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Marshall JCM 800 - "splatty" Lead channel

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  • #31
    JM Fahey

    Here are the results of the tests you suggested above, to the extent that I can complete them.

    Base measured to ground, Clean setting

    Left. .06 Middle. .69 Right. .06

    Boost setting

    Left. .69. Middle. .07 Right. .69

    The Clean channel never exhibited any splatter or gated sounds even when turned up

    The Boost channel was never without the gated sound, output fell off drastically with attack of plucked string or chord at any volume

    I do not have a signal generator or scope so I couldn’t apply the sine wave.

    Note: “left” “right” and “middle “ transistors correspond to “TR1 “ “TR2” and “TR3” on the circuit board art and Also are the order, top to bottom, of the first three pictures I posted yesterday.

    Earache

    Comment


    • #32
      Alexradium

      That is exactly the schematic that I am working from. I asked earlier in the posts for a clarification on that “now .022”
      Note on the schem. It only appears in one place; there’s another note that says “now .1”

      Haven’t seen crappier schematics anywhere. Poorly drawn, poorly reproduced, difficult to follow.

      Would the switching not work reliably or well with the 330uf caps and that’s why the change to .022?

      Earache

      Comment


      • #33
        Pedro outlined the function and value of those caps back in post #17.
        I took it that the 330uF caused too much latency in switching time, and .022 was insufficient.
        Originally posted by Enzo
        I have a sign in my shop that says, "Never think up reasons not to check something."


        Comment


        • #34
          Originally posted by earache View Post
          JM Fahey

          Here are the results of the tests you suggested above, to the extent that I can complete them.

          Base measured to ground, Clean setting

          Left. .06 Middle. .69 Right. .06

          Boost setting

          Left. .69. Middle. .07 Right. .69

          The Clean channel never exhibited any splatter or gated sounds even when turned up

          The Boost channel was never without the gated sound, output fell off drastically with attack of plucked string or chord at any volume

          I do not have a signal generator or scope so I couldn’t apply the sine wave.

          Note: “left” “right” and “middle “ transistors correspond to “TR1 “ “TR2” and “TR3” on the circuit board art and Also are the order, top to bottom, of the first three pictures I posted yesterday.

          Earache
          Thanks.

          Not too sure we agree on transistor labelling and schematic says nothing (while silkscreen does) , so letīs try this:

          Click image for larger version

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          I called them Left - Center -Right as shown on schematic.

          You tell me which is TR1 - 2 - 3 as on silkscreen

          Also measure (P)late and (K)athode voltage at V1b and V2a

          As a side test, Diode scale measure clipping diodes, all of them,letīs call them D1 - 2 - 3 - 4 left to right and top to bottom.

          No need to post values measured if normal (high one way, 0.6V the other) *unless* 1 or more are abnormal, just tell me "all pass" or whatever you find.
          Last edited by J M Fahey; 09-05-2019, 03:14 AM.
          Juan Manuel Fahey

          Comment


          • #35
            Originally posted by J M Fahey View Post
            Thanks.

            Not too sure we agree on transistor labelling and schematic says nothing (while silkscreen does) , so letīs try this:
            I was also unsure whether you were both talking same Q's, but I did note that the logic levels seem reversed.
            You said clean would have two Q's turned on, gain one Q turned on.
            His measurements seem to show one Q on for clean, two on for gain ch.
            Originally posted by Enzo
            I have a sign in my shop that says, "Never think up reasons not to check something."


            Comment


            • #36
              JM

              Quick voltage readings of V1 and V2. I haven’t quite deciphered which half of either 12ax7 is a or b but this should tell you some info

              V1 pin 1 plate. 145v
              V1 pin 6 plate. 82v
              V1 pin 2 grid. 0v
              V1 pin 7 grid. 0v
              V1 pin 3 cathode. .98v
              V1 pin 8 cathode. .62v


              V2 pin 1 plate. 117v
              V2 pin 6 plate. 245v
              V2 pin 2 grid. 0v
              V2 pin 7 grid. 117v
              V2 pin 3 cathode. .89v
              V2 pin 8 cathode. .25v


              As for the labeling, coincidentally what you have as “Left” is TR1
              “Center” is TR2
              “Right” is TR3

              Where there are 4 diodes shown together on the schem, in reality there are only two.
              They measure about .56 on the diode scale of my meter, whichever way you measure them

              Comment


              • #37
                Can you re-do those readings?
                I think V1 pin6 should be 182V, also V2 pin8 reading is not right.
                Originally posted by Enzo
                I have a sign in my shop that says, "Never think up reasons not to check something."


