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Why did my PA fuse blow?

Discussion in 'General Electronics Discussion' started by dietermoreno, Apr 19, 2013.

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  1. dietermoreno

    dietermoreno

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    Dec 30, 2012
    Why did my PA fuse blow?

    Note: this thread has NOTHING to do with radios or radio building, and if I start going off topic about that please lock the thread and ban me again.


    Okay, so I have a PA amplified speaker that my friends were using to play music from my laptop while singing over it.

    Unfortunately, the microphone was having feedback at high volume of the music from the laptop.

    The music from the laptop connects to the PA with an RCA red and white unbalanced and unshielded stereo audio line.

    The mic connects to the PA with an XLR balanced mic line for use with a Shure SM-48 caridoid dynamic mic.

    The PA is 500 watts. The PA has 3 channels (mono unbalanced audio line ins) and one RCA red and white stereo audio line in. The third channel is shared by the RCA line in.

    The mic is connected to channel 1. The laptop audio interface is connect to RCA line in, which is channel 3.

    The vocalist is facing the opposite way that the speaker cone is facing. The speaker is about 15 feet away from the vocalist. The mic pickup is really good and his voice can be picked up by the mic even when he is holding the mic 2 feet away from his mouth (this might be a bad thing for where feedback is concerned).




    When we turned up the volume on everything up to 80%, the fuse on the PA blew.

    The fuse was 3.15 amps.

    I bought a replacement 3 amp fuse from the local hardware store and it blew as soon as I plugged in the PA into mains outlet even with the gain on all channels turned to zero!




    So I think that 3 amps is no good and I need exactly 3.15 amps or it won't work. So I think I will need to order the exact current amount (3.15 amps) fuse on Amazon or else it won't work.

    So I didn't order the fuse yet on Amazon but I will soon. I actually think I'll wait a few days for advice from you guys before I waste $0.10 and then $5 shipping if I'm doing something wrong.




    So why did the fuse blow the first time with the gain not all the way up?

    Why did the fuse blow the second time with a replacement fuse only .15 amp off as soon as I plugged it into mains even with all channels gain set to zero?

    Is it possible that regenerative principles from the feedback of the mic could have caused the fuse to blow the first time with the gain not all the way up?

    by regenerative principles, I mean the feedback when tickled with its own input (the unshielded unbalanced RCA line could have tickled it maybe), the output is greatly increased, and the output could be increased greater than the amount of current that the circuit is designed to handle.

    Would using a balanced stereo line to transmit the music from the PC audio interface to the PA help at all to remove the feedback?
     
    Last edited: Apr 19, 2013
  2. Harald Kapp

    Harald Kapp Moderator Moderator

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    Nov 17, 2011
    That's unlikely. Then there would be no more headroom for turning up the volume. My guess is that the reason for blowing the fuse in the first place is a short in one (or more) of the output transistors. You will first have to locate the fault, repair it, then insert a new fuse.
     
  3. BobK

    BobK

    7,682
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    Jan 5, 2010
    Time to use that dim light bulb trick.

    Bob
     
  4. galaxy

    galaxy

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    Nov 3, 2012
    Maybe the original fuse was a slow blow.?
    Big capacitors in the power supply, sudden inrush of current etc.

    As to why the original blew...are you sure the 3.15 amp fuse is what is supposed to be there?
    I don't know where you are so what is the mains voltage?
     
  5. (*steve*)

    (*steve*) ¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥd Moderator

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    Jan 21, 2010
    Do you still have both blown fuses?

    A photo of them may actually tell us a lot (it may reveal the type of fuse, and the nature of the fault).

    In many cases a fuse that has a moderate overload and melts looks different to one that has had a massive overload and vaporized.

    Also we should be able to tell if either were slow-blow fuses.

    It would also be useful to know exactly what is written on both of the fuses and on any input voltage specifications or on the board (depending on where the fuse was located)
     
  6. dietermoreno

    dietermoreno

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    Dec 30, 2012
    Unfortunately, I threw out both fuses.

    I didn't think that .15 amp would make a difference.

    So your saying its not the .15 amp making a difference, it may be a power transistor short.

    Well I'll tell you the symptoms: before the fuse blew the first time I smelled smoke then the fuse blew with a pop and when I touched the chasis it was very hot.

    The first fuse blew out with a big pop and the whole inside of the glass was black.

