No Sound/Low Volume - Marshall MG250dfx

Discussion in 'Marshall Amps' started by Gerran, Oct 24, 2021.

  1. Gerran

    Gerran New Member

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    Hey guys,

    Super new to the forum. I just picked up a Marshall MG250dfx on CL. I know that this amp isn't very popular/isn't well liked, but it suits my needs right now.

    Anyway, it worked fine for a little while, but after I played for 20 minutes or so, the sound dropped to point where I can barely even hear my guitar with the volume on 10. I know that these apps can have problems with the fans, but I don't think the issue is overheating/fan issues because I took the amp apart and exposed the electronics directly to the air and waited an hour after this happened, and the problem persisted.

    Wondering if anyone has any experience with this issue, and if so, what you did to fix it, besides replacing the fan. And possibly where you got the parts.

    Thanks in advance.
     
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  2. StrummerJoe

    StrummerJoe Well-Known Member

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    :welcome: Sorry about your amp problems...that sucks. Hopefully, and likely, someone with more knowledge will be along to give advice.
     
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  3. Mitchell Pearrow

    Mitchell Pearrow Well-Known Member VIP Member

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    The only big MG we had was the 100 watt head, I had never experienced this problem with it.
    But a mg30 had a volume drop, but it went away on it’s own.
    Does your amp have the 3 preprogrammed voice change channels ?
    I would check that as it could be on another setting that the volume is not coming through.
    I really don’t have much experience with your amp, and if the fan is working I doubt that you overheated it.
    And Welcome To The Forum
     
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  4. South Park

    South Park Well-Known Member

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    Not much you can do solid state is not easy to work on .
     
  5. Gerran

    Gerran New Member

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    It does suck! But thanks so much... I hope so too!
     
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  6. Gerran

    Gerran New Member

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    Thanks so much for this info. As an update, I tested again 10 minutes and while I was playing the volume suddenly returned to normal (and scared the crap out of me). I just wish I knew what the issue was so that I could replace the part and move on.

    To answer your question, no there aren't 3 preprogrammed voice channels. Yeah it def was not overheated. I appreciate your input!
     
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  7. Gerran

    Gerran New Member

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    Yeah, I mean I have a moderate level of experience working with electronics and stuff... I just wish I had a diagnosis. Not sure where to start...
     
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  8. PelliX

    PelliX Well-Known Member

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    Well, even though it might not be getting hot now, the output transistor could be damaged. If you have a component/transistor tester, it should be easy enough to verify that. Also, start with the simple stuff, loose connections, bad contacts in jack plugs, a sticky switch somewhere, etc. Check the voltages on the board and around the output stage. I'm not sure what the output IC is off the top of my head, but you should be able to check that (component itself or schematic) and verify it's operating voltage to see if it's getting that. I believe those amps have a little digital FX daughterboard (hence the name). Not sure if you can 'easily' bypass that to rule it out - got a schematic handy?

    EDIT: Considering the volume returned, I'm not pointing a finger at the output IC - when they fail, they generally stay failed in my experience. It could be the board it's mounted on. Check for continuity on it, and verify there is no continuity where there should be none. They get fairly hot, and I believe I've encountered a cooked one. Whether that one experienced a fan failure, I can't remember, honestly.
     
    Last edited: Oct 25, 2021
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  9. Ken Underwood

    Ken Underwood Well-Known Member VIP Member

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    I mean I have a moderate level of experience working with electronics

    Can you tell us to what level this is?
     
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  10. Gerran

    Gerran New Member

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    I can read a wiring diagram, I understand the basics, I'm good at soldering, and I've worked on/repaired various electronics in the past
     
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  11. Gerran

    Gerran New Member

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    Hey! Thanks so much for these suggestions... I really appreciate it. Fortunately, I was able to locate a schematic online. I guess what I'll do is try my best to hunt down the problem, and if I can't find it, I'll just cut my losses...

    And yeah that makes sense about the output IC. Again, thanks so much for your input!
     
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  12. Gerran

    Gerran New Member

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    Also, you are correct about the daughterboard!
     
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  13. Matthews Guitars

    Matthews Guitars Well-Known Member

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    Solid state is easy to work on, as long as you have good soldering skills and can read the schematics. This notion that solid state is hard to work on never made any sense to me. But I was repairing solid state equipment for quite a few years before I started doing tube circuit repair work. The only time I call equipment hard to work on is when the manufacturer did something stupid like wire up multiple PC boards to each other with discrete soldered wires, not a connector among them that can be disconnected.

    Anything from tube amps to high density surface mounted components in computerized equipment, to me it's all the same. As long as I have the documentation and the tools, troubleshooting is troubleshooting, component replacement is component replacement.

    In fact, I actually enjoy doing surface mounted component work. And I'm good at it. I work to mil spec j-std 001 and IPC-A 610 and IPC-7711 and 7721 standards. (Not certified, but I could pass the tests.) Soldering and electronics assembly is fun...if you don't do it TOO many hours a day, every day!
     
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  14. Ken Underwood

    Ken Underwood Well-Known Member VIP Member

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    My own feelings are that you need a lot more experience to tackle and successfully repair what you have there.
    Being able to do it is all very well but understanding and i mean understanding what you are doing is another thing.
     
