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Marshall JCM 2000 repair issues

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  • #16
    Originally posted by bnwitt View Post
    Well Amp Kat, I don't know about you, but I have a problem buying tubes with misleading brand names on them. I mean really, who would buy a Rolls Royce knowing it was really a Yugo under the hood ornament. I find this recent practice of purchasing old (abandoned or trademark expired) brand names and then slapping them on modern tubes that have nothing in common with the originals quite deceptive and a bit unethical.
    Ditto, especially if you know what a really nice pair or Quad of real Mullards sound like and how much longer they last than Current production tubes .
    KB

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    • #17
      I've replaced several output transformers on the DSL/TSL series amps. The amps have a design flaw which causes the OT to fail. The ceramic cap in the power section fails due to flyback voltage from this design flaw or the load not being correct, not because it's under rated. It's the effect, not the cause. Usually when the cap fails it goes open causing no problems with the amp. If the cap did short it would make the tube function as a triode (same as the triode/pentode switch Marshall uses) and not cause a drastic volume change.
      The high flyback voltages created can cause burnt circuit boards, damage bias supply components, and shorted output transformers. I've seen all these problems. The bias pot solder joints on these amps can break and be a source of bias problems. The very early amps were put together using the wrong value control grid resistors on the power tubes. They used a 220K resistor instead of a 5.6K.
      The early amps had a service bulletin regarding the channel switching LDR which can cause a drop in volume.
      Jerry
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      • #18
        Good stuff Jerry and thanks for that. Do you think this flaw is in the OT itself with relation to it's load or is it strictly a load issue on the primary side ? Obviously there is nothing on the secondary to impede a loading issue and was wondering where the failure was initiated. The LDR seems like possibly a problem the OP may be having as it fits the bill.
        Last edited by Amp Kat; 04-16-2009, 02:28 PM.
        KB

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        • #19
          The flaw is the 16 ohm speaker jack. The 16 ohm speaker jack works as a switch and when somthing is plugged in that jack it opens the ground circuit to the 4/8 ohms jacks disabling them. The problem is when you use the 4/8 jacks the ground is counting on the switch contacts of the 16 ohm jack to do it's job. We all know how those jacks get dirty and fail in the effects loop causing amps to cut out and drop volume, what do you suppose happens if the switch fails when you're cranking your amp thru a full stack? Just like running with no cab and you get flyback voltage which destroys everything in it's path. When that 22pf cap fails, this is why. I have seen board arc arcoss traces and all sorts of other high voltage damage. Many times it will take out the output transformer too. Many shop will change the output transformer but never repair the original problem, the jack. I jumper the 16 ohm jack ground terminals to prevent that from ever happening again.
          Jerry
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          • #20
            I can't believe I never noticed that on the schematic. Good catch. Has Marshall sent a service bulletin on that?
            Warning! Some Electronics devices contain lethal voltages that can kill you. If you do not feel qualified to work with dangerous voltages, refer your repairs to a qualified technician. By giving you online advice, I am assuming no liability for any injury or damages you might incur through your own actions.

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            • #21
              Excellent Jerry and you know awhile back a few of us had a discussion about the effects of running 16 ohm speakers at the 4 ohm setting and vice versa and how it could fry OT's. I never thought this to be the case as I've done it with good success and better tone at the compromise of tube wear but never blew an OT. It was never causing an open load situation like this does and we all know how OT's love that situation especiall when cranked.
              KB

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              • #22
                I'm not a Marshall service center so I don't know if they have a bulletin about it. I suspect not because I've yet to see anyone else fixing them the right way.
                The first thing I always ask when I hear someone has a DSL/TSL with a drop in power or blown fuses is what are you running for speakers. 9 times out of 10 they will be running a full stack with two 16 ohm cabs in the 8 ohm jacks. It's never an issue if you run a 16 ohm cab in the 16 ohm jack.
                Jerry
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                • #23
                  Here is an example of a 16 ohm jack that saw this type of damage. This is the worst one I've ever seen. A lot of times if you look at the ground terminal of the 16 ohm jack it will have a dull gray look instead of the shiney chrome look. That's a sure sign of it being dirty and having heated up.
                  Jerry

