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Marshall JCM2000 DSL100 Reverb Squeal

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  • Marshall JCM2000 DSL100 Reverb Squeal

    I have a Marshall JCM 2000 DSL100 where just barely turning up the Reverb in either Ch 1 or Ch 2, the Reverb is feeding back LOUDLY. Swapped cables, change in sensitivity, but still squeals. I replaced the switching op amp M5201, no difference. Checked the MPS A13, it's fine. Checked the connectors. Conn 1 on the Reverb assy 10-63-02 comes from JCM2-60-00 Conn 8. Output Conn 2 goes to JCM2-61-00 Conn 8. Conn 3 from Reverb board goes to Conn 4 on rear panel PCB JCM2-62-00. Pedal makes no difference.

    I haven't yet replaced the LM348 Quad op amp that drives the tank.

    I don't recall coming across this problem before. The first tank that was installed RMOD-4BB3C1D did the same thing, though it had one of its' two decay springs broken. Just received the new replacement, and it does the same thing.
    Logic is an organized way of going wrong with confidence

  • #2
    You probably already did, but...

    Pans these days are configurable, make sure the INPUT jack is NOT grounded to the frame.

    Feeds back how? Acoustically as in gain so high it acoustic feedbacks? Or feeds back electrically as in even on a dummy load so nothing acoustic?

    Dos it still feed back if you disconnect the drive cable from the INPUT jack on the pan?
    Education is what you're left with after you have forgotten what you have learned.

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    • #3
      I verified both the previous tank along with this new one that the input jack is NOT grounded to the frame.

      Feedback how? Just cracking the Reverb control a touch leads to loud howling. If I disconnect the input, I can get Splash. Checking what I get by plugging in directly to the tank from my generator.....500mV 400Hz Warbled-Sine, I get nothing. Then, plugged directly into the Reverb Return connector of the Reverb Board, and also get nothing...same level/source.

      I've removed the Reverb board JCM2-63-02, replaced the M5210 with JCMC 2120 (replacement for the M5210), replaced the LM348 with an MC33079, added bypass caps to the supply, along with replacing TR1 MPS A13.. Replaced the M5210 on the JCM2-61-02 Preamp PCB IC1, along with TR4 MPS A13. Nothing has changed. Crack the volume to about 1 on the hash marks, and it's howling, while starting to give you Reverb. Both channels are doing the same thing.

      I haven't yet checked the feedback resistors on the Reverb Tank Drive circuit, nor on the recovery circuit IC2. Likewise the resistors around IC1 of the Preamp PCB assy.
      Logic is an organized way of going wrong with confidence

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      • #4
        Since it sounds like a possible ground fault, have you verified the new cables? Not likely to find 2 bad sets, but worth checking.
        "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

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        • #5
          YUP....replaced the cable set, no difference.

          I have the Reverb board out at the moment, staring at it. Only difference I see between it and the schematic I have is the absence of ZD1 & ZD2 2.7V back-to-back zeners. Issues 3 PCB assy. Just got thru checking all of the resistors on this board.

          I don't recall ever coming up against this problem before on a JCM 2000. I didn't have an LM348. All I had were some LM324's (same pinout), and these faster MC33079's. I didn't hear any difference (with this howling problem) between those two IC's.

          Checked the new tank's Ground strapping for the input and output. Input floats, output is grounded to the tank chassis. I can swap the cables around, and still get the same howling, and right before that, a touch of reverb, not proper, so before it breaks into this screaching howling jag, there's a touch of reverb.

          Guess I'll pull the preamp board out again, and check the two reverb pots. And the resistors around IC1...Reverb return ckt...switching IC.
          Logic is an organized way of going wrong with confidence

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          • #6
            I just pulled the Preamp PCB assy out, removed the two Reverb pots to check and clean them inside (pulled them apart). These are both marked B20K....which is 20k Linear...measures 18k. Shouldn't these be Audio Taper?

            Just looked at the schematic for this JCM2-61-02 Preamp PCB Assy. Called out as B22k....so they are Linear. They behave as though they should be Audio Taper. Just nudging the knobs up to around 1, or a touch greater, you get reverb, and that next nudge, nowhere near '2' on the dial, you've got howling and screaching.

            I checked Studio Sound Electronics to see what pot values they had to fit the Marshall JCM2000's. They have 10k Audio, then 100k Audio. There already is a 10k resistor ahead of the top of each pot in the circuit.