    The next time when I put a fuse in with the mains power connected and all of the gain turned to zero, as soon as I turned on the power supply there was a big fat blue spark that could be seen arcing out of the fuse holder and then I smelled smoke again and when I removed the fuse it was all black on the inside again.

    Do you really need a photo? Do you want me to buy a 3 amp fuse for $3 from the local hardware store again just to look at what the fuse looks like after it has blown?
     
  7. (*steve*)

    (*steve*) ¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥd Moderator

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    Jan 21, 2010
    Not with that explanation :) (It was very good)

    The way those fuses blew is indicative of a major fault, probably an output transistor (or transistors) being shorted.

    The smoke you smelt *may* have been the transistors letting out the magic smoke. They would have been in the process of going short circuit. the fuse still blew fairly violently.

    The second fuse, with the transistors already short circuited, blew in an even more dramatic manner.

    If I were you, I would

    a) never throw out any removed components until the repair is complete
    b) measure across emitter/collector or source/drain of the output transistors and see if they are shorted.
    c) try to determine *why* they failed -- it is most likely that there is another fault.
    d) replace the output transistors and fix the underlying fault
    e) try the amplifier again with a series light bulb
    f) be prepared to change bias settings in some cases.

    It would be extremely useful to have a schematic for the amplifier.

    NOTE: fixing the amplifier will require that you work in a logical manner and not skip steps or "just try stuff".

    Note that my hypothesis may not be correct. The fault could be elsewhere. The first things (see (b) above) is to determine what has actually failed and then to see if that fits the initial hypothesis.
     
  8. dietermoreno

    dietermoreno

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    Dec 30, 2012
    Well am I measuring with the multi-meter with the mains power on with no fuse?

    With no fuse won't there be no current in the transistors?


    Well so I guess step 1 is buying a mulit-meter for $10 from the local hardware store before wasting money on more fuses.

    So maybee should I put some aluminum foil in the fuse holder to make the connection and then test with the multi-meter with all of the gains at zero while the PA is connected to mains and PA power turned on?
     
  9. (*steve*)

    (*steve*) ¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥd Moderator

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    Jan 21, 2010
    Do your measurements with the unit unplugged.

    Don't even think about plugging it in until I tell you.

    If you put aluminium foil in the fuse holder I will ban you to protect us all from your stupidity.

     
  10. dietermoreno

    dietermoreno

    238
    0
    Dec 30, 2012
    I'm sorry if this is an extremely noob question, but when I connect the leads of my multi-meter together with the battery in it nothing happens, but when I connect one lead to a 12 V 500 mA DC power supply negative and then connect the other lead to positive then it shows the max amount of current that it "understands", which the scale on the analog meter only goes to 250mA. Yes the battery is in the correct way.

    So I can't test the circuit if the leads when connected don't show the current of the battery, I think or I might just be being an idiot.


    Edit: I took out the battery from the multi-meter and it still showed the max current that the scale will show when I connected the leads across a 12 V 500 mA DC power supply. When I reverse the leads to the battery terminals, the needle moves backwards, as is expected.

    Then I measured the battery itself and it also showed the max current that the scale will show.

    So the battery is good.

    What is not good is my intelligence on how to use this device.



    Yes there are 2 huge capacitors connected to brightly colored insulated thick gauge wires connected to the mains input of the PA.
    The value that the 2 huge capacitors each say is 4700 micro Farad. I live in the U.S. so the mains voltage is 110-125 V RMS or 160-175 V peak. The 2 huge capacitors are the size of half the height and same width of toliet paper rolls. The 2 huge capacitors say "HI RIPPLE".

    Next to the 2 huge capacitors are 2 huge diodes that are each the size of a thumb nail in height and width.

    Next to the 2 huge diodes are two smaller capacitors that are the size of a thumb that are labled 1000 micro Farad each. Next to the 1000 micro Farad capacitors are two capacitors the same size that are unlabled that just say "65 * C VENT" (where the * is a degree).

    Next to the four 1000 micro Farad size capacitors are four diodes half the size of a pinky nail.


    Interestingly, there is actually a second fuse that was there the whole time that didn't blow. It says "T3.15AL 250V", which is the same as the other fuse. That's weird that there are two fuses and only one of them blew.

    I also see two large metallic objects next to each other with one longer than the other and has the high voltage thick gauge wires running into it, I am guessing that is the power supply transformer with the coils hidden inside of a shield to protect you from electric shock.