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                  • #24
                    Jerry,
                    This is great info and thanks again for sharing it. I've found that when the 22pf cap shorts, the output section is majorly imbalanced and hums like crazy. I've not seen an output jack fried like that in any of the units I've serviced, but I'll be cleaning and jumping the ground switch/sleeve terminals from now on as they come through.
                    Last edited by bnwitt; 04-16-2009, 07:33 PM.
                    Warning! Some Electronics devices contain lethal voltages that can kill you. If you do not feel qualified to work with dangerous voltages, refer your repairs to a qualified technician. By giving you online advice, I am assuming no liability for any injury or damages you might incur through your own actions.

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                    • #25
                      I just ran into this today on a JCM 2000 50 watt head. Thanks for the heads up. The 16 ohm jack was gray looking at the ground contact, measured very high ohms unless I pushed it hard against the contact. jumpered all the ground together and no more problems.

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                      • #26
                        Originally posted by billyz View Post
                        I just ran into this today on a JCM 2000 50 watt head. Thanks for the heads up. The 16 ohm jack was gray looking at the ground contact, measured very high ohms unless I pushed it hard against the contact. jumpered all the ground together and no more problems.
                        How does it goes to jumper the ground? Any pictures? My 16 ohm Jack looks grey too.

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                        • #27
                          JBlackout... basically all they are saying is the switch in the jack is not up to it so they short that switch part .
                          If you look at the attached diagram JS1 and JS2 the switchable 8 or 4 ohm parallel outs are dependent on getting the ground connection from the switching contacts of JS3. The thick black lines.
                          If a bit of grit got in or failure of the contacts in JS3 (the 16 ohm jack)
                          the amp would see no load ... no speakers connected in a marshall is a receipe for disaster.
                          The eaisest way to solve this is to jumper (bridge) a piece of wire over the
                          offending 16 ohm jack.
                          I've pinched JerryP's photo to illustrate this!
                          The other way would be to remove + flip the socket board and wire the grounds together.
                          The down side of this is you must not plug in other speakers to the parallel
                          outs when using the 16 ohm connection.
                          Attached Files

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                          • #28
                            marshall jcm 2000

                            i have fixed loads of these amps underrated resistors are the main culpret for loss of volume.look at the anode resitor 100k on the preceeding valve to the phase splitter these just stop working, this valve feeds your tone controls,you get some sound but very fizzy replace with 1 watt resistor

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                            • #29
                              Is it the green bit that just needs wiring on the diagram?

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                              • #30
                                Newbie repair questions

                                Hey guys,

                                New here, but have a quick couple of questions. I have a JCM 2000 DSL 401 which has been faithful for the past 10 years, I think I got in mid 2000. It's been serviced a few times, the last by an awesome tech in Venice about 5 years ago. I only paid him to swap out the tubes (JJ's), but he may have done some of these mods to the board... Because my amp still sound way better than the day I got it. And it still needs a speaker swap Avatar cab to fill it's void!

                                So finally to my questions...
                                1. do you know of any DSL techs in the Seattle area?
                                2. Are there any schematics specifically for the DSL 401? I've seen many for 50's and 100's...
                                3. Are these still the best settings for the DSL series, mainly my DSL401?
                                Change R31 to 27k ohms
                                Change R36 to 33k ohms
                                Set bias to 550 millivolts.
                                - from: Bought used DSL 401 - Marshall Amp Forum
                                4. Should I just send my amp in to Voodoo Amps for the mods, or is it that hard if you just take it slow? Need to brush up on my soldering though...
                                5. Read that you should replace caps every 10 years yadda yadda. Should I even though it's working fine?
                                6. And if I do decide to take this thing on. What else do you recommend having on hand? Gloves, Conductor Sprays, etc?

                                Thanks for any suggestions for this surgery! Long overdue.

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