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Carver av705x w/a gimpy fin

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  • Carver av705x w/a gimpy fin

    tomzarbo didn't seam to have much luck fixing his friends Lepai, I wonder if I can fix my 5 channel Carver.

    Short story. The oem? power cord was about a foot too short and wimpy. Replaced it with some 14awg sjew about a foot longer and a good three prong plug. Upon opening the case I discovered the ground of the original power cord was not attached to anything. Hmm ... sometimes the amp hums unacceptably loud with certain combinations of pre/minidsp.

    I'll fix that! Grounded the replacement cord to the amp chassis.

    Not good. Not good at all.

    The speakers let out an awful squall and the amp let out its magic smoke.

    Anyhow, the protection light is lit on only two channels. I assume they are the ones affected.

    Here's a pic of the boards. The first pic is a good one on the outside of the amp, the next board is the toasty one. c21/r47 looks crunchy. What are the odds of fixing this amp by replacing only this or is it likely when the board is removed there's going to be more damage?

    Click image for larger version

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    Attached Files
    Last edited by Kornbread; 09-02-2018, 04:33 PM.
    http://techtalk.parts-express.com/fo...khanspires-but
    http://techtalk.parts-express.com/fo...pico-neo-build
    http://techtalk.parts-express.com/fo...ensation-build

  • #2
    Gotcha.

    Comment


    • #3
      "if it ain't broke, fix it til it is"

      Actually those amps are very good sounding. Had one for many years, sold it to a friend who still uses it

      Good luck

      Comment


      • #4
        No burnt areas on the back of the board and the only toasty part on the front is that one, what, resistor? Best I could tell it says; RCD 135, 2R7ohm5%, H 9616-1.

        What is it and where do I find it?

        Why is there plastic tape under the output devices? Not much thermal grease under it either.

        Click image for larger version

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        http://techtalk.parts-express.com/fo...khanspires-but
        http://techtalk.parts-express.com/fo...pico-neo-build
        http://techtalk.parts-express.com/fo...ensation-build

        Comment


        • #5
          Originally posted by ontariomaximus View Post
          "if it ain't broke, fix it til it is"
          Good luck
          Um.... did that with the Subaru and it's still sitting in the driveway.
          http://techtalk.parts-express.com/fo...khanspires-but
          http://techtalk.parts-express.com/fo...pico-neo-build
          http://techtalk.parts-express.com/fo...ensation-build

          Comment


          • #6
            Not an expert, but are you sure R 47 is toasty? Hard to tell from the pics, but in my experience it doesn't look too bad. Usually a toasted resistor is not readable and the board would show some evidence of heat.

            R47 is a 2.7 ohm 2W according to the schematic from this link. Do a quick test to see if it's ok.

            Does there appear to be any points where the PCB might have been shorting to the chassis?

            Comment


            • #7
              That plastic tape is a pretty typical grey thermal interface material, it doesn't need grease though it's not quite as effective as older thin mica insulators + grease. The devices have to be electrically isolated from the heat sink, the whole power supply voltage is between the back side of the + and - transistors.

              Originally posted by Kornbread View Post
              No burnt areas on the back of the board and the only toasty part on the front is that one, what, resistor? Best I could tell it says; RCD 135, 2R7ohm5%, H 9616-1.

              What is it and where do I find it?

              Why is there plastic tape under the output devices? Not much thermal grease under it either.

              Click image for larger version

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Views:	1
Size:	116.9 KB
ID:	1385777Click image for larger version

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Views:	1
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ID:	1385778
              Electronics engineer, woofer enthusiast, and musician.
              Wogg Music

              Comment


              • #8
                Thanks for the info guys.

                Originally posted by GTPlus View Post
                Not an expert, but are you sure R 47 is toasty? Hard to tell from the pics, but in my experience it doesn't look too bad. Usually a toasted resistor is not readable and the board would show some evidence of heat.

                R47 is a 2.7 ohm 2W according to the schematic from this link. Do a quick test to see if it's ok.

                Does there appear to be any points where the PCB might have been shorting to the chassis?
                No, I'm not sure but it looks toasty when compared to the same location in the first pic and I can't find any dark spots on the board. Doesn't it have to be removed to test?