                Comment


                • #38
                  Originally posted by g1 View Post
                  I was also unsure whether you were both talking same Q's, but I did note that the logic levels seem reversed.
                  And they are.
                  Here transistors are channel killing switches, not channel enabling.
                  So transistor ON (0.6V on Base) means Channel OFF and vieversa.
                  You said clean would have two Q's turned on, gain one Q turned on.
                  Exactly.
                  Clean ON means Left/Q1 OFF (Vbe=0) and Center/Right ON (Vbe=0.6)
                  His measurements seem to show one Q on for clean, two on for gain ch.
                  a) rethink that considering my "reverse Logic" explanation.

                  But I am still concerned because we are not talking the same yet.

                  According to earache we have:

                  Base measured to ground, Clean setting

                  Left. .06 Middle. .69 Right. .06

                  Boost setting

                  Left. .69. Middle. .07 Right. .69
                  Which is impossible because Middle and Right Bases are connected together so must find same voltage on both.

                  So re-re-recheck everything.

                  Also not sure about:

                  Quick voltage readings of V1 and V2. I haven’t quite deciphered which half of either 12ax7 is a or b but this should tell you some info
                  Easy peasy if you pay some attention

                  * V1a plate has a 100 k load resistor, cahode 1k//.1 to ground

                  * V1b plate has a 220k load, 2k7//.68 to ground

                  * V2a plate has 220k load and a short jumper to V2b grid, equite visibl.

                  * V2b plate goes straight to +V rail , 100k to ground, and even more visible, a .1 cap towards the diodes.

                  much easier to identify (all have different connections) than, say, a Twin Reverb where "almost everybody" has 100k plate, 1k5 cathode.

                  V1 pin 1 plate. 145v
                  V1 pin 6 plate. 82v
                  V1 pin 2 grid. 0v
                  V1 pin 7 grid. 0v
                  V1 pin 3 cathode. .98v
                  V1 pin 8 cathode. .62v
                  V2 pin 1 plate. 117v
                  V2 pin 6 plate. 245v
                  V2 pin 2 grid. 0v
                  V2 pin 7 grid. 117v
                  V2 pin 3 cathode. .89v
                  V2 pin 8 cathode. .25v
                  Like G1 said: most look reasonable, V1-6 looks low, V2-8 looks impossible; it should be within 2 V from V2-7 so around 119V.

                  Of course, V2B might be dead (ummmm .... does its filament light? does it contact Socket contacts properly? ... might have an open plate or open cathode connection)

                  Just in case pull tube and tighten socket contacts with a needle.
                  Juan Manuel Fahey

                  Comment


                  • #39
                    JMF and G1

                    I just redid the voltage readings per your suggestion

                    V1 pin 6 is 82.v really!

                    V2 pin 8 is 122.v this was a mistake in my earlier post

                    I took these measurements again this morning before work. When I get back home later I'll check the tubes and sockets

                    I apologize if I am a little ham-handed in the execution of testing or my descriptions.
                    I come to this forum for help, like many of us (I assume), because we do these repairs as a hobby and aren't as well versed or had a lot of formal training in electronics repair.

                    I have had difficulty from the beginning of working on this particular amp because of my inability to decipher these schematics versus this particular circuit board.
                    I find this circuit hard to trace without the reference designators on the schematic.
                    There is a lot of flipping from front to back of the board while I trace the circuit, which sometimes involves undoing the pots from the front of the chassis if I had just previously powered the amp to test it.
                    There are places on the pcb where traces were replaced with wire or the leads of replaced components were left long, bent over and used to reinforce the circuit trace by a previous repair person.

