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Adcom GFA-555 II with Blown Left Chanel

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  • #16
    Originally posted by AlexRivera View Post
    Basic question, what's recommended to test in-circuit vs off-circuit ?

    I don't think I have to desolder everything in order to test it do I?
    Since you have a working channel to compare against : Resistors, Diodes and Transistors can be tested without removal.
    With a Capacitor - one side may have to be lifted. ( fwiw: I've not seen many cap failures )

    https://learn.sparkfun.com/tutorials...e-a-multimeter
    http://en-us.fluke.com/training/trai...damentals.html
    "Not a Speaker Designer - Not even on the Internet"
    "If the freedom of speech is taken away, then dumb and silent we may be led, like sheep to the slaughter."

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    • #17
      What's the symptom of the "blown" channel? If' there's no audio whatsoever, and there's no blown fuses, then I'd be headed straight to the protection circuits. Sadly, that adcom doesn't seem to have too much in the way of protection circuits and relays, which might make this more complicated. Still, I'd be checking to see if you can measure S101 (located on the heatsink) and see if there's a dead short across it. Compare against S151 (the working channel). Also, I'd be looking at the supply rails. Make sure that you're getting the same voltages across both channels (with a small tolerance). Check this by looking at the components attached to the +V and -V supplies. Pay close attention to this on the transistor boards (the ones attached to the heatsinks). More than likely - your failure is going to be in a transistor somewhere. Good luck.
      DARPA Jr - 2015 InDIYana Winner - RS180-8 + RS100P-8 + ND25FA
      The Aria's - RS150-4 (or RS150-8) + XT25SC90
      The Mariposa's - TEBM65C20F-8 + ND16FA
      The Canzonetta's - RS100P-8 and ND16FA
      AudioSource AMP-100 Mods OR Pyle PAMP1000 Mods

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      • #18
        Awesome!!! Thanks folks, I'm getting a more advanced DMM tomorrow and I'll start tackling this. Sent from my iPhone using Tapatalk

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        • #19
          BTW, the symptoms is no symptoms, no sounds, no noises, no lights on, nada... Sent from my iPhone using Tapatalk

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          • #20
            Originally posted by AlexRivera View Post
            ..., I'm getting a more advanced DMM tomorrow ...
            Be sure to get an arrangement to clamp the ground lead to the amp's ground point - this will allow safe one-handed testing ( sit on the other ) when measuring powered-up.
            "Not a Speaker Designer - Not even on the Internet"
            "If the freedom of speech is taken away, then dumb and silent we may be led, like sheep to the slaughter."

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            • #21
              Little Update.

              I got the DMM today and had 5 free minutes so I tested the easiest, the 4 Big Power Caps and they all seem to be good from 16.35 mF to 16.65 mF, no fundamental reason to change them right?
              Only that performance "improvement" that new caps should provide?

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              • #22
                Originally posted by tyger23 View Post
                Still, I'd be checking to see if you can measure S101 (located on the heatsink) and see if there's a dead short across it. Compare against S151 (the working channel).
                Hi Tyger23, please forgive my ignorance but, how can I test these?

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                • #23
                  Update:

                  I did notice some heat signs under the Q105, Q107, Q108, Q155, Q157 and Q158 transistors and they all seem to be good, except for Q105 which when I put the leads in the 2 exterior pins it gives me continuity which none of the others do, is that a real sign of a shorted transistor?

                  If I need to change this Sanyo 2SA1210 Transistor, do I have to change anything else?

                  Thanks for all your support again!

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                  • #24
                    Ok... did you measure with power applied or power off? Also, I assume you checked Q155 under the same conditions. If the condition of Q105 does not match the condition of Q155, then you're on to something. Personally, I would remove most of the transistors form the board at this point to check the individual status of each. Usually when a transistor fails, it will take other components out with it.

                    Even if Q105 is the only true failure, I'd still replace the matching low-side fet (Q108). You might even want to replace the same pair on the right side. Since the Sanyo transistors originally used in that design are long since EOL, you'll need to replace them with the KSC3503/KSC1381 pair.

