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Marantz 18 receiver needs help

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  • #16
    It's not biased at the midpoint of the swing as it doesn't need to be.

    With the smaller signal scope the output and the input to the power amp.
    Experience is something you get, just after you really needed it.

    Comment


    • #17
      Signal going into the amp from the driver board is normal on 10K, 10J, & 10G and about equal signal strength
      Output at speaker jacks is normal until turned up past 1/2 way
      then it looks like this:
      Click image for larger version

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      Taken on the 1 v per division scale
      What do you make of it?

      Comment


      • #18
        Originally posted by bigdrums925 View Post
        Signal going into the amp from the driver board is normal on 10K, 10J, & 10G and about equal signal strength
        Output at speaker jacks is normal until turned up past 1/2 way
        then it looks like this:
        [ATTACH=CONFIG]47382[/ATTACH]
        Taken on the 1 v per division scale
        What do you make of it?
        Do you have any dc offset on the output terminals. If there is no dc offset, what does it sound like through speakers with program music? You may have it fixed and chasing your tail.

        Comment


        • #19
          About 1 mV DC offset, music thru sounds terrible and no gain after about 1/3 on volume
          Pic of sine wave thru with speaker connected (load)
          Click image for larger version

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          Also at 1v per division
          Nope, not fixed. Doesn't even sound good at low volume

          Comment


          • #20
            Originally posted by bigdrums925 View Post
            About 1 mV DC offset, music thru sounds terrible and no gain after about 1/3 on volume
            Pic of sine wave thru with speaker connected (load)
            [ATTACH=CONFIG]47383[/ATTACH]
            Also at 1v per division
            Nope, not fixed. Doesn't even sound good at low volume
            The simple fact that both channels are distorted usually indicates a power supply problem since it’s common to both channels. I would go back to the power supply section and check all of the supplies. Make sure they are within reason, symmetrical, and have no ripple. Check any and all current limiting resistors. Use a multimeter to start.

            Comment


            • #21
              ^^ Def check this things that olddawg mentioned. If you remove the load completely what does the output look like?

              Edit: Ah I see you already did that.
              Experience is something you get, just after you really needed it.

              Comment


              • #22
                Difficult to say for certain that all the voltages are there cause it's point to point wiring in the PS, But I think they are.
                The ripple seems ok except on the terminal strip:
                Click image for larger version

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                the voltages are ~220v and `-72v where the scope shots that follow were taken:
                Click image for larger version

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                Click image for larger version

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                the rest of the power supply points show a flat line on the scope so I don't know what these are

                the left speaker output
                Click image for larger version

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                right speaker output:
                Click image for larger version

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                Comment


                • #23
                  All the scope shots are almost meaningless without knowing the vertical scale and the horizontal scale. On one the vertical scale is far to sensitive as the signal if off the edges of the screen.

                  Just get you DMM out and measure the volts on the emitters of Q11, Q12, Q13 and Q14, the associated resistors might be an easier test point. If any of those are significantly different from 36V then measure on the other side of the associated resistors.
                  Experience is something you get, just after you really needed it.

                  Comment


                  • #24
                    If you have a clean signal from the line out.. check your power supply voltages where they enter the output amp section. If something is off follow it back. I would recommend using a Multimeter or DVM if you are not totally familiar with using a scope for accurate AC/DC measurements.

                    Comment


                    • #25
                      The voltage on the output devices are all closely matched at +/- 37.5 v
                      going back to my previous question regarding ripple, all the filtered PS voltages show a flat line on the scope
                      except the 2 in the post above and the 2 pics. Any one understand why I would see a static pattern on those 2 points on the terminal strip shown in pic 1?

                      Comment


                      • #26
                        You cannot interpret the scope shots without knowing (1) The Vertical scale (2) Where you are probing on the schematic. Sometimes the horizontal scale is needed too. "Terminal Strip" means nothing - i have no idea where that is on the schematic.

