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Marshall JCM2000 DSL100 Reverb Squeal

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  • #16
    I dont know if this would cause it but , have you pulled pcbs out and looked for cracked solder joints? EVERY Dsl / Tsl Ive worked on had many cracked .

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    • #17
      If you can forgive me some more idle thinking:

      If the red return RCA jack won't feed signal through, possible open in the L1, R15,C15 might leave the input of IC2 wanting more, and when we turn a reverb control up past a certain amount, circuit crosstalk is sufficient to start feedback. MAybe?
      Education is what you're left with after you have forgotten what you have learned.

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      • #18
        Originally posted by g1 View Post
        R2 the 150R is good? Have you tried replacing C5?
        I haven't tried replacing C5. R2 measured 47 ohms. I'll have a look at that, and see if there's any cracks in the traces/pads on any of the boards. I haven't had the main amp board out, but that wouldn't be involved in this from what I can see.
        Logic is an organized way of going wrong with confidence

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        • #19
          Your schematic says R2 is 47 ohms. Mine says it is 22 ohms. Yours is issue 4 mine is issue 5.
          Education is what you're left with after you have forgotten what you have learned.

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          • #20
            Originally posted by Enzo View Post
            Your schematic says R2 is 47 ohms. Mine says it is 22 ohms. Yours is issue 4 mine is issue 5.
            Of course those are the schematics. The PCB had 47 ohms in it. Interesting that we now have 150 ohms, 47 ohms and 22 ohms as the candidates for the value. I haven't yet checked to see that I can pass signal thru the Tank drive circuit. (though I was getting burst pink noise passing thru up to where it began shrieking & howling). While I didn't get signal appearing thru the amp when I injected signal directly into the Reverb return amp's RED RCA jack, I didn't check to see if there was signal coming out of the M5210 circuit (now using JRC equivalent). Hadn't thought about the possible cracks in the PCB, nor issues with the wire passing thru the ferrite beads on the input to the Reverb board. More fun for tomorrow.
            Logic is an organized way of going wrong with confidence

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            • #21
              Just for reference here is issue 5. The zeners are gone and the input circuit is a bit different
              Attached Files
              Education is what you're left with after you have forgotten what you have learned.

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              • #22
                Originally posted by Enzo View Post
                Just for reference here is issue 5. The zeners are gone and the input circuit is a bit different
                I can't open this file for some reason. I get an "invalid file specification" message
                Logic is an organized way of going wrong with confidence

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                • #23
                  Oh crap, sorry. This system hates most of my files anymore. Try this

                  Attached Files
                  Education is what you're left with after you have forgotten what you have learned.

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                  • #24
                    And issue 6 is in post #12
                    "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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                    • #25
                      OK...new day, and everything works. I hope. I stopped off at our Rental Depot on the way in, fetched an assumed-working Marshall JCM2000 DSL 100...one of our older units from the Asset code. Brought it into the shop, set it up, removed the rear grille panel, reached in and unplugged the RED Reverb Return Cable and plugged in my Burst Pink Noise source cable. Powered it up, and within a few moments, noticed the middle left power tube wasn't glowing. Hmmm....Powered back down, swapped left with middle left, and the problem didn't move. What wasn't glowing now is. Heater fuse, no doubt. AND, further justification to resume PREVENTATIVE MAINTENANCE on everything on the shelves! Ready for rental service, indeed!

                      Anyway, Switched out of S/B, tipped the cabinet up enough to see the control panel and dialed in what I had on the other amp, then turned up the Reverb Level. Nothing. Turned the source all the way up (source is patched into HP 353B Patch Panel, which has a step attenuator and an output xfmr), dialed up 0dB Atten. Still no signal....well, very faintly in the background with the Reverb level turned up full.

                      OK....WHY wouldn't I be able to get test signal thru this path, when we can get the very low level Reverb Tank signal thru this first gain stage, then pass it along to the next one, which follows the FP Reverb level control. Oh, I do love a good mystery.
                      Logic is an organized way of going wrong with confidence

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                      • #26
                        Looking thru the maze of CONN connector cables that's enough to drive you nuts without an accurate guide to lead you correctly.....I see the Source to the Reverb Board JCM2-63-00 is CONN 8. That comes from the plate of V2 Pin 1 (4th tube stage), decoupled via 22nF C23, fed via 220k R37, thru jumper R43 to CONN 8. The Return from the Reverb board CONN 2 is sent to CONN 8 on the Preamp PCB JCM2-61-00. That feeds both REV A and REV B pots and on to IC1. Output of IC1 is sent back to the Tube PCB via CONN 9. It arrives to the Tube PCB JCM2-60-00 at CONN 7. It is fed to the Grid of the next tube stage V3 Pin 7 via R42 2.2k & R41 4.7k. It is summed with the signal from the plate of V2 pin 1 & it's circuit using a 100k R39.

                        What I'm reading between CONN 8 and CONN 7 on this tube board is 20k. I see a 100k resistor, along with the 2.2k and what looks more like 47k (R41) but that might just be pigment of the ink.

                        So, the problem of this horrid screech and howling may be the oscillation caused by what I'm reading on the tube board. I'm about to attempt unsoldering R39. I'd rather NOT have to gut the chassis to extract the tube board to do that right. Might be better to just cut R39, then tack-solder a known real 100k resistor. Maybe just cut one end, and measure again to see if the reading changes?

                        So much for my earlier statement that the Tube board probably isn't involved. Horse pucky.
                        Logic is an organized way of going wrong with confidence

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                        • #27
                          Um, any chance the reverb return IC2 is turned to the off position by TR1?
                          Education is what you're left with after you have forgotten what you have learned.

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                          • #28
                            Are you on the second unit now, or the original unit?
                            "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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                            • #29
                              Originally posted by Enzo View Post
                              Um, any chance the reverb return IC2 is turned to the off position by TR1?
                              NO....IC 2 wasn't being turned off by TR1, though I did get that alternate action with the foot pedal when plugged in/engaged. I saw solid signal out of IC2 this morning.
                              Logic is an organized way of going wrong with confidence

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                              • #30
                                Originally posted by g1 View Post
                                Are you on the second unit now, or the original unit?
                                I did verify I had working Reverb on the second DSL100 that I borrowed from our Rental Depot. Still mystified not being able to get signal thru IC2 to appear on the output of the amp, as we can always do on Fender Amps via Reverb Return circuit. Separate issue.
                                Logic is an organized way of going wrong with confidence

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