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Marshall AVT 150 Valvestate

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  • Marshall AVT 150 Valvestate

    My Valvestate AVT 150 has no output whatsoever. The red LED lights up and the transformer buzzes. I have checked and there is no evidence of burnt out components. The 2 TDA chips on the output board are not burnt out as far as I can tell though there is lots of corrosion on the PCB for one of the TDa's.
    I read on this forum that this problem can be related to the mute circuit. Where is it in the main board? Or is it more likely to be the TDA chips have failed.
    If anyone has a schematic then please email.
    With thanks. Happy to be the newest member.
    Last edited by Disco Dave; 03-01-2008, 07:44 PM. Reason: spelling error

  • #2
    If it's the same layout as the AVT275, then all channels also go through the valve stage, which would be my first suspect.


    • #3
      Avt 150

      Cheers for your tip. Since posting I have also done some homework and this amp is particularly prone to problems with the output amp. There are two TDA chips and I may replace these and see if it cures the problem.

      Disco Dave


      • #4
        I've repaired many of these and it was almost always the TDA devices. Both needed to be replaced at the same time to avoid another failure.

        However, the symptom was usually a fuse blowing which you do not seem to have. I would disconnect both output amp boards, feed a signal to the input and see if you have a signal from the preamp at the input to the amps.



        • #5
          Efx send & return ok?

          try a plugging a guitar cable into the effects send and return
          Do you get sound thru headphones?
          Do you get emulated line out sound


          • #6
            Avt150 schem

            I can send schem but need email


            • #7
              I'm also currently repairing one of these amplifiers. So far I have diagnosed a failed fusible resistor and a blown TDA7293 output chip.

              quazcha, if it's not too much trouble then I'd greatly appreciate a copy of the schematic. I can send you my email address by PM if that's OK with you?

              Many thanks


              • #8
                Much obliged quazcha, for your swift, efficient and friendly response. A true gent!

                Best regards...


                • #9
                  Avt 275

                  I just got avt275 from my brother in law. It has the same symptoms as Disco Dave described.
                  I tried quazcha's suggestion but I'm not getting any sound from headphones or emulated line out.
                  There is no burn signs anywhere.

                  Does it mean it might be tube?
                  I also read some posts about checking mute circuit but I don't know where it is.
                  quazcha could I have copy of the schematics?

                  Last edited by mgreczek; 04-09-2008, 08:22 PM.


                  • #10
                    12AX7 as common as...

                    The tube is a 12ax7, very common audio preamp tube (is there an orange glow in the tube? is it warm- the filament could be shot), check any "old-TV" shop if you can find the tube locally or maybe even borrow one from a friend's amp it is used in marshall, fender, mesa etc - even a music store will have a new or used tube you could buy for under $10. I sent the schematics- keep me posted on your progress and good luck!


                    • #11
                      quazcha, thank you for schematic I just got email.

                      Yes tube has an orange glow and it is warm.
                      I'll stop by few stores today to see if I could pick it up locally if not I'm sure I could get it online. In either case I want to get spare.

                      I would be working on that tonight. I'll keep you posted on the progress.


                      • #12
                        emulated line out fixed

                        Just a quick update.

                        I did'nt have too much time to mess with things recently but I got my preamp working.
                        Negative 15v voltage regulator was bad. I replaced L7915CV and it fixed my preamp problems. Now I have nice clear sound thru headphones and emulated line out.

                        I still dont have main sound sound but I traced feed to the imput of each amp. Hopefully replacing both TDA7293 is going to fix that problem.

                        Back to work......


                        • #13
                          Output Ic's blown?

                          is the fuse or output Ic's blown? with the connectors unplugged check voltages from the two signal and two power conns to the output Ic's as compared to schematic. I had a pc trace open up going from pin 1 of one signal connector to the other, unfortunately didnt find until my 3rd pair of TDA's got smoked keep us posted...and good luck


                          • #14
                            Check mute circuit?

                            If the output Ic's are not smoked and the fuse does not blow with all connectors connected check the voltage going to the mute pin on the power amp boards


                            • #15
                              marshall avt 150 valve state

                              Hello there,

                              I had a laney valve state with low o/p and poor preamp drive fault and after a few voltage checks I found dried up filter caps, dry joints.
                              the pre drive 12ax7 was passed it's best so I uesd ecc83 by jj and once all corrections were finished it worked great... job done.
                              wire up a 60w bulb in mains lead plug in and if it stays on your amp is pulling current! a working unit the bulb should flash and stay out if all's well.