"Haaky" wrote in message
oups.com...
On Feb 4, 10:40 am, "Arfa Daily" wrote:
"Eeyore" wrote in message

...



Haaky wrote:

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.

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

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

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