Power Amp Repair

[Jamie]

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.

sounds like a bad bridge or caps.

 

[Jamie]

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.

are you using a Slow blow fuse?

 

[Jamie]

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.

T = Time Delayed.
you need a slow blow.

 

[Eeyore]

That's the funny part. It wasn't running loud at all when it went.

That points more to a possible design fault like ultrasonic oscillation.

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.

The Behringers are reliable and pretty much unbeatable value. They are based on the
QSC RMX models btw and are very similar.

Graham

 

[Eeyore]

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.

I suspect I know what the problem is. I'll bet it does use a toroidal transformer
and the cheapskates haven't bothered fitting a surge-guard device.

This all points to poor/skimpy design I'm afraid.

Graham

 

[Bob]

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.

Let me suggest that perhaps the idling current is set too high. Then
the output transistors start to warm up and after a while there is
thermal runaway and the fuse pops.

Check the idling current and adjust it if needed. Another possibility
is that the temperature sensor, usually a transistor or diode or
thermistor, is not properly attached to the heat sink and thus doesn't
react to warming output transistors.

Bob

 

[Haaky]

Let me suggest that perhaps the idling current is set too high. Then
the output transistors start to warm up and after a while there is
thermal runaway and the fuse pops.

Check the idling current and adjust it if needed. Another possibility
is that the temperature sensor, usually a transistor or diode or
thermistor, is not properly attached to the heat sink and thus doesn't
react to warming output transistors.

Bob

Would that be the cause of the fuse blowing at Power On though? That
seems to be the problem as of now. As soon as the unit recievies that
first initial
surge of power the fuse pops.
I'm keeping track of all the suggestions if i would have had this info
at the start i would have by-passed the repair shop and did this on my
own.
I'm a newbie at this but i want to learn as much as i can. I find it
really facinating. And if i can learn to fix stuff like this on my own
even better

 

[Meat Plow]

Would that be the cause of the fuse blowing at Power On though? That
seems to be the problem as of now. As soon as the unit recievies that
first initial
surge of power the fuse pops.
I'm keeping track of all the suggestions if i would have had this info
at the start i would have by-passed the repair shop and did this on my
own.
I'm a newbie at this but i want to learn as much as i can. I find it
really facinating. And if i can learn to fix stuff like this on my own
even better

Did you say if you were using a slo-blo fuse or not?

 

[Haaky]

Did you say if you were using a slo-blo fuse or not?

No i believe it a "T" timed fused i think that's what there called?

 

[DaveM]

Would that be the cause of the fuse blowing at Power On though? That
seems to be the problem as of now. As soon as the unit recievies that
first initial
surge of power the fuse pops.
I'm keeping track of all the suggestions if i would have had this info
at the start i would have by-passed the repair shop and did this on my
own.
I'm a newbie at this but i want to learn as much as i can. I find it
really facinating. And if i can learn to fix stuff like this on my own
even better

In one of the earlier posts, you stated "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."

If the original fuse was rated for 7A, and it's being replaced by a 6.3A fuse,
that could be your problem. I suggest that you try the correct fuse before
replacing any more components. If no more fuse blowing, then you might be good
to go. If the correct fuse blows, I would suggest that you replace all of the
output transistors at the same time. They are all available from MCM
Electronics.

http://www.mcminone.com/product.asp?catalog_name=MCMProducts&product_id=2SA1186
and
http://www.mcminone.com/product.asp?catalog_name=MCMProducts&product_id=2SC2837

If you can scan the schematic and post it on abse, we can probably come up with
some pretty good suggestions for you to go with. Does the schematic that you
have give the instructions for setting bias for the output stages?

--
Dave M
MasonDG44 at comcast dot net (Just substitute the appropriate characters in the
address)

Some days you're the dog, some days the hydrant.

 

[Arfa Daily]

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.

OK. That's definitely a slow blo fuse that it needs, and if it is on the
back, then it is almost certainly the line power fuse. If there are no signs
of distress inside the fuse, just a broken wire with no signs of melting,
then this would normally indicate that it is breaking just because of metal
fatigue, which is exactly the symptom you get when a fast blo is wrongly
fitted in this position. A big tranny is almost a short at power up for a
few cycles, until the core gets magnetically 'charged'. If you have a
straight fuse in there, you can watch the wire sag until it almost touches
the glass at power up. A " T " rated fuse is designed either not to do that
( spring-loaded types ) or to withstand the effect ( spiral types ). So for
a start, I would make sure that the next fuse you fit, is definitely a " T
".