                        Remove Q11, Q12, Q13 and Q14 and test them. https://vetco.net/blog/test-a-transi...04-12-25-37-07
                        Experience is something you get, just after you really needed it.

                        Comment


                        • #27
                          Not really getting into this, sorry but very short of time, but one point which scratches me the wrong way is that each power amp is split into TWO boards, a "driver" one (11-1002) with the differential pair front end (Q1003/1005) up to Vas (the main voltage gain stage which generates the rail to rail swing) Q1006, its active load Q1002 and "to make things interesting" Q1002 which in most amps receives just some fixed bias (1 to 2V) here it is turned into a "current mirror" by getting out of plase current fed to its base, courtesy of Q1004, which is fed out of phase signal (out of phase relative to what Q1006 receives) courtesy of Q1003 .

                          This drive board generates the main rail to rail swing which is then current amplified by the "current gain transistors"
                          Left Positive : Q8/12
                          Left negative: Q7/11
                          Right Positive: Q9/13
                          Right Negative: Q10/14

                          To boot, driver and output blocks are NOT shown side to side, to further obscure a circuit which already suffers from overcomplication.
                          I would redraw driver and current halves on a single sheet and the usual way , so the standard power amp which is hidden in there is recognizable.

                          You show distorted output waveforms, one of them unsymmetrical, (**please always** state each peak to peak value, donīt make us count screen divisions and wonder where the multiplier switch is set), which worsen a lot when trying to drive a load, so I suspect:

                          * the driver board.
                          Check all transistors get supply voltage (they should, but check anyway) but mainly remember that:
                          a) all NPN transistors have Base some 0.6V more positive then Emitter. Never negative or, say, 5V away from Emitter.
                          b) all NPN transistors have Collector more positive than emitter, always.
                          c) all PNP transistors do exactly the same as a and b but with the opposite polarity.
                          * Only transistors where it is normal to have zero or very low (a few mV) voltage BE or CE are the short protection ones Q3/4/5/6 because, well, they *should* not be activated until you have a real short in process.

                          You should get full rail to rail swing (minus a couple Volt losses) on Q1006 collector and on Q1002, both are the same thing just separated by 2.8V bias.
                          There are two driver boards, both apparently share same part numbers. The more confusion the merrier, huh?
                          If the driver board canīt swing, the output pairs wonīt either, as they have no voltage gain, are basically high current buffers.

                          * the output/current ŋboard? (is it on PCB or wired PTP? )
                          * IF one driver fails, coupled power transistor wonīt drive a load because it missed "push" from the driver.
                          * IF the power transistor blew open , or blew shorted but killed its emitter resistor in the process, the driver transistor can swing unloaded but lacks muscle to drive a speaker load. Might still drive phones though, which is confusing.

                          Ok, back to sleep, itīs 4AM here.

                          Good luck hunting.
                          Juan Manuel Fahey

                          Comment


                          • #28
                            Looking for the +/- 1.4v being feed from the driver boards I find only pin 10G on left channel is delivering.
                            Pin 10K on both boards not working and pin 10G on right channel not working. Each of those 3 read 0.0v
                            So check Q1002 & Q1003?

                            * the output/current ŋboard? (is it on PCB or wired PTP? ) Its PTP

                            Comment


                            • #29
                              * the driver board.
                              Check all transistors get supply voltage (they should, but check anyway) but mainly remember that:
                              a) all NPN transistors have Base some 0.6V more positive then Emitter. Never negative or, say, 5V away from Emitter.
                              b) all NPN transistors have Collector more positive than emitter, always.


                              Look at the schematic of the driver board 11-1002, Q1001
                              As you pointed out base is .6v more positive than emitter. readings I got E 31.8v B 32.8v C 1.2v
                              The schematic and my measurements are not in agreement with "all NPN transistors have Collector more positive than emitter, always"
                              Am I misunderstanding your statement?

                              Comment


                              • #30
                                Q1001 is an NPN type transistor.

                                Q1002 is a PNP type.
                                Your voltage readings, if for Q1002, are good.

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