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Marshall JCM 25/50 Low Volume & Distorted

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  • #46
    Originally posted by mac dillard View Post
    Keep on checking. Your are penning down the problem. FWIW, I have much of the same problem with an AB165 Bassman. Low output and distortion. Put the scope on it and the signal started getting bad at the PI. Problem turned out to be bad OT..Someone on here explained how the OT could cause the readings we were getting.
    I do have a scope and a signal generator. I will give that a try as well. I did try it initially on my dummy load after injecting a signal in the amp but it might be worth another look.

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    • #47
      yes those will tell a lot more about what's going on.
      if your tubes idle current are both close then ,
      another possibility, return jack might need closer inspection check for worn contacts
      if you didn't already try to clean it with some deoxit and run a plug in several times.
      they seem to be notorious in these amps. just a thought.

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      • #48
        Aerostoon, you still around?

        here is your schematic with the DC voltages on them, don't worry about preamp grid voltages, they are not important and are usually erroneous due to voltmeter loading of the hi Z node, this you can see on your inverter grid voltages which are off by about 10 volts,

        the health of an amplifier is determined by the bias voltage, we need the bias voltage across that 470 ohm resistor hanging off the inverter,

        all the DC voltages look good,

        do you have a signal generator? run about 25 millivolts into the input jack at 1000 hz and take down the AC voltages for the test points circled in red,

        if no generator, just strum the guitar nice and easy with all the controls set at 50% and take some readings, you will need to use some alligator clips ahead of time to free up your hands, if you can do this then finding the trouble will be a walk in the park,

        Click image for larger version

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        Attached Files
        Last edited by cjenrick; 07-21-2017, 08:37 AM.

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        • #49
          Originally posted by cjenrick View Post
          Aerostoon, you still around?

          here is your schematic with the DC voltages on them, don't worry about preamp grid voltages, they are not important and are usually erroneous due to voltmeter loading of the hi Z node, this you can see on your inverter grid voltages which are off by about 10 volts,

          the health of an amplifier is determined by the bias voltage, we need the bias voltage across that 470 ohm resistor hanging off the inverter,

          all the DC voltages look good,

          do you have a signal generator? run about 25 millivolts into the input jack at 1000 hz and take down the AC voltages for the test points circled in red,

          if no generator, just strum the guitar nice and easy with all the controls set at 50% and take some readings, you will need to use some alligator clips ahead of time to free up your hands, if you can do this then finding the trouble will be a walk in the park,

          [ATTACH=CONFIG]44154[/ATTACH]
          Thanks cjenrick, your schematic with some voltages shown will really help! I do have a signal generator so I will do your recommended tests this weekend and record the results.

          Comment


          • #50
            Originally posted by cjenrick View Post
            Aerostoon, you still around?

            here is your schematic with the DC voltages on them, don't worry about preamp grid voltages, they are not important and are usually erroneous due to voltmeter loading of the hi Z node, this you can see on your inverter grid voltages which are off by about 10 volts,

            the health of an amplifier is determined by the bias voltage, we need the bias voltage across that 470 ohm resistor hanging off the inverter,

            all the DC voltages look good,

            do you have a signal generator? run about 25 millivolts into the input jack at 1000 hz and take down the AC voltages for the test points circled in red,

            if no generator, just strum the guitar nice and easy with all the controls set at 50% and take some readings, you will need to use some alligator clips ahead of time to free up your hands, if you can do this then finding the trouble will be a walk in the park,

            [ATTACH=CONFIG]44154[/ATTACH]
            Thanks cjenrick, your schematic with some voltages shown will really help! I do have a signal generator so I will do your recommended tests this weekend and record the results.

            Comment


            • #51
              Click image for larger version

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              I injected a 25MV 1000Hz signal in. Set all controls half way and recorded the AC MV readings in red. I checked the DC volts across R40 with no signal and the volume controls at 0. It reads 0.70VDC. The high/low selector was on high.

              Definitely different readings on each output transformer lead. What do you think of the results?

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              • #52
                plug a patch cable from send to return and see what happens, you are losing signal voltage in that area, check vol control grounds,

                try a patch cord again from send to return and take AC measurements from there (V2b)onward to the phase inverter, (V3 pins 1 and 6)

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                • #53
                  Originally posted by cjenrick View Post
                  plug a patch cable from send to return and see what happens, you are losing signal voltage in that area, check vol control grounds,

                  try a patch cord again from send to return and take AC measurements from there (V2b)onward to the phase inverter, (V3 pins 1 and 6)
                  I will give that a try and record the results.

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                  • #54
                    Well I feel pretty silly. I tried jumpering the effects loop and measuring the voltage. No change. Then I thought maybe I should swap the power tube positions and see what AC measurement I get. The imbalance followed the tubes. One of my power tubes was shot. I put some new EL84's in and it works just fine. I tried the new tubes early on but I have done quite a bit of work and must have corrected the issue.
                    Oh well. Now to check the bias and then I'll have a look at the output sine wave on my scope. I sure chased my tail on this repair...

                    Comment


                    • #55
                      The amp is definately louder than before but is probably a third as loud as it should be. With my Eurotube Bias probe I am getting 40MA plate current at 440 volts on pin 3.
                      With the scope connected to the dummy load leads I can turn the clean channel up to max volume and I'm at 50MV per division on my volts/div control. The sine wave takes up most of the screen but no clipping.

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                      • #56
                        Hey glad you got some of it sorted.
                        curious what kind of AC voltage you are getting
                        now around PI C30 C31 at the plates or are those voltages still the same?
                        and the Plate of V2b also.

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                        • #57
                          I will check those out and record the results.
                          Do you think my bias is in the ballpark? Here is my plate dissipation calculation:
                          0.040 X 440 = 17.6 watts.

                          Comment


                          • #58
                            well 70% is 39.7 , 40ma close enough really , it does depend on how you want it to sound and how often you want to change tubes.
                            I'll just add that 70% usually works but nothing wrong will less, i usually bias my marshall between 60 and 65%. but thats just me.
                            Last edited by dstrat; 07-25-2017, 04:10 AM.

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                            • #59
                              "'im at 50MV per division on my volts/div control. The sine wave takes up most of the screen but no clipping..."

                              probably 8 squares on your scope? 8 x 50 mv = 400 mv P-P, still very low, s/b at least 12 V-rms into 8 ohms, with lots of clipping at full volume,

                              jumper send/return with patch cord, record ac voltages with all controls on 11 and record voltages on chart after send/return portion of schematic, use volts instead of mv for this far down in the signal chain,

                              bias current don't matter that much once you crank max volume, by then the AC signal is dominating the DC grid voltage, to see this inject a 100 mv signal with controls maxed and turn pot as you are watching output voltage into dummy load,

                              Comment


                              • #60
                                Originally posted by cjenrick View Post
                                "'im at 50MV per division on my volts/div control. The sine wave takes up most of the screen but no clipping..."

                                probably 8 squares on your scope? 8 x 50 mv = 400 mv P-P, still very low, s/b at least 12 V-rms into 8 ohms, with lots of clipping at full volume,

                                jumper send/return with patch cord, record ac voltages with all controls on 11 and record voltages on chart after send/return portion of schematic, use volts instead of mv for this far down in the signal chain,

                                bias current don't matter that much once you crank max volume, by then the AC signal is dominating the DC grid voltage, to see this inject a 100 mv signal with controls maxed and turn pot as you are watching output voltage into dummy load,
                                FYI I was using my probe at 10X when doing the visual of the sine wave at max volume.
                                I am just setting up to read the AC voltages at max volume.

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