                Just any 2.7ohm 2W wirewound?

                Not registered at hifiengine, mind sending that schematic in a pm.


                wogg Had to gently pry the output devices? up with an exacto knife so I assume the thermal tape needs replaced just in case it was cut.
                http://techtalk.parts-express.com/fo...khanspires-but
                http://techtalk.parts-express.com/fo...pico-neo-build
                http://techtalk.parts-express.com/fo...ensation-build

                Comment


                • #9
                  Originally posted by Kornbread View Post
                  Thanks for the info guys.
                  ...
                  wogg Had to gently pry the output devices? up with an exacto knife so I assume the thermal tape needs replaced just in case it was cut.
                  Give it a close inspection, you should be able to see if you cut it by bending the material about. If you did cut it, then replace.
                  Electronics engineer, woofer enthusiast, and musician.
                  Wogg Music

                  Comment


                  • #10
                    GTPlus thanks for the schematic. As far as looks go, q15 looks no different than the other transistors. Maybe it's just the dust bunnies in the picture. The insides need a good cleaning.

                    Pulling the thermal tape around reveals no cuts.

                    Ordered the resistors; $1.50 plus $3 shipping ... I'll let you guys know how it goes.

                    All the smoke started when I grounded the chassis and only the two channels that were hooked up went into protection. Why?
                    http://techtalk.parts-express.com/fo...khanspires-but
                    http://techtalk.parts-express.com/fo...pico-neo-build
                    http://techtalk.parts-express.com/fo...ensation-build

                    Comment


                    • #11
                      I curious whether the schematic shows the chassis as being grounded?

                      Comment


                      • #12
                        Originally posted by Kornbread View Post
                        GTPlus thanks for the schematic. As far as looks go, q15 looks no different than the other transistors. Maybe it's just the dust bunnies in the picture. The insides need a good cleaning.

                        Pulling the thermal tape around reveals no cuts.

                        Ordered the resistors; $1.50 plus $3 shipping ... I'll let you guys know how it goes.

                        All the smoke started when I grounded the chassis and only the two channels that were hooked up went into protection. Why?
                        The schematic shows a chassis ground at the speaker output. My guess is there was something shorted that should not have been and the speaker probably completed the circuit. How are your speakers BTW? hope they are okay.

                        I'm hoping some of the EE guys chime in to help you out, as I am far from an expert.

                        Originally posted by philthien View Post
                        I curious whether the schematic shows the chassis as being grounded?
                        It does. Calls out PS, Chassis, and Signal

                        Comment


                        • #13
                          There was a chassis ground at the oem rca which had to be improvised. Here's a link back to the Carver site when I replaced the boinked rca plugs. https://thecarversite.com/index.php?...t-rca-inputs/&

                          Again checked the rca plugs to be sure they were not grounded, all good there.
                          http://techtalk.parts-express.com/fo...khanspires-but
                          http://techtalk.parts-express.com/fo...pico-neo-build
                          http://techtalk.parts-express.com/fo...ensation-build

                          Comment


                          • #14
                            Well, that didn't fix it.

                            Waited till the replacement resistors came in, then pulled the black one out. It checked good. Went ahead and replaced it with the new one anyway.

                            Pulled the rest of the boards out and haven't 'seen' anything that looks like smoke came from it; nor did I smell anything burnt. I'll pull the heatsink from the board this weekend and look for any funny looking areas on the backside of the boards.
                            http://techtalk.parts-express.com/fo...khanspires-but
                            http://techtalk.parts-express.com/fo...pico-neo-build
                            http://techtalk.parts-express.com/fo...ensation-build

                            Comment


                            • #15
                              You're going to want to test those output devices. Those suckers are the first things to go when an amp fails, in my experience it's 99% of the time (fabricated anecdotal statistics are the best!).

                              Here's a page on how to test BJT's with a multimeter. You should be able to test them on the board OK, though the reverse resistance may not look open like it should, but it will be higher than forward. Chances are you'll see a dead short from emitter to collector on one or more devices, indicating that sucker fried internally.
                              Electronics engineer, woofer enthusiast, and musician.
                              Wogg Music

                              Comment

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