electronics?

[antispatula]

I don't know anything about electronics. Bare with me.

So I'm trying to fix a guitar amp. It was only a fuse, but before figuring that out, I broke the on off switch and a capacitor that ran between the ac cords, hot and cold. So to complete the circuit and turn the amp on, all I have to do is connect the hot and cold wires? Thats what a switch does if I'm not crazy. So the cap that was between the two wires broke off, I got a new cap that was at 2.2 uf, while the old one said to get a replacement that had between .0043 and 2.2 uf, so I was kind of at the max. I solder on the new cap, and then I plug in the power, and it's on, because the cap was completing the circuit, unlike the old one. Does anyone get what I'm saying? Then I somehow got the old cap to solder back on, and it didn't automatically turn it on, like it should be. Sorry if this makes no sense. This MAY help....

 

[Ziller]

If you can find the schematic, that would tell you exactly what goes where. However, the power switch usually is inline with one of the incoming AC lines, not across the hot and cold. If you wire it between the hot and cold AC lines, and turn it on, it will short out the AC feed, and that's it for antispatula (or at the least it will trip the breaker and throw some sparks). The cap is wired between two AC lines to pass high freq to ground, so that's probably correct.

Usually it goes

AC 1 -------- Fuse -----Switch ------ Power trans
|
cap
|
AC2 -------------------------------- Power trans

Or something like that. Sometimes the fuse and switch are on difference AC lines, but both are inline and not across the AC lines.

What kind of amp is it?

 

[Henry Mars]

I don't know anything about electronics. Bare with me.

So I'm trying to fix a guitar amp. It was only a fuse, but before figuring that out, I broke the on off switch and a capacitor that ran between the ac cords, hot and cold. So to complete the circuit and turn the amp on, all I have to do is connect the hot and cold wires? Thats what a switch does if I'm not crazy. So the cap that was between the two wires broke off, I got a new cap that was at 2.2 uf, while the old one said to get a replacement that had between .0043 and 2.2 uf, so I was kind of at the max. I solder on the new cap, and then I plug in the power, and it's on, because the cap was completing the circuit, unlike the old one. Does anyone get what I'm saying? Then I somehow got the old cap to solder back on, and it didn't automatically turn it on, like it should be. Sorry if this makes no sense. This MAY help....

I have some advice. If you don't know what you are doing take it to someone who does. You could kill the amp and kill yourself in the process. Electricity kills when misunderstood.

 

[ggunn]

I don't know anything about electronics. Bare with me.

So I'm trying to fix a guitar amp. It was only a fuse, but before figuring that out, I broke the on off switch and a capacitor that ran between the ac cords, hot and cold. So to complete the circuit and turn the amp on, all I have to do is connect the hot and cold wires? Thats what a switch does if I'm not crazy. So the cap that was between the two wires broke off, I got a new cap that was at 2.2 uf, while the old one said to get a replacement that had between .0043 and 2.2 uf, so I was kind of at the max. I solder on the new cap, and then I plug in the power, and it's on, because the cap was completing the circuit, unlike the old one. Does anyone get what I'm saying? Then I somehow got the old cap to solder back on, and it didn't automatically turn it on, like it should be. Sorry if this makes no sense. This MAY help....

STOP! You have already made things worse by trying to tackle a job that is beyond your skills.

Take your amp to a qualified tech and stop trying to muck about with it yourself. There are LETHAL voltages on many nodes in an amplifier circuit; a brushing touch with a finger can land you in the emergency room or the morgue. This is not an exaggeration.

It is stupid to risk your life by trying to save a few bucks.

 

[gbdweller]

I usually do the tongue touch method to test a circuit...Just kidding. Get out of there before you fry yourself. Many amps carry lethal voltage for quite awhile even after being disconnected from the power source. Don't find out if yours does by becoming a human toaster.

 

[antispatula]

woah........woah.........That's scary. I could be dead right now.....

Sooooo what am I supposed to do if I want to learn electronics? I'm not doing it to save money, it's my friends amp, and I thought it sounded fun to try and fix it. In fact, all I need really is a new switch, I just really want to understand how and why, you know?

The amp I'm playing with is really cheap and SMALL, does that make any difference? It's like 15 watts.

Thanks a BUNCH for the warnings guys. Again, what do I do if I'm interested in electronics. Is having a job with electronics really that dangerous? That's kind of depressing, I'm pretty interested in it......