                    Unfortunately for this particular ADCOM design, there's virtually no DC offset protection for your speakers. So, typically when one of those transistors goes, it puts DC offset on your outputs and takes out your speaker too.

                    Spend some time reading over this thread:
                    http://www.diyaudio.com/forums/solid...r-rebuild.html
                    DARPA Jr - 2015 InDIYana Winner - RS180-8 + RS100P-8 + ND25FA
                    The Aria's - RS150-4 (or RS150-8) + XT25SC90
                    The Mariposa's - TEBM65C20F-8 + ND16FA
                    The Canzonetta's - RS100P-8 and ND16FA
                    AudioSource AMP-100 Mods OR Pyle PAMP1000 Mods

                    Comment


                    • #25
                      Originally posted by tyger23 View Post
                      Ok... did you measure with power applied or power off?
                      Power Off, I will remove the transistors and measure them and I agree that is a good idea changing them all, I have even read people recommending finding a way to put a heat sink on those, definitely a good idea to change them all.

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                      • #26
                        Originally posted by tyger23 View Post
                        Unfortunately for this particular ADCOM design, there's virtually no DC offset protection for your speakers. So, typically when one of those transistors goes, it puts DC offset on your outputs and takes out your speaker too.
                        I'm glad you bring this up, I'm aware of the problem and fortunately that has not happened to me but it could certainly happen, I own 1 GFA-535, 3 GFA-545 II and 1 GFA-555 II among others but are all these Adcoms in the same boat or the smaller ones have some protection?

                        Any ideas how can the kit in the link below be integrated into these amps?

                        Any other/better way to do that?

                        Once I thought about the Adcom Speaker selectors but they do not protect the speakers but the amp instead so that would be an easy solution but unfortunately is not designed for that.

                        Thanks Again,
                        Alex
                        Velleman K4700U 2-Channel Loudspeaker Protection Kit

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                        • #27
                          Alex; I've not used that Vellman, but I have successfully used the Yuanjing UPC1237 board in several of my NAD amplifiers. It works well:
                          https://www.yuan-jing.com/Speaker-Pr...ection-board-b

                          It looks like integrating this board would be easy for you. You would just tap off the AC power from the transformer output that goes to the fan control board. That should be 22VAC from what I've read, which is just about perfect for this board.

                          Enjoy.
                          DARPA Jr - 2015 InDIYana Winner - RS180-8 + RS100P-8 + ND25FA
                          The Aria's - RS150-4 (or RS150-8) + XT25SC90
                          The Mariposa's - TEBM65C20F-8 + ND16FA
                          The Canzonetta's - RS100P-8 and ND16FA
                          AudioSource AMP-100 Mods OR Pyle PAMP1000 Mods

                          Comment


                          • #28
                            Originally posted by tyger23 View Post
                            If the condition of Q105 does not match the condition of Q155, then you're on to something.
                            Tyger23

                            I got a "better" tester and finally tested both Q105 and Q155 and the results are definitely different so I think I'm into something here. Q105 is not being identified as a PNP like Q155.

                            Thanks Again,
                            Alex

                            Comment


                            • #29
                              Quick Update Folks,

                              Before I ordered the parts and since I had the transistors out for testing I decided to put the left channel together and I was able to confirm that by replacing Q105 the channel works just fine like the right channel does under the same circumstances, so only Q105 is bad, basically a 50 cents repair...

                              I measured the DC offset in the speaker terminals and at rest both channels are around 3 mV, so I think they are good unless somebody suggests differently?

                              Thanks,
                              Alex

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                              • #30
                                Alex, as I said before - I would replace the matching low-side fet as well. Since the Sanyo part that was original is not available, you'll need to replace it with a different part. You should buy the matching pair for the low side to ensure proper operation and minimal D.C. Offset.
                                DARPA Jr - 2015 InDIYana Winner - RS180-8 + RS100P-8 + ND25FA
                                The Aria's - RS150-4 (or RS150-8) + XT25SC90
                                The Mariposa's - TEBM65C20F-8 + ND16FA
                                The Canzonetta's - RS100P-8 and ND16FA
                                AudioSource AMP-100 Mods OR Pyle PAMP1000 Mods

                                Comment

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