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  15. PelliX

    PelliX Well-Known Member

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    You're probably right, but let's not forget, that's how many of us learned it. Doing a bit, reading a bit, rinse, repeat. None of us learned to play a guitar from a book without holding one in our hands, right. Thesedays with the Internet you can literally figure out any part of a circuit relatively easily. No dusting off books at the library, anymore. Datasheets for components a mouse click away.:shrug:
     
  16. Ken Underwood

    Ken Underwood Well-Known Member VIP Member

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    Sorry mate cannot agree with you on that one, if that is the case then all i have to do is look on the internet and in no time at all i can become a Brain Surgeon.

    If you are going to do it do it properly
     
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  17. PelliX

    PelliX Well-Known Member

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    I started noodling around with electronics and gear well before I was of an age where I could even potentially pursue an education in that field. So I picked up bits of info from my dad, library books, etc. Buying kits, building them, modifying them. Then the father of a friend of mine gave me basically his entire school material from when he learned about that stuff classically. I quit school very early and have not a single certificate, diploma or license to my name. I work as a senior network/infrastructure architect/engineer for a tech multinational, and never followed a course for that, either. Nobody learns to become a surgeon without cutting around in corpses (and eventually the living, I guess), and I think there is a big ethical difference between poking around in an amp and poking around in a person. That's why I'm an engineer, not a doctor, maybe.

    I understand what you're saying, but some of us learn by getting our feet wet first. I learned to write hacky scripts well before I progressed to actual good application design and more complex structures. It's my experience that you can save time by learning something 'in class', but with enough perseverance you can get there the other way, too. You said that if this were true, you could become a brain surgeon in no time - no, indeed - that's not possible. But you could learn a fair deal about neurology and human anatomy with the resources available. Surely, Ken, as someone who knows about this you realize that you start by understanding part of a circuit and work your way through. Eight times out of ten the gear ending up on my bench is new to me. Never encountered that model or that manufacturer, but I get the basic gist of what it does and how it works. The rest you work out, because you have the knowledge to go from there. So maybe OP knows just enough to understand what the transformers and rectifier do. Why "stop" there? I've seen people in their fifties decide to pick up programming all of a sudden and actually become quite decent at it. :shrug:

    Of course this is different in a professional environment. I deal with that aspect every day, but I also make a point of giving someone a (controlled) shot at solving a problem that's well above their 'level' or outside their field of expertise if the situation permits it. Sometimes somebody takes the ball and runs with it, so to say, sometimes they drop it. Two of my best engineers are exactly that type that grabbed their chance and proved themselves. Those guys are worth infinitely more to me, the team and the company than many of the certified know-it-alls that I interview.

    Reminds me of when my girlfriend told me to leave it up to the professionals to fix her car (after two garages failed) because I don't have a license. I then proceeded to diagnose the fault, ordered and replaced the part and done. We both learned something then, and after that I was in charge of fixing the car. That's not a snide remark, by the way - I really mean no disrespect, I'm just trying to provide another point of view here.
     
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  18. Matthews Guitars

    Matthews Guitars Well-Known Member

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    I believe formal education (even if it's self-education) is required to really understand a circuit, but when I took electronics classes, troubleshooting was integrated into learning. As soon as the class was taught a new subject, the next phase of learning was to understand how that new component or circuit can fail and what the symptoms were. And we were tested on it.

    So we were taugh a circuit...and then had to figure out what failed in it when given an example.

    It isn't like you have to learn EVERYTHING before you troubleshoot and repair ANYTHING. But if you don't know what the components are, what their values are, and what they do, then you can't be expected to troubleshoot a circuit that has them in it. Not effectively, anyway.

    And, logical troubleshooting processes do allow you to effectively repair equipment even if you don't FULLY understand it. But you must have some basic knowledge.
     
  19. PelliX

    PelliX Well-Known Member

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    Totally. The thing is, you can simply look up what an inductor is or how to measure a capacitor and what it does now in seconds. So let's say you don't know how a valve amp works. You open it, <blank look>, then start reading and getting a schematic for it, etc. Does that make you an expert? No. Can you learn to service amplifiers that way? Yes, with some practice, reading and perseverance, you could.

    Also, case in point, it's a solid state amp. If you can identify the resistors, diodes, capacitors, op-amps and transistors in it and test/measure them you're pretty much there... and if you don't know how to test a transistor or how to measure a cap - what a great time to learn if you're so inclined. I love electronics, but I learned it mostly for practical purposes like fixing my stuff or making stuff that I could not otherwise afford. Same for computers and networks. Same for carpentry, etc. Heck, if I had a Jacuzzi I'd damn well learn how it works, too. :rofl:
     
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  20. Matthews Guitars

    Matthews Guitars Well-Known Member

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    If you can trace voltages and signal paths, and have knowledge of where they should be and their levels, you're 3/4 of the way there.

    The biggest feather in my electronics troubleshooting cap is the one I got for repairing a 7 million dollar flight simulator for the US Marine Corps. Fault isolation troubleshooting and watching that circuit breaker was all it took to find the failed instrument in the cockpit, remove it, and replace it.

    In terms of system complexity and dollar value, it was my biggest troubleshooting job. In terms of difficulty, it was among the simpler ones. "What is making the circuit breaker pop?" "Use the manuals, determine what instruments are attached to that circuit breaker, systematically unplug those instruments one at a time until the fault is cleared. The last one you pulled out of circuit is the failed instrument."
     
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