 

[Light]

First of all, as has already been said, take it to someone who knows what they are doing. There are parts in a guitar which can carry a leathal charge for YEARS after it has been unpluged. Just don't muck aroung with it, take it to a professional.

woah........woah.........That's scary. I could be dead right now.....

Sooooo what am I supposed to do if I want to learn electronics? I'm not doing it to save money, it's my friends amp, and I thought it sounded fun to try and fix it. In fact, all I need really is a new switch, I just really want to understand how and why, you know?

Buy some books. Start with, Electronics for Dummies (that is NOT a personal attack, it's a good book which will help you with some important basic concepts).

Then, after that, if you feel like you have something of a handle on the basics, you can start thinking about bigger and better projects. Tino Zottola's Vacuum Tube Guitar and Bass Aplifier Theory is a very good sorce of basic information on Tube electronics.

Most importantly, take information you read on the internet with a very LARGE grain of salt, at least things which seem to be opinion. You will hear a lot of talk about various caps and resistors "sounding better." Don't buy it. While everyone on the internet is talking about how their Orange Drop capacitors are so great, they don't notice that more than half of them turn out to be junk (microphonic, way outside of tolerance, leaky from the get go, or just falling apart).

Finally, if you really want to learn about electronics, find a teacher. You don't necissarily have to talk a class or anything (though it wouldn't be a bad idea), but even if you can just find a repair guy who will take you under his wing for a bit. Particularly with guys who are knowledgable about tubes, they tend to like to find people who are interested in learning about this stuff, because a lot of it is becoming a lost art.

The amp I'm playing with is really cheap and SMALL, does that make any difference? It's like 15 watts.

No, it makes no difference.

Thanks a BUNCH for the warnings guys. Again, what do I do if I'm interested in electronics. Is having a job with electronics really that dangerous? That's kind of depressing, I'm pretty interested in it......

Not once you know what you are doing, though I do have to say that I know very few professional electronics guys who have not had at least one serious shock in their lives. You have to learn good work habits. For instance, you need to get into the habit of always dischargeing the filter caps, or any time you are working inside a live amp, you only use one hand, and keep the other hand in your pocket. This means that if you get shocked, it is much less likely that the current is going to go through your heart, which is how electricity usually kills people. For a much similar reason, you should rest the wrist of the hand you ARE using on the chassis, if at all possible. Then, if you get shocked, the charge is probably going to go through your hand and back into the chassis, in which case your hand will go numb for a while, but you will be alive. (Don't do that though if their is any chance of a like high voltage wire touching the chassis, cause YEOUCH!)



Light

"Cowards can never be moral."
M.K. Gandhi

 

[Ziller]

and I thought it sounded fun to try and fix it

It is fun, and not that big a deal. Just take basic precautions and you'll be fine.

1. Make sure the amp is unplugged, and
2. Drain the high voltage caps

Cap (capacitors) are like batteries that can hold a charge, sometime very high voltage, for a long time. These should be drained before you stick your hands in there. Usually they are large cylinder shaped components with numbers like 20uF 450V.

All very good advice from Light, information is key. There are plenty of good books out there. But don't get scared off, it aint rocket surgery (unless, like me, you are rewiring an old ampeg rocket, then I guess it is rocket surgery! ha ha)

 

[mshilarious]

The amp I'm playing with is really cheap and SMALL, does that make any difference? It's like 15 watts.

What makes a difference is whether or not it's a tube amp. If it is, voltages inside could be 300V or so. A solid-state amp will usually not have more than 40V.

Either way, the biggest danger is from live power, so absolutely make sure that the amp is unplugged before you open it.

The next danger is charged capacitors. A modern amp design should have resistors that discharge them for you, but don't ever presume they are discharged.

For a solid state amp, get a 1K ohm 1W resistor. I hold the resistor in linesman's pliers, with the pliers holding the body of the resistor and NOT touching the leads, and the my fingers safely on the plastic handle. Touch one lead to the chassis, and one lead to the positive side of the capacitor. That is the side that the arrow on the cap points AWAY from, not towards like you might think. That will discharge a 2200uF cap in about 11 seconds, but give it 30 just in case If there are several big power supply caps, discharge each one separately, although they are likely wired in parallel so the first one you discharge might discharge all the rest. Don't assume that though.

