Why 4.7 Ohm?

Cojo · 19th May 2006

Hi and good mornig!

This winter I had a small DJ that went up in smoke as I manage to short the outputs. When I unscrewed the mixer I found two small resistors that had blown. They were 4.7 Ohm each!

My question (since I don't have any shematics) is why do they put such a small resistor in the path (remember that this is no high end product)? My only thought would be for "safety" if the current gets to high!? But I'm only grabbing in the dark here.

Also, would it be possible to just bypass the resistors since they are so small?

/Cojo

tINY · 19th May 2006

I wouldn't short them. They may be there to tie ground to saftey ground.

-tINY

FunkyOllie · 19th May 2006

Well, assuming we're talking about a small DJ mixer, not just a DJ of below average height, and assuming the resistors are in series with the signal ouputs then you are along the right lines. They are there to protect the output devices from blowing up when the outputs are accidentally shorted and the output device tries to deliver more current than it can. The thinking being that its better to blow up a couple of cheap resistors than a relatively expensive IC. Lets hope they worked!

I wouldn't just link them out. For the small trouble of replacing them you could end up saving a lot more time and money in the long run.

Cojo · 19th May 2006

Quote:

Originally Posted by tINY

I wouldn't short them. They may be there to tie ground to saftey ground.

Ok. I won't shorten them!

Quote:

Originally Posted by FunkyOllie

Well, assuming we're talking about a small DJ mixer, not just a DJ of below average height, and assuming the resistors are in series with the signal ouputs then you are along the right lines. They are there to protect the output devices from blowing up when the outputs are accidentally shorted and the output device tries to deliver more current than it can. The thinking being that its better to blow up a couple of cheap resistors than a relatively expensive IC. Lets hope they worked!

I wouldn't just link them out. For the small trouble of replacing them you could end up saving a lot more time and money in the long run.

Great! Thanks for that info!

And yes, it was not a small DJ...

...the "mixer" must have dissapear in the heat of typing!

I have new resistors that I'm going to replace today. Hopfully the only thing that happned to the mixer was the resistors burning. We'll see tonight!

/Cojo

Cojo · 19th May 2006

Ok, this is not looking good...

I've replaced the two resistors but they burned once again! Nothing connected to the mixer. Hmm, I'm lost here. What is the cause to this?

What usually brokes if you short an output? The strange thing is that when it broke, it still passed audio. Very distorted, without the lowest frequences but audio none the less!

Any more ideas, anyone?

/Cojo

tINY · 19th May 2006

There is another fault somewhere - you may want to have a tech look at it.

-tINY

Cojo · 20th May 2006

Quote:

Originally Posted by tINY

There is another fault somewhere

Most likely!

/Cojo

**Tim Farrant** · 21st May 2006

Are you sure they are in the output circuit? 4.7 ohms is very small, should be more like 47 ohms. And, if they were burnt, how did you find out the value? I would say they are more than likely part of a power supply line filter for the output chip. If they burnt again, then the IC is more than likely dead.

Cheers
Tim.

Cojo · 21st May 2006

Quote:

Originally Posted by Tim Farrant

Are you sure they are in the output circuit? 4.7 ohms is very small, should be more like 47 ohms. And, if they were burnt, how did you find out the value? I would say they are more than likely part of a power supply line filter for the output chip. If they burnt again, then the IC is more than likely dead.

Cheers
Tim.

Hi Tim, thanks for replying!

Well... I also thought it would be hard to see the rings (and it was) but when I desoldered them the value was printed on the board under the resistors. It was printed 4.7 so I assumed they must mean 4.7 ohm. I also think it is very small so I thought maby they mean 4.7K ohm but when I looked around I found that other resitors was clearly labeld 4.7K so I think they must really mean 4.7 ohm.

Any Idea what IC to search for? Would it be mounted next or close to the resistors? If so, then it would be rather easy to find.

/Cojo

**Tim Farrant** · 21st May 2006

Well you need to trace the PCB traces, see if they go to the power supply pins of the IC. Are they connected to electrolytic capcitors which then connect to ground? What is the output IC part number?

Cojo · 21st May 2006

Thanks!

I'll have a look tonight!

/Cojo

19th May 2006	#1
Cojo Lives for gear Joined: Dec 2005 Location: Gothenburg, Sweden! Posts: 1,471 Thread Starter	Why 4.7 Ohm? Hi and good mornig! This winter I had a small DJ that went up in smoke as I manage to short the outputs. When I unscrewed the mixer I found two small resistors that had blown. They were 4.7 Ohm each! My question (since I don't have any shematics) is why do they put such a small resistor in the path (remember that this is no high end product)? My only thought would be for "safety" if the current gets to high!? But I'm only grabbing in the dark here. Also, would it be possible to just bypass the resistors since they are so small? /Cojo __________________ Conny Johansson [ www.asteroidkiller.com ]

19th May 2006	#3
FunkyOllie Gear interested Joined: Oct 2005 Posts: 17	Well, assuming we're talking about a small DJ mixer, not just a DJ of below average height, and assuming the resistors are in series with the signal ouputs then you are along the right lines. They are there to protect the output devices from blowing up when the outputs are accidentally shorted and the output device tries to deliver more current than it can. The thinking being that its better to blow up a couple of cheap resistors than a relatively expensive IC. Lets hope they worked! I wouldn't just link them out. For the small trouble of replacing them you could end up saving a lot more time and money in the long run. __________________ Ollie Ex - Technical Engineer Funky Junk London

21st May 2006	#8
Tim Farrant Moderator Joined: Jun 2002 Location: New Zealand Posts: 2,410	Are you sure they are in the output circuit? 4.7 ohms is very small, should be more like 47 ohms. And, if they were burnt, how did you find out the value? I would say they are more than likely part of a power supply line filter for the output chip. If they burnt again, then the IC is more than likely dead. Cheers Tim. __________________ "Come on, make my day" Dirty Harry (Clint Eastwood) Visit Buzz Audio

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19th May 2006	#2
tINY Gear Guru Joined: Jul 2004 Location: Orygun Posts: 10,230	I wouldn't short them. They may be there to tie ground to saftey ground. -tINY

19th May 2006	#5
Cojo Lives for gear Joined: Dec 2005 Location: Gothenburg, Sweden! Posts: 1,471 Thread Starter	Ok, this is not looking good... I've replaced the two resistors but they burned once again! Nothing connected to the mixer. Hmm, I'm lost here. What is the cause to this? What usually brokes if you short an output? The strange thing is that when it broke, it still passed audio. Very distorted, without the lowest frequences but audio none the less! Any more ideas, anyone? /Cojo