            I think I need to drive over to our Rental Dept and pull out a JCM 2000 to listen to. I don't recall having this extreme touchiness on the Reverb pot settings. Checked to see what Reverb tank is normally used. 4BB3C1D 150 ohm input, 2250 ohm output, input isolated, output grounded, Long tank, 2 spring, Long Decay, Vertical (wall) mount, connectors face down (mounts to the back side of the cabinet front grille above the control panel).

            I looked thru my zener diode stash, as the two 2.7V Zeners (back-to-back) are not on the Reverb board. Closest I had were 3.3V 1N746A, and installed those. Didn't make any difference.
            Last edited by nevetslab; 05-20-2022, 10:53 PM.
            Logic is an organized way of going wrong with confidence

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            • #7
              Oh my. Does it still behave this way if you disconnect the cable from CON8 near the reverb controls and IC1?

              Are the two controls interactive? And does either one trigger this?


              I just wonder... the two pots are sorta in parallel as far as AC is concerned via the two caps and 10k resistors. Perhaps that could explain the taper phenomenon?

              I used to be into radio, and we had regenerative receivers. You turned up the RF gain until it oscillated, then backed off a hair. As the amp was about to break into oscillation, it had a narrow band of very high gain. I wonder if this circuit is already on the verge of oscillation and kicking the knob up to 2 was what it takes to break it out. That more than a taper issue. Perhaps?
              Education is what you're left with after you have forgotten what you have learned.

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              • #8
                If I disconnect CONN 8 (which comes from the Reverb PCB assy, output from Return circuit), I get no oscillation/screaching.

                Now, THIS IS interesting. Plugging a signal into the Reverb Return connector of the JCM2-63-02 Reverb PCB assy, I cannot get that signal to come up thru either Ch A or Ch B Reverb control. Always on other amps, patching signal into the Reverb Return path will pass that signal on and be able to mix it via the Reverb Control. Not so on this amp.

                I've tried different signal sources, and signal levels up to 1V. Sweep the oscillator up past 5kHz, and then you're getting that signal faintly in the background via leakage, but NOTHING direct as one would expect.

                The two controls are NOT ntereactive, when I have the Reverb reconnected.

                I haven't yet been over to our Rental Dept, but will fetch another JCM 2000 DSL100 like this to see what I'm missing (hopefully).

                This is a weird one.
                Logic is an organized way of going wrong with confidence

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                • #9
                  Schematic?
                  - Own Opinions Only -

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                  • #10
                    R2 the 150R is good? Have you tried replacing C5?
                    "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

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                    • #11
                      Originally posted by nevetslab View Post

                      Now, THIS IS interesting. Plugging a signal into the Reverb Return connector of the JCM2-63-02 Reverb PCB assy, I cannot get that signal to come up thru either Ch A or Ch B Reverb control.
                      But you said you could get a splash when you had tank input disconnected? How high did you have to turn the reverb up and how loud was the splash?

                      "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

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                      • #12
                        Originally posted by Helmholtz View Post
                        Schematic?
                        Here's that board issue 6, which does not have the zeners at IC2 input.
                        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


                        • #13
                          Originally posted by Helmholtz View Post
                          Schematic?
                          Here are the schematics for the JCM 2000 DSL100 amp in question.

                          tl10-63-02.pdf

                          jcm26102_ISS4.pdf

                          jcm26002_ISS5.pdf

                          jcm26202_ISS2.pdf
                          Logic is an organized way of going wrong with confidence

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                          • #14
                            Originally posted by g1 View Post

                            Here's that board issue 6, which does not have the zeners at IC2 input.
                            I found that also with Issue 5 board. Not getting anywhere, I was grasping at straws, tried that, but had nothing to do with it.

                            The fact that I can't even get a test signal to pass from the Reverb Return circuit on the JCM2-63-02 Reverb board via CONN 2 to CONN 8 on the Preamp PCB....there's something amiss that I haven't found yet. The Reverb tank circuit together with the Reverb Control appears to be in a self-oscillation loop, the way it's behaving.
                            Logic is an organized way of going wrong with confidence

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                            • #15
                              Originally posted by nevetslab View Post
                              The fact that I can't even get a test signal to pass from the Reverb Return circuit on the JCM2-63-02 Reverb board
                              Check that pin 1 of the 5201 is doing it's thing with reverb footswitch open and shorted. Sounds like it might be stuck in limbo?

                              "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

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