Power Amp Repair

I need some advice guys.

I have an American Audio GX-300 Power Amp i use in my studio. Bought it used off of EBay. Been working like a charm for almost a year. One day while tunning up a guitar the amp shut off. Found it blew a fuse. I replaced the fuse switched it back on and it blew again.

Finially had to take it in . I was told that an Output had gone. Cost me 140 bucks Canadian to replare it.

I get it back a week later. Take it home, plug it in. And bang the fuse blows again right away. I return the amp to the repair shop. 2 weeks later i get it back. They told me the Output the guy replaced was fautly and get this he wanted more money to fix what was suppose to be fixed properly in the first place.

Any i get the amp back, worked fine for a week. Then 1 day i turn it on and it pops the fuse again. I replaced the fuse and it worked fine. A week later turn it on again and it popped the fuse again.

I'm not taking it back the the these guys again. I'de rather try and diagnose the problem myself and fix it.

Any ideas to where and what i should be looking for and were to start looking for the problem?

I've been doing alot of research and am comfortable with electronics to try and narrow down the fault and fix it.

Any help would be greatly appreciated. If you need any more ifo i can provide it. I also have the schematics for the amp aswell. That was a bonus courtasy of American Audio Inc.

Reply to
Haaky
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You could check for cracked solder joints. Audio amps though, particularly the higher power variety, can become real money pits for the inexperienced to work on. Output transistors are often expensive, and they'll pop in the blink of an eye as you've found, if any fault remains when you switch it on.

Reply to
James Sweet

Post the schematic on abse.

Reply to
Homer J Simpson

I'll definitely check for any cracked soldier joints. I figure i paid

175 for the amp. I just put another 140 into fixing it that's probably all i'm gonna put into it.

I'll probably end up buying a new Power Amp, Keep this one and use it as a backup or use it to experiment with.

I'm wondering if it's something in the power supply?. It's seems like it's getting a surge of power when i turn it on. Not all the time just on occasion. I changed speaker wires and changed out the cables going from the mixer into the amp. Just incase there was a short somewhere.

All i get from the repair shop is "maybe you have a short in the speaker wire" or "try plugging it into another outlet"

I'm not impressed with the service of the repair shop. I mean if the guy new what he was doing. I would think he would have tried to find what caused the output to blow. And not just replace the output. There has to be something else wrong somewhere in there.

And it's not like i was in a hurry to get the amp back. I was willing to let them keep it for as long as they needed. I guess there to busy to do a good job:)

Reply to
Haaky

What's abse??

Reply to
Haaky

alt.binaries.schematic.electronics

Reply to
tnom

I would probably start by troubleshooting the things external to the amp given the description you gave. Things like frayed speaker wires, damaged speaker coils, overdriven inputs, etc all could be causing the repeat failures of the amp.

Reply to
dkuhajda

Correction : electronic

Reply to
tnom

Checked and replace the speaker wires. Had the wedge monitors and speaker wires hooked up to another amp with no problems.

Damaged speaker coils? Didn't check them. How would i check them?

Overdriven inputs?. Was thinking about that. The only inputs going to the amp are from the mixing board.

Reply to
Haaky

Did the repair shop fit the correct replacement part ? Did they re-adjust the bias current ? There are simply so many possibilities.

The fact that it's 'gone' several times suggests some other underlying problem though as others have mentioned.

Does it run hot when idling for example ? When it originally popped was it running hot ? Excessive heat is death for output devices.

Graham

Reply to
Eeyore

Pretty unlikely.

You mean it sometimes goes HUMMmmmmm...... at switch one ?

That's quite normal especially with toroidal transformers.

Graham

Reply to
Eeyore

alt.binaries.schematics.electronics where you can post 'binary files' - attachments in other words.

Google groups don't have binary groups though.

Cogeco appears to have news.cogeco.ca though - that's a 'proper' news server.

Graham

Reply to
Eeyore

You'll tend to know if they're damaged !

Was it running loud ? Decent pro gear is protected against overheating etc but consumer ( and 'prosumer' ) products usually aren't.

From the appearance of it and the price I'd guess it's an 'also-ran' Chinese amp and I wouldn't expect it to be that durable.

OK, American Audio is a DJ brand. DJ brands are normally total crap.

A Behringer EP1500 would be a far better choice btw.