A tube amp uses the same procedure, just with higher value and rating on the resistors, because the voltages are much higher. The tube circuits I build are only preamp stages, so I wing it with the 1K ohm, but really for an amp with power tubes I'd think two 10K ohm 1W resistors in series is better. That will discharge 3 100uF caps in 30 secs, so give it a minute, again discharge each separately just for insurance.

The final danger to a solid-state or hybrid amp is you! Discharge your own static electricity before touching any semiconductor components (opamps, transistors).

If you do not understand any aspect of what I just wrote, you are NOT ready to work on a tube amp. Period. Play around with low voltage circuits until this makes sense.

 

[Light]

Safty recomendations

All pretty good advice, but I would instead suggest soldering that resistor into a clip lead (or just hooking it up between two clip leads), and then clipping one end to the chassis and the other to one of the caps. And then, once you are done, MEASURE the voltage on the caps with a multimeter, just to be sure.

Also, electricity is going to make your muscles contract, so if the first time you touch something, even if you know it is discharged, use the back side of your finger. If you get a shock, your muscles contract and pull you away from the charge, instead of forcing you to grip the charge HARD.


Light

"Cowards can never be moral."
M.K. Gandhi


Light

"Cowards can never be moral."
M.K. Gandhi

 

[mshilarious]

All pretty good advice, but I would instead suggest soldering that resistor into a clip lead (or just hooking it up between two clip leads), and then clipping one end to the chassis and the other to one of the caps.

Oh there are definitely better looking tools than pliers! Maybe the best I heard about was somebody made a "resistor stick" out of insulating material such that one resistor lead stuck out, and the other was attached to a clip for the ground, with your hand safely on the insulating stick.

 

[robin watson]

I'm a little nervous about that diagram - it looks like that switch is wired as a direct short across the mains!

 

[A Reel Person]

!!!

Amatuer electronics repairs/mods can be an enlightening experience for the untrained novice!! Literally!!

 

[robin watson]

240v AC is particularly enlightening.

 

[antispatula]

woah this is an oldish thread.....I just read half the responses.

Woah, not knowing a thing about safety, I touched the leads to the capacitor I was soldering back on like 20 times, and it never did anything, even AFTER I had it soldered on and turned on and then off again.... It never did anything.

 

[crazydoc]

woah this is an oldish thread.....I just read half the responses.

Woah, not knowing a thing about safety, I touched the leads to the capacitor I was soldering back on like 20 times, and it never did anything, even AFTER I had it soldered on and turned on and then off again.... It never did anything.

You are lucky and ignorant. The 2uf cap across the mains voltage probably wouldn't carry enough of a charge to hurt you, for one thing. Secondly, when you turn off the amp the cap may discharge automatically. Thirdly, unless you grabbed both leads at the same time, or were barefooted on a damp concrete slab floor, you didn't complete the circuit through your body or to ground.

But, keep fooling around with this stuff. You will learn a lot, especially if you get a good jolt or two.

 

[Light]

But, keep fooling around with this stuff. You will learn a lot, especially if you get a good jolt or two.

Or you'll be dead. That can happen too.


It really all depends on the cap you were messing with. All things considered, it is pretty clear the one you were messing with was not a filter cap. If it had been, you would have known, and FAST.


Light

"Cowards can never be moral."
M.K. Gandhi

 

[crazydoc]

Or you'll be dead. That can happen too.

Well, yeah. Then he may learn if there is a god or afterlife (or not)

But seriously, if you wear rubber soled shoes, and keep one hand in your pocket when you probe around in a circuit, you may ruin the equipment but you won't get killed. I have what's left of a screwdriver I was using to tighten down the breaker contacts in a 240V 30A circuit when it slipped. The explosion scared the bejesus out of me, but I didn't get hurt because of safety precautions.

 

[antispatula]

well, either way I threw that sucker in the garbage weeks ago, I understand it's dangerous.......So are electricians, technicians, and electrical enginneers all at grave risk becasause of their jobs?

 

[antispatula]

Or you'll be dead. That can happen too.


It really all depends on the cap you were messing with. All things considered, it is pretty clear the one you were messing with was not a filter cap. If it had been, you would have known, and FAST.


Light

"Cowards can never be moral."
M.K. Gandhi

well, it's not as if I know what I'm talking about, but the one I messed with was the one that connected the two wires that came from the ac cord......