It is common practice in a repair shop to scribble " C " and " A " on the
heatsink over the transistors, just to make sure that when you have them all
out, the new ones go back in in the correct places. The transistors are
standard Japanese types and are actually 2SCxxx and 2SAxxx. They always
leave the " 2 " off the on-device print. The " A " is a PNP type, and the
" C " is NPN, so it's very important that they go back in right.

If the company were happy to send you service info just as a result of an
e-mail enquiry, then in my experience, they would probably talk to you on
the phone.

Arfa

 

[Haaky]

In one of the earlier posts, you stated "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."

If the original fuse was rated for 7A, and it's being replaced by a 6.3A fuse,
that could be your problem. I suggest that you try the correct fuse before
replacing any more components. If no more fuse blowing, then you might be good
to go. If the correct fuse blows, I would suggest that you replace all of the
output transistors at the same time. They are all available from MCM
Electronics.

http://www.mcminone.com/product.asp?catalog_name=MCMProducts&produc...
andhttp://www.mcminone.com/product.asp?catalog%5Fname=MCMProducts&produc...

If you can scan the schematic and post it on abse, we can probably come up with
some pretty good suggestions for you to go with. Does the schematic that you
have give the instructions for setting bias for the output stages?

--
Dave M
MasonDG44 at comcast dot net (Just substitute the appropriate characters in the
address)

Some days you're the dog, some days the hydrant.

I was affraid to put in a 7A fuse even though that what is on the back
of the unit. Ever since i've owned the amp it has had a 6.3 fuse it.
I might try a 7 if it keeps popping. Thanks for the link to the
websites. I might order me some just to have as spares

I'll post the schematics of the amp

 

[Eeyore]

sounds like a bad bridge or caps.

No it sounds like magnetising current actually. Toroids are notorious for it
because of the low reluctance of the magnetic circuit.

Graham

 

[Eeyore]

T = Time Delayed.
you need a slow blow.

Actually he needs a T type to IEC specs.

Slow-Blow is unique to the USA and has different characteristics. In fact all US
fuses are different to IEC ones.

Graham

 

[Eeyore]

No i believe it a "T" timed fused i think that's what there called?

That is the correct type to use.

Graham

 

[Eeyore]

If the original fuse was rated for 7A, and it's being replaced by a 6.3A fuse,
that could be your problem

Pretty unlikely actually. 7A is not a preferred value so it'll be hard to come by. You
could always try 8A though to see if it makes a difference.

Graham

 

[Mark D. Zacharias]

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.

They may have missed a failing bias diode or another bias related part may
be failing. Doesn't sound like they have much experience with these types of
intermittent recurring failures.

Mark Z.

 

[Haaky]

They may have missed a failing bias diode or another bias related part may
be failing. Doesn't sound like they have much experience with these types of
intermittent recurring failures.

Mark Z.

My confidence in this repair shop went down the tubes. I thought they
would be good. They have a good rep, are always busy, very helpfull
Supposedly the repair guy has 30 years experience. But i wonder if
it's in electronic repair?

 

[Haaky]

Pretty unlikely actually. 7A is not a preferred value so it'll be hard to come by. You
could always try 8A though to see if it makes a difference.

Graham

Wouldn't i be running a bigger risk putting in an 8A fuse instead of a
7 or a 6.3?

I'de rather the fuse blow instead of a more expensive part.

 

[GregS]

My confidence in this repair shop went down the tubes. I thought they
would be good. They have a good rep, are always busy, very helpfull
Supposedly the repair guy has 30 years experience. But i wonder if
it's in electronic repair?

I have built things that have had seemingly intermitant problems. A
transformer will pull different currents depending on time
of the AC waveform. You can probably figure what percentage the waveform
will be max, and most of the time it will not blow. Then you hit it and it
blows. Also tends to give off loud noises. This might be part of the problem.
For the price of that unit, a good shop could not afford to play with throw
away equipment, or, your not going to like the bill a good shop will charge.

greg