Graham

Reply to
Eeyore

The first time they fixed it, apperently they didn't use the correct part cause it did the same thing as soon as i got it home. The second time they fixed it, it appears to be the proper part, number match etc. Have no idea if they re-adjusted the bias current or not. I would think they should. But i would bet they didn't

It doesn't run hot now. But when it blew the first time. It seemed to be to hot, actually very hot

Maybe it's my imagination but it doesn't seem to be as loud as it was before. Maybe i'm just used to the other amps now. I was using my Marshal and Fender amps while the power am was in the shop.

Reply to
Haaky

So far it's done it twice since i got it back. As soon as i hit the power switch the fuse in the back popped. When i replaced the fuse and switched it back on it worked fine. Seems to be an intermitten thing, i hope. I don't mind that so much if it only happens on occassion, i'm just affraid that eventually something else will go wrong with it. And i know it will happen when the warrenty they gave runs out.

Reply to
Haaky

Cut your losses, buy a real amp.

If its a POS before it was fixed, its still a POS amp. The only way i fix a fry out is to replace ALL the output devices. Not just the shorted ones. Yes, it cost more. But there is no way to guarantee the other devices where not damaged and will soon fail.

Bob

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Reply to
Bob Urz

amp

That's the funny part. It wasn't running loud at all when it went. Had it turned almost off while one of the guitar players was tuning up. This amp is suppose to have overheating and surge protection circuits build into it. I'm just wondering if it could be that. Behringer EP1500 is pretty good eh?. I'll have to look into it. I was looking at the Crown brands i think the 402 or the 602

It's definitely gonna be replaced with a new unit hopefully very soon. This little amp has given it's all for me. I'm not about to chuck it:) It's actually a pretty good sounding amp. Problem is once something goes on it, you can be that that's just the start. It's usually followed by other thing going too.

Reply to
Haaky

Just a couple of 'simples'. Are you fitting a fuse that is the correct

*type* as well as rating ? Most gear of this kind will require " T " rated types, otherwise unexplained intermittent failure at switch on is the symptom you will get. Also, how is the fuse blowing ? Vapourised and blackened, blob in the middle of melted wire ends, or wire just broken ? This will give you a big clue as to what is causing the failure. What is the purpose of the fuse. Is it the line power fuse ? Are there other fuses either on the secondary windings of the tranny, or downstream of the bridge ? Also, Have you considered calling the manufacturers ? Often, they can be a lot more helpful than you might imagine. If you are not an electronics repair shop, just wing it, and tell them that you are. Sound confident and just say " This is Bloggs Disco and HiFi Repairs in Vancouver. I'm looking for some technical assistance with an XYZ 400. Can I speak with someone in your service department please ? " Worth a try, and at least you will get a proper opinion. It might be something as simple as the bias needing setting, as someone else in the thread suggested.

Arfa

Reply to
Arfa Daily

That's exactly what i would have done if i would have fixed it myself. Everything that i read said that your better off replacing all the outputs Cause whos to say one of the other outputs might not be a little flaky. And if it goes, they charge me to replace it because it wasn't work done already by the repair shop. That's a scam. I would have rather paid a extra and had them replace them all. But when it went the second time it was the same output that they had replaced.

The amp is gonna be replaced with a new unit as soon as i can decide on what to buy. Maybe i'll just use this one for the vocals

Reply to
Haaky

When it went the first time. The fuses where black, really black. I replaced the fuse with the exact same fuse that came out of it. But i have no idea if that first fuse was even the proper one. The guy i bought it from might not have replaced it with the proper type of fuse. On the back of the unit it says the fuse type is 250v T 7A The fuse that was in it was a 250v don't know if it was a "T" or not

6.3A

When the fuse blows now. It's not blackened it just looks like the metal thread is snapped.

I'm guessing it's the line fuse. There's only 1 there and it's connected to the power switch and AC cord. In series i guess.

I emailed American Audio, explained what was going on. They just send me the schematics of the unit. I think that all i asked for.

I figured posting here is a good idea. Get some fresh perspective and some things i would never have thought of. I noticed when i got the amp back from the repair shop. He marked on the heat sink the letters C A A C over top of each pair of i'm guessing that these are the outputs?

When i got the amp back after the first repair. The first set of "A" where different color than the rest and different than what was in there before it went in. When i got it back the second time those different color ones were gone and the ones that are in there look like the rest.

I see there are markings on them 2 are C2837 then there are 2 A1186 then 2 more A1186 and then 2 more C2837 screwed onto the heat sink.

It's actually very educational researching and trying to diagnose the problem. And i'm not affraid to get in there and get my hands dirty and doing the repairs myself.

Reply to
Haaky

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