#1
well, my nephew has a little starter amp (PG-10) and guitar and his amp quit working, so I figured I'd see what I could do. I've built a couple of byoc pedal kits but I don't really know much about amps.

when turned on the amp will make the normal hissing noise of an amp that is on but with nothing plugged in, there's no kind of signal from the guitar. I tried it with my guitar first just to make sure it wasn't his guitar that messed up.

anyway I've got it opened up and I don't really see anything that looks burnt out, though there are a couple of solder joints that look pretty nasty. if I'd made a solder joint like that I'd feel like a noob (well I am, but not at soldering) and probably redo it immediately. also, the input and headphone jacks are like a closed up plastic kind so I can't really tell if there's a problem there or not. (I"ve never seen one like that)

and the whole inside is very dirty and grimy


anyways, what should I look for? what would the first thing for me to try?
#2
You could try fixing the solder joints first. When it's turned on and making the hissing noise, what kind of effects do each of the knobs have on the hiss? I don't have any particularly helpful or specific advice yet, but obviously you know to stay away from the capacitors unless you can safely discharge them, and super obviously, only work on it when it's not got any power going to it.
#3
Quote by Les Paul Ell
You could try fixing the solder joints first. When it's turned on and making the hissing noise, what kind of effects do each of the knobs have on the hiss? I don't have any particularly helpful or specific advice yet, but obviously you know to stay away from the capacitors unless you can safely discharge them, and super obviously, only work on it when it's not got any power going to it.


I guess I'll try fixing the bad looking solder joints. the volume and gain knobs make the hiss louder or softer, as expected. I didn't mess with the EQ knobs. and yea, I know to stay away from the capacitors, and the transformer scares me so I haven't touched it either lol.

even though I probably won't be messing with them, how do you safely drain capacitors? I hear people saying not to mess with them if you don't know how, but nobody ever says how to do it.

Edit: apparently you just short the positive to ground? that's simpler than I expected. do you have to drain things like film capacitors? I thought they didn't have a positive or negative.
Last edited by The4thHorsemen at Nov 20, 2009,
#4
No, don't simply short the positive to ground as there will be a massive spark as soon as you do. Instead, there's such a way to use a huge resistor (as in wattage wise huge) and use that between the positive and ground. This makes it slowly discharge. The exact value and wattage of the resistor needed I don't know though, and I'm pretty certain that it depends on the capicitor you're draining. Just keep looking on google. If you don't find anything that you can trust then maybe just don't fix the amp.

About a year ago I ripped apart a beginner amp for parts. I got loads of parts, but looking back I'm shocked (pun...) that I didn't kill myself. I didn't discharge the capcitors at all or anything, I think I was just lucky with where I put my fingers.

If the amp isn't worth too much, which I'm guessing it's not as it's a starter amp, maybe just treat it as dead and get your nephew to buy a new one/help him buy a new one.
#5
^if it's not a tube amp, then the caps drain as soon as you shut the power off. Transistors keep conducting after current is cut, unlike tubes.
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#6
Your jack probably looks a bit like this:



It's important to get a schematic, or at least identify which pin of the input jack is the "hot". Very often enclosed jacks will fail internally. Their contacts can become damaged. You can't see if the tip of the plug is making contact and it is being connected to the input of the amplifier's circuitry. But you can measure the continuity with an ohmmeter, if you know where it's supposed to connect.

If you plug in a cable and get good continuity from the tip of the loose end of the cable to the correct pin on the board, you can dismiss it as a possible problem. If you can't, you've identified the problem and need go no farther. Replace the jack.
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#7
hey, I went ahead and reflowed all the solder joints cuz I wanted to be sure it wasn't a cold solder joint and it worked but was kinda crackly when I wiggled the cord, so I stuck a screwdriver in there and kinda scratched around in there and tried again and it worked.


anyways, about the cap draining - yea I saw where they were talking about either shorting it to ground or doing so with a resistor to keep it from sparking. but seriously, what's wrong with causing a spark? it's not gonna hurt anything unless you do it right next to something flammable.
#8
I don't think it's really a spark so much as it is a mini lightning bolt.
Current Gear:
LTD MH-400 with Gotoh GE1996T (EMG 85/60)
PRS SE Custom 24 (Suhr SSH+/SSV)
Ibanez RG3120 Prestige (Dimarzio Titans)
Squier Vintage Modified 70s Jazz V
Audient iD22 interface
Peavey Revalver 4, UAD Friedman BE100/DS40
Adam S3A monitors
Quote by Anonden
You CAN play anything with anything....but some guitars sound right for some things, and not for others. Single coils sound retarded for metal, though those who are apeshit about harpsichord probably beg to differ.
#9
you'll eventually destroy the plates in the cap and short it, is why. have you ever seen a cap loaded with 200v discharged by shorting it with a screwdriver? It's pretty scary. It makes a huge spark and usually welds the screwdriver to the terminals, and it's loud as hell.

As said, SS circuits are perfectly save. Not only is the supply voltage a fraction of tube circuits (usually 20 or 30 volts for amps), but SS devices continue to conduct. Tubes stop conducting when the cathodes cool down.

i'm pretty sure the jack is the problem. i've seen those things fail on countless occasions.