    Also there appears to be a voltage regulator transistor labled "U" on the primary winding of the transformer and one on the secondary winding of the transformer. Well actually, not on it for sure, just right next to it.


    So I'm guessing that the power supply circuit skematic is something like this:


    mains>>fuse>U>transformer>U>huge diode--->--->..4700 micro F..--->--->smaller diode-->-->..1000 micro F-->-->smaller diode-->-->65 * C VENT cap
    ..IN....3.15A.....V.......................V...........................................V..........................................................V...................................................V
    .......................V.......................V................................................V..........................................................V...................................................V
    ground--->>>>>V>>..huge diode..--->--->..4700 micro F..--->--->smaller..diode--->--->..1000 micro F-->-->smaller..diode-->-->65* C VENT cap.........V
    ....................................................................................................................................................................................................V...........V
    ....................................................................................................................................................................................................V...........V
    ....................................................................................................................................................................................................V...........V
    ....................................................................................................................................................................................................+...........-..
    .........................................................................................................................................................................................................DC........
    ........................................................................................................................................................................................................OUT........


    Well here is a skematic of a power supply with a ring of 4 diodes, maybee it looks more like that, except the voltage regulator in my PA has one voltage regulator on the primary winding of the transformer and one voltage regulator on the secondary winding of the transformer(if that is even correct):
    [​IMG]

    So maybee my "huge diodes" don't do anything for the power supply and the diodes in use in the power supply is the ring of four small diodes.

    So maybee my voltage regulators aren't actually on the transformer, but just are next to it and the actual wiring is very different from the external appearence without seeing the circuit board and not knowing where those colorful thick gauge wires go.
     
    Last edited: Apr 21, 2013
  11. davenn

    davenn Moderator

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    Sep 5, 2009
    there wont be any voltage regulators on the primary side

    have you traced the circuit correctly from where the AC mains comes into the supply ?
    does it go through a bridge rectifier?, is there a large, ~ 47uF 400V electrolytic capacitor on the primary side ?
    It mite be a SMPS typ PSU

    Time for a photo we dont really wanna play the 20 questions guessing game

    Dave
     
  12. (*steve*)

    (*steve*) ¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥd Moderator

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    Jan 21, 2010
    Dietermoreno, do you remember when I said this:

    I gave you a single simple task. Measure the resistance across the output transistors.

    Did you do it? No.

    What did I say I would do if you didn't follow this advice?

    I said I would ban you to prevent me from having to deal with stupidity.

    And what did you do?

    Did you do what I asked? No

    Did you just "try stuff" Yes

    Did you start fantasizing about how things might work? Yes

    OK. I will give you one last chance.

    Go away and make the measurements I asked for. Do NOTHING else.

    I expect the next post in this thread from you is either the measurements AND JUST THE MEASUREMENTS, or a request for assistance in finding where to take the measurements.

    If you want to provide more, you MUST comply with the request above first. Then, you may also provide some photos of the amplifier.

    If I am not satisfied with your next post, I will ban you.

    Learn to follow instructions, or learn somewhere else.
     
  13. dietermoreno

    dietermoreno

    238
    0
    Dec 30, 2012
    Photos from cell phone camera attached.
     

    Attached Files:

  14. alfa88

    alfa88

    344
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    Dec 1, 2010
    A couple questions: What is the make and model of the amp?, I saw a schematic of a power supply, do you have a schematic of the amp? From the pics it looks like the amp has 4 different power supplies and I figure the 2 fat diodes are feeding the output transistors. I don't think that's your trouble area as Steve has hinted at. Take a nice clear picture of the insides of the entire unit and one of your multimeter while your at it
     
  15. davenn

    davenn Moderator

    13,833
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    Sep 5, 2009
    Virtually all your photos are no good they are too blurry to make out anything

    the the board into some good natural light preferably outside but not direct sun
    do more pics that are not so close up, you phone camera cannot seem to focus that close

    look at your pics BEFORE you post them .... if they are blurry DONT post them
    I want sharp, clear and well lit pics ... I want to see the individual sections of the unit

    Power supply, preamp, power amp stages



    You use the Ohms range, you are looking for resistance readings NOT voltage or current

    Dave
     
  16. dietermoreno

    dietermoreno

    238
    0
    Dec 30, 2012
    Okay well its dark outside now and I'm going to be in school all day and won't get home untill its dark for the next few days, so I'll wait a few days to take a few pictures not so close of the whole insides of the unit in natural light.

