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Adcom GFA-555 II with Blown Left Chanel

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  • Adcom GFA-555 II with Blown Left Chanel

    Hi all,

    I have an Adcom GFA-555 II with Blown Left Chanel that I'd like to repair, I can try to do that myself to make it worthy so I'm wondering if anybody could help me, I don't have much experience in electronics repair but I can certainly learn so where would you guys start?

    There's no evident signs of anything being blown/toasted and all fuses are new.

    I wonder if anybody has done this before and could share their experience in a human readable fashion? Unfortunately all the posts I have found are very high level and use expert language and I'm having a very hard time to understand them.

    I have also seen a lot of people taking this as an opportunity to upgrade this old amp and giving it a refresh, so if anybody knows about a for dummies upgrade guide it will be much appreciated.


    Thanks in Advance!

  • #2
    First Down load this
    "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."

    Comment


    • #3
      Originally posted by Sydney View Post
      First Down load this

      Thanks Sydney, I have that service manual already

      Comment


      • #4
        Good:
        Next Since you have a working channel - with the amp off you can take a Multimeter and compare values of parts of the working channel against the comparable part in the faulty channel.
        Use the Resistor color code. Diodes and Transisitors can also be tested un- powered.
        "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."

        Comment


        • #5
          Would you recommend start from the output side or the input side?

          Comment


          • #6
            I'd work from the input to output
            I just fixed a bad chl. in my 1982 Hafler 220 - a bad 47k resistor on the Q1 transistor.
            * Since I have an EE degree and have fixed numerous SS amps - I'm leading you cautiously.
            RATHER than 1st going HOT
            "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."

            Comment


            • #7
              I would first start checking all the voltages and comparing (Primary, Secondary and Op amps). The rectifiers have been known to go bad, as do other PSU parts because there's no soft-start on these. Once you verify the voltages, I would check on the DC voltage on the outputs - if it's really high voltage on the bad channel, usually means the outputs are bad, if not, then start with the input and mark up the Service manual with your measurements and then check the bad channel crossing them off one by one.
              Paul

              The "SB's" build page
              http://techtalk.parts-express.com/sh...-4-(pic-heavy)

              Comment


              • #8
                Originally posted by AlexRivera View Post
                ..., I don't have much experience in electronics repair ...
                Taking this at face value
                How are your multimeter skills?
                Especially around line voltage

                "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."

                Comment


                • #9
                  Originally posted by Sydney View Post
                  Taking this at face value
                  How are your multimeter skills?
                  Especially around line voltage
                  Yes, this. Try not to get electrocuted.
                  Francis

                  Comment


                  • #10
                    Hahahah, I'm definitely new but I have been electrocuted a few times before so I'm going to be cautious when doing that, I will start taking readings of the safe stuff first and then move to the risky ones, but definitely will do whatever can be done while un-powered ...

                    Comment


                    • #11
                      Originally posted by AlexRivera View Post
                      ...but I have been electrocuted a few times..
                      Technically I think you mean merely shocked
                      Having a schematic is a huge plus. Another of course is knowing the function of the components in the circuit.

                      "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."

                      Comment


                      • #12
                        Originally posted by Sydney View Post
                        Technically I think you mean merely shocked
                        Having a schematic is a huge plus. Another of course is knowing the function of the components in the circuit.

                        Agreed, this is going to be a good learning experience for me, I just hope I have enough time and patience to put into this, I appreciate the help so far.

                        Comment


                        • #13
                          Of course there are many videos and books on Trouble-Shooting. Some are general - how to use a multimeter and run continuity; other more specific, like how to test a diode and transistor in circuit.
                          Because the Hafler amps were available as kits - their manuals included test procedures and had a Table of Specific Test points and acceptable Voltage values.
                          "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."

                          Comment


                          • #14
                            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?

                            Comment


                            • #15
                              Originally posted by bullittstang View Post
                              I would first start checking all the voltages and comparing (Primary, Secondary and Op amps). The rectifiers have been known to go bad, as do other PSU parts because there's no soft-start on these. Once you verify the voltages, I would check on the DC voltage on the outputs - if it's really high voltage on the bad channel, usually means the outputs are bad, if not, then start with the input and mark up the Service manual with your measurements and then check the bad channel crossing them off one by one.

                              Bridge Rectifiers Tested off-circuit and I get the same values from both sides, one less thing

                              Comment

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