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Thread: Marshall JCM 2000 repair issues

  1. #1
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    Marshall JCM 2000 repair issues

    Hi Guys

    First of all, Im very naive when it comes to technology and the associated terminology so bear with me

    Ive had my amp for 9 years from pretty much new and never had it serviced or had any problems with it. then recently it lost a lot of power, no crackels hisses etc, just a loss of volume overall.

    I was told about a guy who did repairs locally, so took it to him, who diagnosed the fault, and said i needed a new set of valves. i expected as much, and although a little disappointed at being quoted 120 for 4 pre an 2 outputs (its a 50 watt amp) as well as fitting, didnt have much in the way of options so went with it. I mentioned that Id heard theyd need biasing, and asked if he could do that, and he said that as he was getting a matched pair of mullards, he wouldnt need to bias them. this worried me, but hey, hes the expert.

    I went today to check it worked with my cab and guitar and to pay and bring it home, but on plugging in there was just crackling and we couldnt get a sound from the guitar. he ummed and ahhhd, took off the back and felt the output transformer which was hot, and he said it looks like your output transformer has gone. He was however getting a signal with his signal generator, but it was very weak, and he said it had been much louder before Id arrived.

    He mentioned the cause might have been that he had been using an 8 ohm load, and me plugging into the 16 ohm cab must have pushed the transformer over the edge.

    Now, I dont have the knowledge to know whats going on, and feel a bit trapped into the situation, and worried about how much Im going to be charged now. Does all this sound ok or should I be concerned? As far as i can tell this guy knows his stuff electronically, and he comes recommended from a friend who i trust very much, but he is not a musician at all and doesnt work solely with amps.

    I have now left it with him to test the output transformer and see where we go from there.

    If anyone could chime in about the type of issue Im having and confirm if the output transformer should be the first suspect, and whether the rest of the work so far sounds reasonable itd really put my mind at rest.

    Thanks in advance,

    Kris.

  2. #2
    Senior Member Mars Amp Repair's Avatar
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    Well,
    First of all, which JCM 2000 is that amp. JCM number are just like a series designation. The other numbers might be a chassis number like 2202 or TSL-100.

    That is a bit weird to have the OT go bad like that, espically if you weren't playing all that loud at the time. I think the jury will be out on that one for a while.

    As far as replacing tubes with matched set not needing biasing, he's got a problem there. The Matched set only means tjhat all four power tubes will be drawing the same current at a specific bias voltage setting. That could be 100ma for one set (which would be low for 4 tubes) or 220ma for another set (which would be too high for 4).

    Either way, the bias being off doesn't ususally take the OT out. If it is of the TSL or DSL variety, there is a common anti-oscillation disc capacitor that shorts & could mimmic a bad OT to an unexperienced tech...so I'd be careful at accepting this 1st diagnosis. get a second opinion from a tech that at least understands how biasing works!

    glen

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    hi thanks for your reply,

    its the dsl variety, 50 watt version, two power valves not four. i assume it still needs to be biased?

    I kinda wish I could just get it back and go somewhere else, but hes already got the new valves and stuck em in, how does the 120 charge sound for new pre and power valves/service/fitting?

    He says the powers are mullards (which my friend doubts as they are hard to get apparantly) and i dont know what the pre amps are, they are still ecc83s tho.

    thanks again

    matt

  4. #4
    Senior Member bnwitt's Avatar
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    If your tech said a matched pair of power tubes in a fixed biased amp doesn't need to be biased, I would be suspicious of his technical skills period.
    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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    You should get your amp back from this guy and take it elsewhere. IMHO. To touch the OT and say it looks as though it is bad... that's an awful assumption to make. A bad OT is last in line when diagnosing such symptoms. It is not to say it CAN'T be the OT... but a bad assumption.

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    thanks guys, wnet and got the amp back, and found a good guy elsewhere. However, before I went to get the amp, i told him to take out the new valves, send them back and put mine back in. Shock horror, it works. Like it did before I even had a problem, like its fixed.

    Could this be to do with the fact that hes probably changed (by accident) the positions of my preamp valves? Im sure Ive heard that one of the four preamp valves deals with the drive or something, I could be completly off with this, but if that one was dying this would make sense. Im still going to take it for a service with the real tech, just pondering thats all.

    Thanks so much for your input

  7. #7
    Senior Member bnwitt's Avatar
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    Quote Originally Posted by wardcband View Post
    thanks guys, wnet and got the amp back, and found a good guy elsewhere. However, before I went to get the amp, i told him to take out the new valves, send them back and put mine back in. Shock horror, it works. Like it did before I even had a problem, like its fixed.