    Okay, know when I switched the mode from voltage or current to resistance and on the 10 times multiplier setting, it shows about 12 ohms x10 = 120 ohms when I connect the test leads together with just the leads not connected to anything else.

    When I connect both test leads to the chassis I also measure 120 ohms.

    When I connect one test lead to the collector and one test lead to ground of the power transistor I also measure 120 ohms.

    The amp has 3 power transistors. The amp has 3 channels. So I think each channel has one power transistor. The power transistors are the big fat ones. I didn't take any photos of the power transistors yet. Next photos in a few days in natural light will include a photo of the 3 power transistors. I don't know where the pre amp transistors are. Presumably the pre amp transistors are smaller and that's why I can't find them yet.

    Note that I am using the words "collector" and "emitter" interchangeably because I don't know the direction of the circuit. All that matters for my testing is that they are not the same, like plus and minus doesn't matter which one, it just matters that its not the same (I think).

    When I connect one test lead to the collector and one test lead to the emitter I measure about 20 ohms x10 = 200 ohms.

    The resistance across collector and emitter for all 3 power transistors is 200 ohms.

    The huge capacitors say 4700 micro F. There are also two 1000 micro F capacitors. The capacitors that I said were unlabled earlier and just said "VENT", I found that I was reading the wrong side of the capacitor and in fact it has a value of 2200 micro F.

    I also found that those little black boxes in photos 0421131928, 0421131928a, and 0421131928b are diodes.

    The two huge capacitors are in photos 0421131922a, 0421131923, and 0421131923a.

    The fat diodes are in photo 0421131919.

    The voltage regulators are in photos 0421131926, 0421131926a, and 0421131927. What I think is the transformer is also in those 3 photos.

    The "extra fuse" is in photo 0421131930a.

    The fuse that I replaced goes in the fuse holder in photo 0421131931a.

    The 4 little diodes are in photos 0421131919a and 0421131920.

    Photos 0421131921, 0421131922, 0421131922a, 0421131923, 0421131923a, and 0421131925 are all capacitors.

    Photo 0421131930 is a view of most of the power supply.
     
    Last edited: Apr 22, 2013
  17. (*steve*)

    (*steve*) ¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥd Moderator

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    Jan 21, 2010
    It sounds like you're using an analog meter (it has a swinging needle, not a digital display).

    If so, when you touch the probes together, you then need to adjust for zero.

    Transistors have 3 connections. Larger ones have a metal tab on them which is connected to the collector. In some really large transistors there is no separate collector lead.

    Once you get us images of the whole board, we can point you toward the output transistors.

    Typically they will have the largest heatsinks on them and be near where the speaker terminals are wired to.

    If you can read the markings on the transistors we can tell you what type they are, and which leads you need to measure across.

    It is possible that you don't have individual transistors. You may have a hybrid module (with many legs) attached to a heatsink.
     
  18. dietermoreno

    dietermoreno

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    Dec 30, 2012
    Well the 3 transistors that I was looking at have no heat sinks, don't have the thick gauge wires running to them, and are right next to the input jacks. So I think those are actually the pre amp transistors.

    There is a 6 inch wide by 6 inch high heat sink on the side of the PA that leads to an area beneath the board. The red and black thick gauge wires that connect to the speaker output jack go to an area underneath the board where the huge heat sink is. So I think under the board by the huge heat sink is where the power transistors are.

    So do I need to remove the board to be able to see the power transistors? The board has screws that can be unscrewed.

    I know you said better photos first, but I can't take a picture of the power transistors if they are below the board without unscrewing the board.
     
  19. (*steve*)

    (*steve*) ¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥd Moderator

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    Just take a photo as Dave suggests. If you need to disconnect anything we'll tell you.
     
  20. Mongrel Shark

    Mongrel Shark

    260
    18
    Jun 6, 2012
    I'm no audio amp expert, But...

    I think that may have something to do with the fuse issue...


    Could you explain in a little more detail exactly how you made the connection between your PA and you guitar, which was plugged into another amp? Does your guitar have two amp jacks? I could only see one in your video... Was that a Live speaker line you where tapping on the strings? If so, why are you trying to destroy your guitar and two amps? You didn't even have all the stuff that could explode on screen. Would be a crappy destruction video if thats what you where trying to do.... You do have a grasp of how much power 500W is right?



    :confused:
     
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