ive had a mg30cfx for a couple years now. the amp randomly cuts out every now and then. volume doesnt go over 3.... any ideas??
Regarding your MG30CFX, we would recommend that you have it looked at by a qualified engineer and without seeing the amplifier it is hard to diagnose.
I had a very similar problem, however, it's a little different. My volume knob works when it's under level 3, but would still cut out randomly regardless (unless I was jamming at full blast). I found a fix to my problem and wrote it on a separate forum here, but I'll copy and paste what I wrote there for your convenience. The original post was for the MG30DFX but I own the CFX like you. I assume your problem is with the volume connections. See steps 2 and 3 below and try them. Let me know if you have any questions. Best of luck.
I have had this same issue with my MG30CFX for years and I think I have fixed it (I assume these suggestions will also work for the MG30DFX series). However, only time will tell if it's actually fixed or if the Marshall gods are just messing with me. Here are various debugging steps I recommend doing - in order. I am a licensed mechanical engineer and have several years of experience with electronics and low power systems, but this was the first time I have ever tried to fix an amplifier (so please take these suggestions with a grain of salt). I'll try to be as in-depth as possible while also using a limited vocabulary so that the average joe can follow.
1. BEFORE YOU UNSCREW ANYTHING - Unplug the amp, then go and grab any cable with a 1/8" (male) audio jack end. It can be any type of 1/8" cable (3.5mm), i.e. an extension cable like, a 1/8" male to 1/4" female adapter, or even a pair of old earphones (which is what I used). Simply plug the 1/8" male end into the MP3 jack (not the headphone jack, the one to the left of it with the iPod-looking icon) and now test if the amp still cuts out. If it still cuts out, try a different 1/8" cable. If this doesn't work for you, go to step 2. If it worked for you and is no longer cutting out, well... congrats. This is the step that worked for me, but this problem is very general and could be a multitude of things. What I did specifically is I grabbed some old earphones and cut them near the audio jack connector and threw some electrical tape on the open copper (this is not a hazard because no current is going through the wires). I then noticed that if I wiggle the audio cable, it begins to cut in and out. So... read step 3 and watch the video on how to open the amp. The MP3 jack was loose, and I think the amp was cutting out because it keeps "recognizing" a signal in the MP3 jack (probably due to the odd wiring in series instead of in parallel). I basically tightened the jack and cleaned it (step 2) and it did the trick.
2. Leave the amp assembled. Go grab a can of electronic dust remover (i.e. Keyboard Cleaner) and stick the nozzle in the input jack (whichever duster you purchase, make sure it's 152 compressed gas, most keyboard cleaners will work). This will get rid of any dust/dirt that is inside the input jack. Try to tilt your amp forward while you do this so that no condensation can drip into the jack. Do the same thing for the headphone and MP3 jacks while you're at it. Next, simply grab some electronics contact cleaner spray (search Electronic Contact Cleaner WD40 on Amazon for the most reliable one) and spray some on the end of a 1/4" guitar cable. Stick it in the input jack and remove it several times, twisting it each time. This will lubricate and "polish" (for lack of a better word) the circuit board connectors that contact the guitar cable inside the jack. You can also do the same for both 1/8" jacks. *Note: this was the first step I did last month but it did not solve the problem (it still might work for you, however).
3. Make sure the amp is unplugged and has been unplugged for about 30 minutes so that the capacitors can fully discharge. Search for this video on YouTube uploaded by Tampatec: 10 WAYS HOW TO FIX A GUITAR AMP AUDIO NOISE, SOUND DROPOUTS, NO POWER (Sorry, I can't put links because I made this forum account like 30 minutes ago). The video is great and it describes most problems that could be wrong with the amp internally. Try all of these steps/suggestions in the video. I watched this video about a month ago and did everything he said. I found a couple of cold-soldered connections and re-soldered the connectors on the input jack and replaced several capacitors and resistors on the board - however, nothing happened for me and was ultimately a waste of time. If you don't know how to solder, then don't solder or remove ANY of the components... but you probably will anyways. If you're intent on soldering yourself but have no experience, I recommend practicing on an old VHS player or any large electronic device you don't use. Another thing you can do that the youtube video does not mention (I don't think) is to spray some of that keyboard cleaner from step 2 everywhere on the circuit board (but beware of any condensation that can drip from the nozzle). Next, take off the knobs and spray them conservatively with some of that electronic contact cleaner from step 2. You may do this for every knob but it's kind of time-consuming (this also did not solve my problem). If the MP3 jack is the culprit as it was for me, disassemble the jack from the front panel and adjust the wire/tighten any components around the jack. If you can solder, I recommend unsoldering the wires attached to it and resoldering it with 40/60 Tin/Lead solder, then clean the entire jack (inside and out) with the tools from part 2. This is what I ended up doing a few days ago and I think this was the main issue for me; I have not had any problems since then.
Another thing I did (a month ago - before figuring out the solution was to bypass the MP3 jack) was that I completely disabled the effects knob by literally cutting the wire that connects it to the circuit board. I DO NOT RECCOMEND THIS (unless nothing else works and you're experienced with soldering and routing). This was not the smartest thing on my part - I had to move two or three wires around and re-solder some connections for it to still work. You could technically unsolder the MP3 jack but I'm afraid if you do this, you won't be able to use the headphone jack because I suspect they are in series with each other (I'm not certain of this, but it would make a whole lot of sense if they were in fact routed in series). But don't do this, only do it if you're desperate and if the technician is charging a lot. I'm just letting you know that my problem was not due to the effects knob. Do steps 1 and 2 before anything else.
One last thing... I noticed that the gain really kind of sucks on this amp. On my VOX, the clean channel still plays when the gain is turned to zero. On the Marshall, zero gain means zero sound - even on the clean channel. There's no fix to this, but I noticed that after I sprayed the gain knob connections and the input jack with the WD40 Electronic cleaner, I got a MUCH cleaner sound with the gain turned (almost) all the way down. It is a bit louder as well which was nice. I also think I actually improved the sound quality of the amp by re-soldering the jacks, but maybe not. Let me know if any of this works for you. I think this is a common issue with Marshall combo amps, as magnificent as they are.