    Could this be to do with the fact that hes probably changed (by accident) the positions of my preamp valves? Im sure Ive heard that one of the four preamp valves deals with the drive or something, I could be completly off with this, but if that one was dying this would make sense. Im still going to take it for a service with the real tech, just pondering thats all.

    Thanks so much for your input
    Sounds more like a loose connection or something, not a dying preamp tube. Could even be loose socket pins. I've got to tell you that these amps are notorius for cracking solder joints at varying locations. This can cause intermittent problems that come and go. I would take the amp to a good tech and have him go through it and beat on it, chop stick it (drop it down some stairs)etc, etc. I just had a DSL401 in here today that had several very poor original solder joints and fatique cracked solder joints on it's board and I see this alot with the JCM2000 series amps. After dissasembling and reassembling the amp for 3 different problems, I just chopsticked the heck out of the board and found two more bad joints at coupling caps. That kept me from having to take it apart two more times or having it come back after the customer pickup. You need a good tech with Marshall DSL experience or you'll be going back and forth for each new problem. I've attached my photo service record on just three of the problems with todays JCM2000 for an example. There were several more solder joint issues and this is usually the case with the JCM2000 series amps. I'm almost ready to buy a stereo microscope to inspect these boards to save some time.
    Attached Thumbnails Attached Thumbnails john-auchman-dsl401.pdf  
    Last edited by bnwitt; 04-14-2009 at 02:57 AM.
    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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    My money would be on the "Mullard" replacement power tubes having been bad, especially if they were new "Mullards" (which are nothing at all like real Mullards apart from also being vacuum tubes).

    Aside from that the JCM2000 series suffers from a long list of known maladies, most of which can be found by doing a search on this forum based on "JCM2000", "DSL", and "TSL". Cracked solder joints (rampant & widespread), a shorted (and under-specified) ceramic cap near the power tubes, and bad 16-ohm jack switching contacts probably top the list but are not the end of the possible issues. Something along these lines could have been the cause of the initial problem, as most usually cause intermittent symptoms which may at this time be temporarily resolved.

    As bnwitt said a tech with a lot of Marshall DSL/TSL/JCM2000 series experience would be a big plus here.

    That being said I think the JCM2000 amps are pretty good-sounding and versatile when working correctly.

    Best of luck!

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    Senior Member bnwitt's Avatar
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    Quote Originally Posted by Mark Black View Post
    ...That being said I think the JCM2000 amps are pretty good-sounding and versatile when working correctly.
    I'd agree with that. The DSL401 I just repaired sounded great clean and had pretty good overdrive tones too. All available via footswitch.
    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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    Senior Member Mars Amp Repair's Avatar
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    Regarding the 22pf under rated voltage cap...I have now had 2 come back with the replacement 1KV cap shorted.
    I am now replacing that little bugger with a 2kv cap...glen

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    Senior Member bnwitt's Avatar
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    Quote Originally Posted by Mars Amp Repair View Post
    Regarding the 22pf under rated voltage cap...I have now had 2 come back with the replacement 1KV cap shorted.
    I am now replacing that little bugger with a 2kv cap...glen
    Wow! What the heck is going on there in that part of the circuit that would blow a 1KV cap?

    Wardcband this is academic to the discussion of your amp as only the 100 watt version has the cap in question.
    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.

  12. #12
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    That amp is pretty succeptable to bad tubes and usually takes out resistors when it doesn't like them and I've seen a few plate resistors go on that amp also. There are two versions of these,one with bias pots on the outside and newer ones without. What I found amaazing is that he blamed the mismatch in loads as the culprit and that isn't the cause for blowing OT's. Now if the speaker came disconnected or open while the amp was cranked for all it's worth then yes that is a cause if the OT is even blown at all. The 1st thing he needs to do is test the transformer and these types are very simple to test either with ohms or voltage from the secondary to primary. I also agree that the Mullard reissues are most likely the whole culprit as I've had problems with almost every pair I've seen. I'd switch techs too.
    KB

  13. #13
    Senior Member bnwitt's Avatar
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    Quote Originally Posted by Amp Kat View Post
    I also agree that the Mullard reissues are most likely the whole culprit as I've had problems with almost every pair I've seen. I'd switch techs too.
    Well the original problem was a loss of power with the original tubes in place so I'm still betting on a bad solder joint, dirty effects jack, or some other intermittent problem that has yet to be discovered. If a good tech doesn't give the amp a once over, I would expect the problem to return.

