Damn MG...Help me fix it.

[roast]

So, was at practice yesterday with my band, and whaddya know, the MG cuts out. Its a marshall mg100dfx, and just wont produce any sound. Has anyone got an idea on how to fix it? I took out the chassis, had a look around, nothing seems to be wrong. Had a look at the speaker, nothin wrong there. Of course, i could have missed something.....im not a genious at electronics. Perhaps something happend the transformer? As it was in a cold room for quite a while?

The amp turns on, ie, the lights come on, the fan whirrs.....blah blah, just no sound comes out. Ocassionaly, i get a click or a buzz as i plug in a cable, but just for a split-second as its plugged in. Any ideas?

[teemuk]

Unless the amplifier shows signs of short circuiting, overheating etc. check if you can get sound out from it by plugging into the FX loop return and by plugging the FX loop send to headphones or something. In other words, try to locate the stage where the signal cuts out. I suspect you have a broken solder joint somewhere.

[roast]

Ok, ill try that. Wait, what do i plug into the FX loop? The guitar?
It dosent look like theres any solder problem, everything looks fine. Is there any chance its a prob with the transformer?

Ill try that with the effects loop anyway.
I plugged headphones into the headphone port, nothing. I plugged an mp3 player into the cd in, and nothing...

[teemuk]

What makes you think there's a problem with the transformer? Is it unusually hot or making a lot of noise or something? That could just indicate a short circuit somewhere. If you doubt the condition of the transformer just go ahead and measure it's rail voltages. No clue what they should be but +- 15V or similar for the preamp section and maybe +- 25 to 40 for power amp section seems like a resonable amount. If substantially lower it's most likely a sign of short as well. Only in the condition of no-voltage at all I'd suspect a tranformer failure. They are quite tough components.

The most usual cause of "no-sound and sudden crackles in the signal"-error is a broken solder joint; the difficulty is that they are tough to locate with bare eyes. You need a magnifier glass or something. You can try to locate it by "shaking" potentiometer knobs and jacks slightly. This will sometimes reveal if they are intermittent. A problem with solder joints is even more certain if you can not see any burned parts.

Anyway, you have to locate the problem in order to fix it. Since the CD input did not work search the problem following signal path upwards (from the CD input) all the way to the power amplifier stage. (Check those rail voltages first). If you can't scope then use a DMM to check the continuity of traces and condition of resistors.

I'm quite sure you can plug the guitar into the FX loop without harm but likely you will not hear it either since the fact that you can not hear the CD-input most likely indicates a broken connection between the preamp and the power amp. I'm too lazy to check whether your amp has a master volume potentiometer but if it does then it's one of the most likely places to have a problem. It's usually the single and only component linking signal paths of power and preamp together.

BTW, in case your amp is really messed up you should prevent the possibility of even further damage that might happen during testing the circuit. You have to limit the mains current. A light bulp in series with the mains cord is a good trick. If the bulp burns brightly there is a short somewhere; luckily the internal resistance of the lamp will limit the short current and likely save you from blowing parts. This cheap device is very easy to build out of scrap lamp holder + cord and mains socket. It has saved me from blowing plenty of transistors. In my opinion, it's as "must have" tool as a multimeter.

[roast]

I really dont understand most of what your saying.....sorry...

There isnt sudden crackles in the signal, theres just a crackle when its turned on and when a lead is inserted. Ive shaken all of the pots, it still dosent crackle.

Yeah, the amp has a master volume pot, if that helps..

[joecool85]

Could be a bad solder on one of the jacks, but I'm guessing more than likely its a bad solder elsewhere.  What I would do is go through and resolder all the pots and jacks, its easy to do and then you don't have to worry about that.

[teemuk]

Sorry if my post was too unintelligible. Fact is, if you want to know how to fix that amplifier then the first thing to do is to find out whats wrong with it - and while doing this things tend to get technical.  While people on this forum might give you help in the troubleshooting process, you are the only one who can freely work on, examine and measure the circuit. You are in the key position of troubleshooting that thing. To start from the basics in explaining what you should do would be an enormous process and primarly waste of time since you should learn the basics by yourself. Google is you friend. Here's, for example, two very good tutorials on troubleshooting electronic circuits:

A shorter one:

http://sound.westhost.com/troubleshooting.htm

And an extensive one:

http://www.repairfaq.org/sam/tshoot.htm

I hope I will not sound too offensive (believe me, I have no such intention) but if you just say that the problem is "no sound", it is not actually very helpful: Based on that symptom I can come up with a multitude of different scenarios that trouble your amp, broken solder joint being one of them. Other scenarios include a defective component on the signal path (open resistor or capacitor perhaps) or a defective component causing a short/open circuit somewhere in the circuit. Now, do you know how to search for problems like these? For example, do you know how to measure transistors or search the internet for transistor datasheets to find out their pin order? Do you possess such knowledge of circuit theory that you can use plain logic to narrow down your search for the problem source?

What I'm saying is, are you sure your skills are up to the task of trying to fix this amplifier? Can you "trace" out a schematic from the circuit board?Are you confident on desoldering/soldering PC boards? Do you know how to work safely with a mains powered circuit or measure a circuit while it's on? Do you have a clue what kind of voltage or current values you should be measuring from a certain part of the circuit?

No offense, but if you answered "no" to any of the above questions it might be better that you give the amp to someone who knows how to troubleshoot the problem and fix it. A "hack" repair can ruin the amplifier totally.

Edit: Joecool85 is right in the fact that you should resolder all jacks and potentiometers. In my opinion, you have a good chance of being lucky and fixing the problem by doing that. If you are confident enough with the soldering iron this is a fairly easy thing to do.

[joecool85]

While I agree with teemuk for the most part, I will say that all of that isn't necessary (for instance knowing how to measure a transistor), these things are important:

- You need to know how to be safe working with 110v AC
- You need to know how to measure resistance and voltage and own a DMM (digital multimeter)
- You need to know how to solder, and have the equipment (a basic soldering iron and solder is fine)
- You need a basic understanding of how electronics work

With these things you should have a good chance at fixing your amp, not being an amp tech.  If you can't fix your amp by resoldering some joints and checking that your voltages are good, odds are it will need to see a professional.