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Marshall JCM 800 - "splatty" Lead channel

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  • #46
    Thanks God who gave us the mighty Scope.

    You have a **gross** case of oscillation, which being around 50kHz isn't audible ... by itself that is, but it overstretches the gain stage big time and SWAMPS the audible signal.

    You can not hear 50kHz but yes what it does to signal.

    In fact most certainly oscillation gets triggered ON-OFF by audio signal and we hear that continued stsrt-stop as "splat".

    Nice little scope, it tries to help and tells what "it" finds.

    I would call that waveform, specially the 8th one: 440 Hz (what you are injecting) mixed with 50 something kHz (oscillation); but I guess TEK just counts zero crossings so it reports the grand total, 50kHz + 440Hz which is basically 50kHz.

    Thatīs why even with sophisticated instruments we still NEED the Human brain to separate chaff from wheat.

    Ok, we need to kill that oscillation and we cure 90% of our problems, if not 100%.

    Too busy now but will open the schematic later and suggest some action.
    Juan Manuel Fahey

    Comment


    • #47
      Gents

      Kind of a noob when it comes to using a scope but very encouraged that you folks see something in the results!

      The noob in me had me read enough of the manual to get the “auto ranging” going.

      The Uber-noob in me wants to know what settings to go for on the scope in manual mode

      I have to go do some stuff but will tackle this again later and get the requested measurements

      Let’s fix this effing thing! I was getting so frustrated, but this international team of technicians will solve this problem and make this Marshall what it once was....mediocre...but working! Hahaha

      Comment


      • #48
        Not entirely sure the schematic can help.?. Well, with the exception that it tells what values to use during a proper restoration. Because certainly the amp didn't have this oscillation when it was sold. The problem is almost certainly due to the work that's been done on the amp. And in the case of PCB construction the layout and lead dress can be critical. Once the designers find a combination that works it can be repeated. Change that and maybe it doesn't anymore. Which I think is the case here. I think that restoring the amp to stock, including the size and nature of the original components and lead dress as close as can be referenced with image searches is the best avenue.

        If that isn't possible because of damage to the PCB then the loop of oscillation has to be identified. It's probably due to lead dress. Some flying leads that are longer than stock or routed other than stock. Other causes can sometimes be found. I remember a thread here once where a guy had replaced the OT in his Marshall and the amp was oscillating badly. We requested photos and discovered that he had neatly laid an unused OT secondary lead along the inside corner of the chassis, well into the preamp end of the amp chassis. He thought it was the tidiest, and therefor best place for it Snip that off and all was fine.

        I think we're looking for a flaw or flaws in any previous work that altered the amp from stock.
        "Take two placebos, works twice as well." Enzo

        "Now get off my lawn with your silicooties and boom-chucka speakers and computers masquerading as amplifiers" Justin Thomas

        "If you're not interested in opinions and the experience of others, why even start a thread?
        You can't just expect consent." Helmholtz

        Comment


        • #49
          G1 - this is Weird.

          V1 pin 6 connects to two resisters which end up in different spots.

          The wire directly from the tube socket pin 6 connects to a 220k resister which measures in spec at 217k

          At the same spot where that wire connects, another wire under the board connects to a 33k resister which measures in spec at around 32k

          In parallel they add up to 28k

          Pin 8 connects to an 820 ohm resister which measures in spec at around 813 ohms

          These resisters look original, being the reddish brown resisters often seen in British amps

          Comment


          • #50
            Remember the can caps 320uf value, which are in parallel that have been used as replacement parts and spoken about earlier?

            Yeah I never got to replacing those with the suggested 2.2value...


            The schem says that they aren't all 320uf, some are .1, and I have had trouble with reading and understanding this schem anyway, so it was hard to know just what goes where...

            I have the replacement parts, and also have received replacement transistors if needed

            Would this fix help with the oscillation?
            Last edited by earache; 09-07-2019, 06:15 PM.

            Comment


            • #51
              Originally posted by earache View Post
              V1 pin 6 connects to two resisters which end up in different spots.

              The wire directly from the tube socket pin 6 connects to a 220k resister which measures in spec at 217k

              At the same spot where that wire connects, another wire under the board connects to a 33k resister which measures in spec at around 32k

              In parallel they add up to 28k

              Pin 8 connects to an 820 ohm resister which measures in spec at around 813 ohms

              These resisters look original, being the reddish brown resisters often seen in British amps
              They may look original, but I can't see them using a 820 on the cathode when there is a 220K at the plate. And that 33K is shown as 68K on the schematic.
              I'm wondering if these are more modifications.
              Originally posted by Enzo
              I have a sign in my shop that says, "Never think up reasons not to check something."


              Comment


              • #52
                Originally posted by g1 View Post
                I'm wondering if these are more modifications.
                BING BING BING BING BING BING!!!!!!!!!!!!!!!!!!

                Restoring the amp components to original and trying to match the stock lead dress as close as possible from pics will likely bring the oscillation under control.
                "Take two placebos, works twice as well." Enzo

                "Now get off my lawn with your silicooties and boom-chucka speakers and computers masquerading as amplifiers" Justin Thomas

                "If you're not interested in opinions and the experience of others, why even start a thread?
                You can't just expect consent." Helmholtz

                Comment


                • #53
                  JM Fahey

                  I have retaken the scope readings of the boost channel.

                  These pictures show the volume being turned up progressively.

                  I started the scope in AutoRange to acquire the initial reading and to center it on the display, then shut the AutoRange off, and took the picture.

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                  Attached Files

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                  • #54
                    G1

                    I can't completely tell what is original and what is a mod as I haven't had this amp it's whole life, nor did I do the previous repairs.

