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Why 4.7 Ohm?
Old 19th May 2006
  #1
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Cojo's Avatar
 

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
Old 19th May 2006
  #2


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





-tINY

Old 19th May 2006
  #3
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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.
Old 19th May 2006
  #4
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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... heh ...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
Old 19th May 2006
  #5
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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
Old 19th May 2006
  #6


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



-tINY

Old 20th May 2006
  #7
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Quote:
Originally Posted by tINY
There is another fault somewhere
Most likely!

/Cojo
Old 21st May 2006
  #8
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Tim Farrant's Avatar
 

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.
Old 21st May 2006
  #9
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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
Old 21st May 2006
  #10
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Tim Farrant's Avatar
 

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?
Old 21st May 2006
  #11
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Thanks!

I'll have a look tonight!

/Cojo
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