MOSFET TROUBLE

Discussion in 'General Electronics Chat' started by misael.mata, Oct 9, 2014.

  1. misael.mata

    Thread Starter Member

    Aug 26, 2014
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    hello there, i had a question? help would be greatly appreciated . i have this kicker amplifier im trying to repair, switched out some transistors and now my irfz44s are overheating, what causes this? i checked resistors diodes and caps and all seem okay. help please
     
  2. Lestraveled

    Well-Known Member

    May 19, 2014
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    Hey man, please post a schematic or something we can work with. Love to help fixing a guitar amp especially.
     
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  3. Papabravo

    Expert

    Feb 24, 2006
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    What causes them to overheat is spending too much time in the ohmic region and poor response to changes in the input signal. When you "switched out" the IRFZ44's did you replace them with OEM parts, NTE "close equivalents" or cheap Chinese imitations. It makes a big difference in high performance amplifiers. Tell us again why you decided to switch out those parts. Did you observe anti-static protocols when you did this?
     
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  4. misael.mata

    Thread Starter Member

    Aug 26, 2014
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    i couldnt find a scematic for this model but i found one something like it. and yes i replaced the faulty transistors with oem, i pulled them out another amplifier my friend has, but what would really cause this overheating, they overheat in a matter of seconds
     
  5. misael.mata

    Thread Starter Member

    Aug 26, 2014
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    here is the shematic
     
  6. misael.mata

    Thread Starter Member

    Aug 26, 2014
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    i actually wear an antistatic wristband wenever i work on this type of stuff just to make sure
     
  7. misael.mata

    Thread Starter Member

    Aug 26, 2014
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    so what else would cause my irfz44 to overheat?
     
  8. Papabravo

    Expert

    Feb 24, 2006
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    A couple of surprises

    1. Kicker is the name of the manufacturer, not an adjective describing the performance. Capitalization helps alot in these cases.
    2. It is a class D amp reather than class A or AB
    3. A schematic for a similar amp is really not much help especially because I can't see where the IRFZ44's would be placed.

    This suggests to me that the switching properties of the parts you took out of the other amp were damaged in the process. Slow switching in a Class D amp is a guaranteed finger burner. Sorry but, that's the way the Mercedes Benz. I'd try to score some brand new parts if I were you. Maybe nothing else got damaged.
     
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  9. misael.mata

    Thread Starter Member

    Aug 26, 2014
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    so that means that the problem is in the transistors, even if they test good on multimeter. and the amplifier is actually a class ab,
     
  10. misael.mata

    Thread Starter Member

    Aug 26, 2014
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    so that means that i got to replace all the transistors on the board so that way the z44s dont burn up
     
  11. Dr.killjoy

    Well-Known Member

    Apr 28, 2013
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    Try drawing it out first and take some measurements first before jumping to any answer....
     
  12. misael.mata

    Thread Starter Member

    Aug 26, 2014
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    What causes it to get 12 volts at the emitter and collector?
     
  13. Lestraveled

    Well-Known Member

    May 19, 2014
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    Did you change U4, the HIP4080A??
     
  14. misael.mata

    Thread Starter Member

    Aug 26, 2014
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    Is that the 494 ic in my amp?
     
  15. Lestraveled

    Well-Known Member

    May 19, 2014
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    No, it is the driver chip for the output Mosfets. It is labeled," replace with each repair" on the schematic.
     
  16. Lestraveled

    Well-Known Member

    May 19, 2014
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    Oh, surprise. The schematic you posted is not a schematic of your amplifier.
     
  17. misael.mata

    Thread Starter Member

    Aug 26, 2014
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    The times that you've had problems with transistors or mosfets overheating, how was it that you fixed the problem???
     
  18. #12

    Expert

    Nov 30, 2010
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    I can explain part of this. A class D amplifier is what I call a "fast switcher". The mosfets run way above audio frequency and chop the power up into tiny slices. Those slices get applied to the speaker which is way too slow to move at the switching frequency, so it just moves at the audio frequency that is represented by the slices of power. The point is, there is no way a volt/ohm meter can measure how fast the mosfets can switch.
     
  19. rogs

    Active Member

    Aug 28, 2009
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    I'm confused.....so the class D schematic presented is not of the amplifier being repaired?.....and it's not even certain whether the actual amplifier being repaired is class D or Class AB?

    So if, for example, the amplifier is class AB and uses lateral Mosfets in the output stages, then replacing them with vertical Mosfets like IRFZ44 is going to cause problems...like the wrong G-S bias causing major overheating, for example........

    EDIT: on their website, it looks like the Kicker amps are all Class D, so this probably not the cause then........
     
    Last edited: Oct 10, 2014
  20. ScottWang

    Moderator

    Aug 23, 2012
    4,853
    767
    So when you trying to asked for help to fixing amplifier, at least that you should attach some clear pictures for PCB side and parts side, otherwise something may happen is that a blind man leading some blind men, it may leading to the Cliff.
     
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