                    Additionally because this amp has pots mounted directly to the pcb, the weight of the board is held up solely by the front of the pots where they mount to the chassis.
                    Two of the pots have been replaced before I got this to work on.
                    This area of the amp may still be suspect.

                    Know that I am grateful and humbled by the willingness of the various folks on this forum to take the time to dig in and help with the details of a problem.

                    Thanks again!

                    Earache

                    Comment


                    • #40
                      Oh, donīt worry about that.

                      In general Forum members trying to help become some kind of "precision Nazis" for the very good reason that your measurements are all we have to work with, being many miles away, from tens to thousands.
                      Hey, even being 1 block away is far enough !!!!!!

                      One new problem: 122V on V2b cathode is fine (I expected something very near to 117V found on grid)
                      Good news: that stage seems to be normal or almost.
                      Bad news: itīs still splatty so we need better tools.

                      If no physical scope is available, youīll need to load some freeware scope and build an attenuator , a simplified fixed one , just to test this dog of an amp.

                      Basically a .02 or .01uF x 600V or better capacitor to stop DC (which anyway canīt be seen on a software scope since soundcards do not recognize it) , a 470k/470 ohm 1000:1 attenuator (so tube World 300 Vpp possible swings get reduced to Soundcard friendly 300 mVpp) , a couple 1N4002 protection diodes clipping anything above 700mV peak plus needed plugs, crocodile clips, etc.

                      Will repost the specific drawing since I have no access to TGP where the original project is.
                      Juan Manuel Fahey

                      Comment


                      • #41
                        JM

                        I got my hands on a scope, a battery powered Techtronix digital scope, circa 2000.

                        I have a signal generator but I'm not sure how well it works.I made it from a kit years back. I once used it to try to "inject" a signal into a live circuit and hit the wrong point and cooked it partially. I think it only produces a square wave...I will investigate it and see what it does.

                        Comment


                        • #42
                          Originally posted by earache View Post
                          JM

                          I got my hands on a scope, a battery powered Techtronix digital scope, circa 2000.

                          I have a signal generator but I'm not sure how well it works.I made it from a kit years back. I once used it to try to "inject" a signal into a live circuit and hit the wrong point and cooked it partially. I think it only produces a square wave...I will investigate it and see what it does.
                          Cool.

                          The difficult part is the scope, hope yours still works.

                          The test tone is easy, just download and play this: https://www.mediacollege.com/audio/t...6bit_30sec.mp3

                          Any smartphone or cheap MP3 player puts out 100 to 200 mV through the earphone jack, which is perfect.

                          Set player to Loop or Continuous or Repeat 1 because tone lasts only 30 seconds.

                          First inject signal through Clean channel, all tone controls on 5 , then repeat through dirty one, slowly rising Gain/Distortion.

                          "Normal" would be a rising sinewave until itīs clipped by diodes; anything else is weird, post screen capture showing it.
                          Juan Manuel Fahey

                          Comment


                          • #43
                            JM

                            Here's the results of the tests you prescribed. The scope is in "Auto Ranging" mode.
                            These first 4 pix are of the Clean channel as the volume and master volume are progressively raised:

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                            These 4 are of the Boost channel, again as the Volume, Gain and Master Volume are progressively raised:

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                            The Boost Channel never got loud during the test.

                            I've noticed another symptom. I have been poking around with a chopstick and get a lot of noise at the tube socket for V3. Pin 6 and 7 are especially noisy. I've tried retensioning the socket and resoldering the connections to the socket pins and also the circuit board, but neither of these things seem to help.

                            earache

                            Comment


                            • #44
                              Can you turn off the auto-ranging?
                              It appears that boost ch. is oscillating at around 50Khz, but the auto-ranging is making it "look" like something we might expect.
                              (or is the 50Khz reported being superimposed upon the 440Hz, I'm not sure)
                              Originally posted by Enzo
                              I have a sign in my shop that says, "Never think up reasons not to check something."


                              Comment


                              • #45
                                Can you measure the resistors connected to V1 pins 6 & 8 ?
                                Should measure around 220K and 2K7.
                                Originally posted by Enzo
                                I have a sign in my shop that says, "Never think up reasons not to check something."


                                Comment

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