged Shottky rectifier, a short circuit on the Power supply output lines or a short circuit in the power amplifier.
                        Where did you measure 125volts? The rail voltage is + - 55volts!

                        Edit; You cannot measure the output Fets with a DVM successfully, except for a short circuit.
                        Support for Fender, Laney, Marshall, Mesa, VOX and many more. https://jonsnell.co.uk
                        If you can't fix it, I probably can.

                        Comment


                        • #13
                          Originally posted by ca7922303 View Post

                          I recently was able to resurrect a 6 pro. Is there alot of differences between those two?
                          Correction: I should have said, I was involved in resurrecting a 6 pro, the expertise came from members of this forum(G1, Nevetslab, others).

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                          • #14
                            Click image for larger version  Name:	168519868770052733558143850894.jpg Views:	0 Size:	4.23 MB ID:	983038 C177Click image for larger version  Name:	16851981907384822651221865835166.jpg Views:	0 Size:	4.08 MB ID:	983035
                            Originally posted by Jon Snell View Post
                            The issue with U2 will not cause one rail to fail.
                            It is caused by either a damaged Shottky rectifier, a short circuit on the Power supply output lines or a short circuit in the power amplifier.
                            Where did you measure 125volts? The rail voltage is + - 55volts!

                            Edit; You cannot measure the output Fets with a DVM successfully, except for a short circuit.
                            U8/11(-156vdc)
                            D12/60(-156vdc)
                            R46(-156vdc)
                            R49(+156vdc)
                            Q2(-156vdc)
                            Q20(0.00vdc)
                            Q10/11/12(0.00vdc)
                            ​​​​​
                            Looks like anything northwest and west of T3 is (0.0vdc)

                            What can I use to measure the output FETs? Click image for larger version  Name:	16851979774985190116851622562585.jpg Views:	0 Size:	6.04 MB ID:	983034 Photo of positive probe on R46, black probe on chassis ground. Variac @ 115vac. Click image for larger version  Name:	16851981907384822651221865835166.jpg Views:	0 Size:	4.08 MB ID:	983036 Looks like R46/U2(2)/C177 or C26 failed and we're replaced?
                            Attached Files
                            Last edited by ca7922303; 05-27-2023, 03:03 PM.

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                            • #15
                              So ... the psu is faulty yes or no.
                              U8 U11 ... schematic of your board would be useful, save us guessing.
                              You state 156volts. From where to where?
                              Support for Fender, Laney, Marshall, Mesa, VOX and many more. https://jonsnell.co.uk
                              If you can't fix it, I probably can.

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