                    It just seems weird that the resisters in question (see arrow in pic below) are the same reddish-brown color as all the majority of the resisters that are of the original build.

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                    • #55
                      Everyone

                      A couple questions regarding some suggestions you all have made and the mods that have been done before I began working on this amp.

                      Please reference the picture below.

                      Click image for larger version

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                      In the photo, hiding to the right of the pot is a .0022 cap.

                      It's a Mallory 150 style, not original to the amp.

                      On the schematic it comes from the middle leg of the dual Gain pot for the boost channel.

                      This is at one of the places where the schematic says, "330uf now .022"

                      Would the seemingly incorrect value of this capacitor cause me any problems?

                      Also in the picture - note how the leads of the can caps that make up the 320uf are in parallel with each other.

                      It's kind of ugly. I wonder if the long legs of these caps have anything to do with the oscillations on the boost channel?

                      Just wondering aloud. Please comment and thanks in advance

                      earache

                      Comment


                      • #56
                        Well in actual photo's of stock amps I don't see a capacitor physically next to that dual ganged pot. Nor do I see the long flying shielded leads from the board that your photos show. It's very likely there are several modifications. Some may be more than simple component value changes. It's very difficult from a remote location to identify such things. We can't tilt the photo to see on the other side of a pot.

                        I believe your commitment to the project needs to go deeper. I think you need a model representative schematic and as many gut shot photos as you can find and take the necessary time removing leads and components to properly trace and check the circuit. And return the amp to stock in both component values and lead dress. It is truly impossible for anyone that doesn't have the amp right in front of them to do this or even advise on the effort effectively. Example:

                        In the photo, hiding to the right of the pot is a .0022 cap.

                        It's a Mallory 150 style, not original to the amp.

                        On the schematic it comes from the middle leg of the dual Gain pot for the boost channel.

                        This is at one of the places where the schematic says, "330uf now .022"

                        Would the seemingly incorrect value of this capacitor cause me any problems?


                        That cap isn't supposed to be there. Does a circuit board trace go to another cap that is possibly of the correct value? And so does that mean that the Mallory 150 cap was added rather than substituted? And so does that mean it's part of a circuit alteration? And so does that mean that a 330uf cap HAS NOT been replaced with a much smaller film cap? Is that .0022uf or .022uf? The way you've written the post it seems you indicated both. And simply indicating the cap is connected to the middle leg of the dual pot for the boost channel doesn't really tell us the circuit function. And we don't have all amp schematics memorized. Even a preacher takes his Bible to the pulpit.

                        YOU on the other hand CAN trace the circuit with some effort. In the end this will be the only way to fix the amp.
                        Last edited by Chuck H; 09-08-2019, 04:29 PM.
                        "Take two placebos, works twice as well." Enzo

                        "Now get off my lawn with your silicooties and boom-chucka speakers and computers masquerading as amplifiers" Justin Thomas

                        "If you're not interested in opinions and the experience of others, why even start a thread?
                        You can't just expect consent." Helmholtz

                        Comment


                        • #57
                          Hi Chuck

                          I appreciate your interest, thanks!

                          From my research I've found that there are two or more versions of this amp in the1980's when it came out.

                          I have an earlier one with discrete transistors.

                          There is a later version where there's a 14 legged chip instead.

                          This cap we spoke of is really part of this amp. It has a reference designator on the board artwork.

                          Can you share the photo(s)that you have of the amp you speak of? I'm having lots of trouble finding ones of the version that I'm working on.

                          earache
                          Last edited by earache; 09-08-2019, 11:26 PM.

                          Comment


                          • #58
                            See post #20. I can see a ceramic cap there.
                            Originally posted by Enzo
                            I have a sign in my shop that says, "Never think up reasons not to check something."


                            Comment


                            • #59
                              Originally posted by earache View Post
                              JM Fahey

                              I have retaken the scope readings of the boost channel.

                              These pictures show the volume being turned up progressively.

                              I started the scope in AutoRange to acquire the initial reading and to center it on the display, then shut the AutoRange off, and took the picture.
                              Thanks.
                              No Internet or Phone linbe since yesterday, just crawling by connecting smartphone to notebook as a crude Modem canīt even load the full size pictures, will try again tomorrow.

                              But in principle waveforms look clean , no HF oscillation, you must have changed something.

                              But in any case, you must restore amp to original, even if it takes some time.

                              Putting some burns ointment on your hand is not a solution if first you donīt take it away from the flame thatīs burning it.

                              Just print 2X size schematic on 2 sheets taped together, and check part by part, connection by connection, ticking checked parts along the way.

                              Any difference you correct on the spot until amp is Factory original.

                              Schematic is suppossed good and tracks were put there at Factory; any tacked n components, flying wires, etc. are suspect.

                              You canīt imagine how I HATE "Forum suggested Mods", doubly so those who donīt "waste time" with schematics, just go: "cut track joining C34 and R22, run a wire from C34 to R47, then cut ....." OMFG, the potentil for destruction is immense.
                              Juan Manuel Fahey

                              Comment


                              • #60
                                Originally posted by g1 View Post
                                See post #20. I can see a ceramic cap there.
                                Well I'll bet it's not a 330uf ceramic cap So that's not what the board or schematic is showing. And herein are the problems with making corrections from a remote location.

                                And you're right earache. Most of the images I saw were with the chip. So...

                                Does the designation on the board for that cap actually say 330uf?
                                "Take two placebos, works twice as well." Enzo

                                "Now get off my lawn with your silicooties and boom-chucka speakers and computers masquerading as amplifiers" Justin Thomas

                                "If you're not interested in opinions and the experience of others, why even start a thread?
                                You can't just expect consent." Helmholtz

                                Comment

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