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GK RB1001 Mk II no output

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  • GK RB1001 Mk II no output

    https://elektrotanya.com/gallien-kru.../download.html

    Hey y'all,

    Got a GK RB1001MK II with no output, but also no DC on the output.

    FX send to another amp yields good signal, full functionality. The problem is in the Power amp.

    I was able to trace the signal through the preamp into the "WOOFER IN" molex into the power amp board;

    The woofer empties out into the PA at q2, 2SC3381. A simple audio probe shows good sounding signal on pin 1, but that's it. None of the resistors show open around there and I do get voltages;

    2sc3381, voltages taken with limiter

    BASE Pin 1: 0 MVDC
    EMITTER Pin 2: 50 VDC
    COLLECTOR pin 3: .78VDC
    SUBSTRATE pin 4: - 5 MVDC
    COLLECTOR pin 5 - .78 VDC
    EMITTER pin 6 - 50 VDC
    BASE pin 7 - 0VDC

    So if these voltages look good, then why would I lose signal after the output of 2sc3381? Or am I approaching this in the wrong way?

    Thanks

  • #2
    Any npn BJT needs its base to be 0.6V to 0.7V more positive than its emitter (best measured directly between base and emitter) for proper operation.
    So your voltages look wrong.

    BTW, capital M means Mega (= times one million) and lower case m means milli (= divided by 1000).
    - Own Opinions Only -

    Comment


    • #3
      Thank you Helmholtz;

      So if the voltage (measured base to emmiter) is bad, then

      1. Our supply is bad
      2. The component is bad

      Since we have 0v on EITHER base from 2 separate supply lines, then it's likely our component is bad?

      I am unsure of this since the schematic is pretty elaborate; but ideally if our base needs to be more positive than our emitter we should be seeing something around 51V on the bases?

      To be honest I'm A little unsure how to proceed, but my gut is telling me to replace the part.

      Comment


      • #4
        Just noticed that you mixed up pins. Please measure again between pins1/3 and pins 7/5 and mind polarity. Black meter lead goes to emitter (pins 3 and 5).
        I think Q2 is good.
        - Own Opinions Only -

        Comment


        • #5
          Originally posted by Helmholtz View Post
          Just noticed that you mixed up pins. Please measure again between pins1/3 and pins 7/5 and mind polarity. Black meter lead goes to emitter (pins 3 and 5).
          I think Q2 is good.
          Whoops! I think I mislabeled them.

          Pin 3-5, black on Emitter= .78V
          pin 5-7, black on Emitter= .78V

          Re-probed to be sure, turns out I do get a signal on the emitters and after the dual NPN, but it is very weak compared to the incoming signal to the base.

          I get that same weak signal on the collectors of q4 / q5, BUT NOT q9 / q10.

          Comment


          • #6
            First of all, it is a good sign that you have no DCV at the output. Might be something related to the muting/fault circuits.

            The schematic gives lots of test voltages (maybe a dozen). I'd check them all. Don't forget to verify zener voltages.
            To be honest I don't have much experience (and patience) with troubleshooting SS amps and I know that there are at least half a dozen guys here who might know better than I.

            So maybe someone suggests a more efficient strategy?
            Last edited by Helmholtz; 04-17-2021, 09:20 PM.
            - Own Opinions Only -

            Comment


            • #7
              Lets get that numbering on the Q2 straightened out. Datasheet attached matches numbers on schematic.
              Post DC voltages for EBC on both sides of dual package.
              Attached Files
              "Everything is better with a tube. I have a customer with an all-tube pacemaker. His heartbeat is steady, reassuring and dependable, not like a modern heartbeat. And if it goes wrong he can fix it himself. You can't do that with SMD." - Mick Bailey

              Comment


              • #8
                Originally posted by g1 View Post
                Lets get that numbering on the Q2 straightened out. Datasheet attached matches numbers on schematic.
                Post DC voltages for EBC on both sides of dual package.
                Sorry about the confusion, here is a more complete list of voltages from the component; I am very new to the world of SS repair so if I'll include as much detail as possible, including probe locations.

                I also noticed that at power up, the voltage on the collectors starts very low (>5VDC) before drifting up. I had the unit on a limiter during these tests but after about 5 minutes it had stabilized somewhat around 64VDC, but was still creeping up slowly milivolt by milivolt.

                BASE Pin 1, measured from ground(black) to Base (red) = 0
                COLLECTOR Pin 2: Measured from ground to Collector = 66 VDC after 5 min
                EMITTER pin 3: Measured from BASE (RED) to EMITTER (Black) = -.78VDC
                SUBSTRATE pin 4: = Measured Ground (black) to SUBSTRATE (red) 5 mVDC
                EMITTER pin 5 - Measured from BASE2 (Red) to EMITTER2 (Black = -.78VDC
                COLLECTOR pin 6 GROUND (Black) to Collector (RED)= 66 VDC after 5 min

                A test I'm more familiar with is the diode MM test across base(black) ---> emitter(red), in which both sides show .657V across with no power applied. I feel like watching that voltage creep up may be a clue as to what's happening...

                Comment


                • #9
                  Those all seem good. If you look above and below Q2 on schematic, there are a bunch of resistors with DC voltages listed. 5 at the top and 4 on the bottom. Check if the voltages are all in the ballpark. (they will all be relatively low as you are on the bulb)
                  "Everything is better with a tube. I have a customer with an all-tube pacemaker. His heartbeat is steady, reassuring and dependable, not like a modern heartbeat. And if it goes wrong he can fix it himself. You can't do that with SMD." - Mick Bailey

                  Comment


                  • #10
                    Thank you for your patience, busy week;

                    Here are the test voltages

                    TOP SECTION
                    Voltage Across R16, R15, R24, R31, R33 = All 0 VDC
                    Voltage AT d7 = 0VDC

                    Positive rails measure ok at 97 VDC / 42 VDC



                    BOTTOM SECTION
                    (I got some small mv readings, possibly a meter error? without probes connected the meter voltage test shows 135mv, close to some readings...)

                    R22, R21 = 0mv

                    R20 = 21mv
                    R25 = 187mv
                    D1 / D5 Junction = 187mv

                    Negative rails healthy at -97 / -42 VDC

                    U2 receives 15VDC


                    So, with these results, (healthy rails / no voltage) should we begin to examine the circuit around q20 and U2 for a mute malfunction? I have it on a limiter, but it's not pulling a lot of current. I'm just paranoid while taking measurements in a PA like this, wouldn't want to accidentally short something.... since it's not pulling current, and there's no DC on the output, healthy signal from the preamp and no output from the back, that would support the theory that a mute is faulty.

                    Comment


                    • #11
                      Are those transistors getting their supply voltages? Look at the top and bottom rails of the schematic, there are 2 fusing resistors, R39 along the top, and R17 on the bottom.
                      Check they are both good.
                      "Everything is better with a tube. I have a customer with an all-tube pacemaker. His heartbeat is steady, reassuring and dependable, not like a modern heartbeat. And if it goes wrong he can fix it himself. You can't do that with SMD." - Mick Bailey

                      Comment


                      • #12
                        Originally posted by g1 View Post
                        Are those transistors getting their supply voltages? Look at the top and bottom rails of the schematic, there are 2 fusing resistors, R39 along the top, and R17 on the bottom.
                        Check they are both good.
                        Appreciate the reply but the amp owner surprised me and snagged it back. WIsh I could have gotten to the bottom of it but I still appreciate your efforts!

                        Comment

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