Okay, I'm just going to start off by being totally honest and be at your mercy here. I am not an electronics guy, though I would very much like to be. I am fascinated and dumbfounded when I attempt to learn about this stuff.
I am, however, a musician; a musician with an electronics problem. I have a Marshall G50R-CD solid state guitar amplifier that started acting up about a couple months ago. The signal was phasing in and out and I was losing most of my volume. But it would only do this when the volume was turned up around 4 or so... I opened it up and found a component that was loose. The solder joint had come apart, so I re-soldered it and it worked as good as new... for a while anyway. A couple weeks ago it started doing it again, only this time the signal wasn't just fading, but cutting out completely. I opened it up again to find a little wiggle in my solder job, so I re-re-soldered it and put it back together again. This time when I hooked it back up it sounded perfect on the distortion channel but was scratchy or staticy when I would strike a string on the clean channel. (I'm assuming it actually does this on the distortion channel as well but the distortion hides the static.?) I don't know what the part is that keeps coming loose, but it's a white rectangular component about 1/2" tall and wide by about 3/4" long near the end of the circuit very close to where the wires go out of the main speakers. There are two other identical to it right by it and surrounded by a couple of large cylindrical capacitors. I hope you guys can help me. Everyone else keeps telling me "take it to an amp tech," but they usually like to work for money and I don't have any of that lately. I sure hope you guys can help me.

 

Post Photos.

 

Yes, you should post photos.
and for future reference, amp usually doesn't mean amplifier on an electronics forum

 

magnet18- Right... I did mean a power amplifier... not amperage. Sorry, I'm a musician so I tend to use the musician's vernacular. I'll try remember where I am from now on.
I will try to get some pics on here ASAP.
thanks

 

I hope these pics help. I'm not sure what you're looking for but the component that keeps coming loose is one of the white rectangular ones; the one closest to you in the second 2 pictures. there is a group of 3 right there and they are marked:
5W3309(ohm)J
D A E DO

anything else you need just let me know. I also have a small cheap multimeter (the kind with the analogue needle.)

And I do thank you for your quick responses so far.

 

Ouch! Those pix are hurtful on the old eyes. Despite the fact that they're too blurry to read the markings, they look like power resistors.

 

Those are POWER resistors and probably get very warm when you crank it up. Lots of power to dissipate. Try this. Fill the area beneath the resistor with a 'sandwich' of small chips and pieces of a similar circuit board. I'm sure you can find a trashed electrical appliance of some sort with a circuit board. You don't need to completely fill it. The idea is to have support. Then use some regular Elmers glue to fix them in place. Cover the edges of the resistor as well. Let it DRY for at least 24 hours. The resistors get very warm and this in combination with the vibrations of loud music, works its destructive macic on the solder joint. Hopefully the 'bracing' will prevent the resistor from vibrating loose again.

 

The easiest tests you can make are the 'tap, wiggle, and poke' tests. This done with the power on using a none conductive object, like a tooth brush handle.
We also need better photos and a shot of bottom of the board.

 

as far as pics, I'm afraid that's the best I can do. I was using my camera phone because that's the only digital camera I have. (Well, I do have a polaroid digital camera that I bought when the Flintstones were having a garage sale. Wilma said Fred didn't like it anymore cuz it was too old.)
and as far as the tap, wiggle and poke test... it seems solid to me.

 

....and as far as the tap, wiggle and poke test... it seems solid to me.

Are you sure you understood what that is? It has little to do with everything looking solid.

 

Are you sure you understood what that is? It has little to do with everything looking solid.

I'm really not "sure" about anything but I did just hook it up to see if I could get a better description of what it's doing and lo and behold.. it works perfect again... I cranked it up so loud that my 2 year old daughter went running and screaming out of the room. (sorry, baby doll) I was wondered if maybe putting a piece of electrical tape on it to hold it down might help stabilize it incase the solder broke again last night and that was when it was staticky. I took it off to take the pictures and plugged it in after that and sounds fine now. could the tape have possibly had anything to do with it? Wow, you guys are probably laughing so hard at me right now. But, hey, I'd rather ask the stupid questions trying to learn something than try to act like I know it all and really screw something up.

 

The 'tap, wiggle, and poke' tests are tests that professionals use to find an intermittent connection. It typically done with the power on so the tech can hear any crackling from any components making and breaking connection. Keep in mind that a component doesn't necessarily have to appear loose on the PCB to have an intermittent connection. Solder joints can develop 'halo rings' that most times can't be seen with the naked eye.

 

Perhaps a little soldering tutorial will also help: http://www.youtube.com/watch?v=I_NU2ruzyc4.

 

... I was wondered if maybe putting a piece of electrical tape on it to hold it down might help stabilize it ...

In case you were ignoring it. I suggested a GOOD method of bracing the component that can be accomplished by anyone with access to glue.

Give it a try. I know from wherefore I speak. I worked as a 'soundman' for a local area band for several years and have some experience making repairs with no budget for parts.

 

I know this is an old thread, well two days old but I just saw it. I'll throw my two cent worth in. Let me echo CDRIVE. Show the bottom of th board. I spent hours on an intermittent motorboating audio amplifier once years ago. I found the problem was fine coal dust on the bottom of the board. Now, I know you are going to say, "Coal dust, I ain't got no coal dust." What I am getting at is you soldered the resistor, twice, or was it three times? Did you use flux? Did you clean it real good after soldering? It can make a big difference just how you soldered the resistor. Also Kermits's suggestion is excellent. Give those big boys some support.
Roland