    Barry
    Audiotexan likes this.
    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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    Old Timer Amp Kat's Avatar
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    Quote Originally Posted by bnwitt View Post
    Well the original problem was a loss of power with the original tubes in place so I'm still betting on a bad solder joint, dirty effects jack, or some other intermittent problem that has yet to be discovered. If a good tech doesn't give the amp a once over, I would expect the problem to return.

    Barry
    You may be right and I shouldn't have said whole problem because it definitely has other problems. Obviously when it gets moved a certain way is when it acts up so yes a connection,cold solder or even tube sockets and loop jacks could cause it. Those tubes are also problems but I doubt they would cause power loss unless bad out of box which many are.
    KB

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    Senior Member bnwitt's Avatar
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    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.
    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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    Quote 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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    Member JerryP's Avatar
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    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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    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 at 03:28 PM.
    KB

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    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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    Senior Member bnwitt's Avatar
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    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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    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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    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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    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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    Senior Member bnwitt's Avatar
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    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 at 08:33 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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    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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    Quote 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.

  27. #27
    Senior Member oc disorder's Avatar
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    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 Thumbnails Attached Thumbnails 2000-output.jpg   tsljack2.jpg  

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

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    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.

  31. #31
    Senior Member oc disorder's Avatar
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    Welcome to the forum hemorrhage.
    You can find the schematic on this page.
    historiclespauls.com - piller.at MUSIC - Marshall DSL201/401
    It's not necessary to fit a heat sink but it worked for him..
    Start a fresh topic with 401 in the subject.
    I havn't seen one for a while so hopefully someone in the USA
    can chip in.
    If you have limited experience and minimal tools it may be
    rather challenging for your first time major project.
    Its up to you to weigh up the pro's and con's but if something goes awry
    it may cost a lot more to get it fixed.
    eg Small champs etc are much eaiser to get into as you can see all the components and there is tons of info about these.
    Maybe a kit for a small practice amp would be good where you can take your time and enjoy it .
    Just my thoughts... anyway I've been over posting today and I need some decent sleep.
    Good luck !

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    1. do you know of any DSL techs in the Seattle area?
    The guys at Aviator Audio in Edmonds have extensive JCM2000 (and specific DSL401) experience.

    Disclaimer: Yes - I have worked there. That's how I know...

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    Hi, thanks for all this useful info!
    My dsl 50 was doing the same thing so added the jump and tried it and indeed, it works!
    I feel like I may have done a bad job soldering the wire though, I took a picture.
    http://i19.photobucket.com/albums/b1...r/DSCF6801.jpg

    On the subject, while inside the amp I notice all the internal fuses, and noticed one was missing?
    I've had this amp for almost 2 years, and minus this weird output thing haven't had any problems.
    Is this an active place for a fuse, or is this something like Marshall using the same board for the dsl 50 and 100 and it is unused in the 50 but used in the 100?
    Here's a picture.
    http://i19.photobucket.com/albums/b1...cular/fuse.jpg
    Attached Thumbnails Attached Thumbnails dscf6801.jpg  
    Last edited by shibbz; 04-21-2010 at 07:10 AM.

  34. #34
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    Quote Originally Posted by shibbz View Post
    Hi, thanks for all this useful info!
    My dsl 50 was doing the same thing so added the jump and tried it and indeed, it works!
    I feel like I may have done a bad job soldering the wire though, I took a picture.
    http://i19.photobucket.com/albums/b1...r/DSCF6801.jpg

    On the subject, while inside the amp I notice all the internal fuses, and noticed one was missing?
    I've had this amp for almost 2 years, and minus this weird output thing haven't had any problems.
    Is this an active place for a fuse, or is this something like Marshall using the same board for the dsl 50 and 100 and it is unused in the 50 but used in the 100?
    Here's a picture.
    http://i19.photobucket.com/albums/b1...cular/fuse.jpg
    Same board for the 100 and the 50 watt so that fuse is not used in your amp.
    Jerry

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    I've got a busted DSL100 that I'm digging into, and was wondering if the symptom I'm seeing is a result of the dirty 16-ohm jack. I open it up and the resistor outlined in red in this figure was complete roasted. We're talking big black spot, nothing left except the leads. Can someone explain to me the function of this resistor, and what could have caused that kind of damage? Also, I've read a bit about "flyback voltage" in this thread, but haven't really found a good explanation of what it is. Is that similar to what would be called a 'reflection' on a transmission line, caused by a mismatched impedance? Thanks!


    poppedresistor.bmp
    Last edited by PepeSilvia; 08-02-2010 at 03:01 PM.

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