Troubleshooting amplifier

Discussion in 'General Electronics Chat' started by kaae, Oct 2, 2013.

  1. kaae

    Thread Starter New Member

    Oct 2, 2013
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    Hi there, i've got an amplifier for free (almost). It's an Mackie Tapco Juice j2500.
    the problem is that the amp is only playing on channel B, when powered up it also only blink in the led below that channel, same for the signal led which is also only flashing on the working channel.

    i've opened it up to discover that only Channel A got power, (did measure all the wires going from the power supply )
    other than that i noticed that one of the capasitors on the faulty channel might be leaking - picture?
    and also some of the resistors on the working channel gets really hot, like 80-100 celcius, in a matter of seconds. Marked with red ring on one of the pictures Below.

    can any of you come with a sugestion what might be faulty in the amplifier ?

    http://imageshack.us/photo/my-images/94/osrp.jpg/
    http://imageshack.us/photo/my-images/59/tk1u.jpg/
    http://imageshack.us/photo/my-images/703/x68f.jpg/
    http://imageshack.us/photo/my-images/10/9xi0.jpg/
    http://imageshack.us/photo/my-images/31/8x43.jpg/
     
    Last edited: Oct 2, 2013
  2. studiot

    AAC Fanatic!

    Nov 9, 2007
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    What do you mean it is only working on channel B and only channel A has power?

    I can't see a picture of the alleged faulty capacitor.

    There resistors noted as overheating are mounted well clear of the board to enhance airflow, which suggests they are expected to get hot. There are a suprising number of power resistors for a two channel amp, however.

    Do you have any circuits or manuals, perhaps inside the case?

    How well is the working channel working?
     
  3. shteii01

    AAC Fanatic!

    Feb 19, 2010
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  4. bountyhunter

    Well-Known Member

    Sep 7, 2009
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    As I often say, first thing I was taught is that when electronic equipment blows up:

    1) Always suspect the power supply first

    2) Always assume old electrolytic caps are bad

    People don't realize that semiconductor devices have typical life spans in the hundred thousands of hours and electrolytic capacitors have life spans of a few thousand hours.
     
  5. kaae

    Thread Starter New Member

    Oct 2, 2013
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    sorry for the mixup, only channel B is working and is actually working perfectly, loud and clear. same for the power, only channel B has got electricity and hot resistorer.
    The capacitor that i suspected for being faulty is the one to the right on the second last picture, it could look like it's leaking? for the power supply, i can't read any power going from the power supply and to the faulty channel, weirdly non of the wires has power, which is 6 wires, all completely dead.
    no circuit manuals sadly, i've been looking for one but haven't been lucky,
     
  6. RobeeJ

    New Member

    Mar 19, 2013
    24
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    It could have leaked, just make sure its not glue often added to these bigger caps! Though you can easily tell if its that usually, because all of them have the same gunk underneath. :) Doesn't look like glue to me, but photos can be deceptive.

    When you remove it, clean the mess up with some isopropyl alcohol.

    Does the PSU have separate rails for +/- for each channel? If not its a bit strange that one channel is getting power and the other not. If it does have four rails, you should be able to compare them.
     
  7. Alec_t

    AAC Fanatic!

    Sep 17, 2013
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    Check all the electrolytic caps. Replace any that have bulging ends.
     
  8. kaae

    Thread Starter New Member

    Oct 2, 2013
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    non of them have that, all i can see is this dark hard thing in the button, probably glue.
     
  9. kaae

    Thread Starter New Member

    Oct 2, 2013
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  10. Mike33

    AAC Fanatic!

    Feb 4, 2005
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    I would think you'd do well to find "which parts make up the working channel B", and then find the same for channel A. By some probing, you should be able to determine if you have power anywhere in channel A. The name of the game is to find out where the power stops, so to speak.

    Something as simple as an open or shorted cap could cause your problems.

    I mean, it's not clear whether any of your components in the A channel have ANY power....do any of the opamps and so on? Once you're sure it's totally dead, you can then state that you probably have a power supply problem.


    The other way to work this is to unplug the power supply from the boards (if you can, looks like it may be possible on this). Then try to measure your voltages there, see if any are just "bad". If something is blown on the board, it may pull down your voltage to that channel in service, but not show anything if you disconnect the PS from the boards...it is pretty important to have an idea of how power feeds to each channel, though, so you don't get confused!
     
  11. studiot

    AAC Fanatic!

    Nov 9, 2007
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    kaae, pictures are good, but they are unlikely to be enough.
    You need to tell us explicitly what resources you have to carry out this diagnosis and repair.

    Multimeter?
    Soldering skills?
    Use of multimeter skilss?
    Flying leads (+ croc clips)?
    More sophisticated equipment (signal tracer / scope etc + skills to use them)?
    etc
     
  12. kaae

    Thread Starter New Member

    Oct 2, 2013
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    i'll try and measure the psu once i get the time, i will let you know what i have measured.
    when it comes to using multimeter and soldering i'm quite good, and i might as well have some crocodile clips lying somewhere, also i got pretty much every tool needed.
    I haven't got a signal tracer, nor used one before, not that i sound too difficult though. but if needed i could get a signal scope at my work.
     
  13. kaae

    Thread Starter New Member

    Oct 2, 2013
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  14. kaae

    Thread Starter New Member

    Oct 2, 2013
    13
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    You cant really see it in this picture but behind some of the wires there are like 8 fuses or so, Half for each of the rails. All the fuses are blows on one of the sides. And if i Change Them theyll just Blow Again, but this only happens if the wires going from there and to the amplifier rail are connected, if i pull Them out the fuses Wont Blow. http://imageshack.us/photo/my-images/33/ab3x.jpg/
    Can be some of the transisters that has a short, or what Can make these fuses Blow, they are 230 v 5 a and massive sparks appear When they blow..
     
  15. tubeguy

    Well-Known Member

    Nov 3, 2012
    1,157
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    A common failure is shorted output transistors along with the associated driver transistors. Some of the associated emitter resistors may be open also.
    (The 8 whitish square resistors standing up and the 8 large transistors mounted to the large heat sink). If I counted correctly.
    There are likely some other smaller devices mounted on the heatsink which are driver transistors.

    With NO power applied, measure the resistances between the center and end legs on the transistors. Compare with the working channel.
     
  16. adamclark

    Member

    Oct 4, 2013
    472
    6
    you got a tapco amp for free??? you gotta fix that one man.. good find
     
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