any amp techs?

Discussion in 'General Electronics Chat' started by needhelp, Sep 6, 2005.

  1. needhelp

    Thread Starter New Member

    Sep 6, 2005
    4
    0
    had some good respone from these guys , but i want as much input from differant people as possable

    http://www.dutchforce.com/~eforum/index.php?showtopic=4391

    if you cant accses the lin it says this inaway

    Hi... any one? =)

    inaway... thought this would be a great place to start looking for help as its what my problem is based on. Basicly my car amp hsa stopped working.. the red protection light is comeing on when i turn the amp on. i have taken the back on the amp to see if anything is burnt or broke away from the board, but everything seems to be intact. any body know were i could start to look? i was thinking if i start to change 10 or so resistors ect.. at a time and then keep trying it after changeing 10 or so to see if it will work


    any input would be great, thanks
     
  2. needhelp

    Thread Starter New Member

    Sep 6, 2005
    4
    0
    anyone?
     
  3. mozikluv

    AAC Fanatic!

    Jan 22, 2004
    1,437
    1

    hello,

    have you check the fuse? if okay, turn on the unit, then check the power amp ic or the power amp output bjt transistor or power amp output mosfet if there's any supply coming in. if it is an ic just be sure you know which pin is the supply pin & ground pin. if it's bjt's check the colletor pin, and if mosfet check the drain pin.

    if that is an ic amp it would be better if you can tell us the code #, likewise with the bjts or mosfet.

    moz
     
  4. needhelp

    Thread Starter New Member

    Sep 6, 2005
    4
    0
    erm,, sorry i dont no anything about electronics all i no is its going into protection and i dont want it to :( fuse are ok, nothing inside smells / looks burnt
     
  5. mozikluv

    AAC Fanatic!

    Jan 22, 2004
    1,437
    1
    hi

    usually when the unit goes into protection mode, like what it's doing now, it has sensed abnormal voltage/current at the output. these abnormal voltage/current comes from external source (speaker side or from self-powered subwoofer) or from internal source w/c is the unit itself. if by removing the speaker connection or the subsystem, the unit still goes into protection mode, then the unit is the culprit. again usually the fault comes from the output transistors. as i have said before check the collector or drain pins of the output device. it should read about the same level as the supply voltage. likewise check if there is any dc voltage at the output, it should read 0v.

    bypassing the protection circuit would be a fatal approach specially on the speaker side. if you want to do this approach you should substitute the speaker with a 4ohms or 8 ohm 100w resistor.

    moz
     
  6. needhelp

    Thread Starter New Member

    Sep 6, 2005
    4
    0
    could you please run throw how i make these checks, and what i will need to replace inside my amp? i can give you a link to some pics and maybe you could circle what needs changeing?
     
  7. mozikluv

    AAC Fanatic!

    Jan 22, 2004
    1,437
    1
    hi

    sorry for not responding right away, been very busy. i've followed your link including the forum. it seems that the consensus as to what kind of output transistors are used is MOSFET. just by looking at the pics could not give us any conclusion whether they are MOSFETs or BJTs. likewise i can't try to circle the parts seen to be checks due to non availability of a schematic diagram. that would tantamount to guess work w/c it is not our objective here.

    i will deal with the testing of a MOSFET. MOSFET stands for Metal Oxide Semiconductor Field Effect Transistor. but i interpret the initial FET into FIRE EMITTING TRANSISTOR. why? becoz dying MOSFETs sometimes if not often emit flames! usually a MOSFET that looks okay is okay but it could also be a dud or dying.

    oftentimes MOSFET failure is cause by an internal short circuited drain-to-gate. what happens is the drain voltage is reverted back into the gate w/c is fed via a gate resistor & into the driver transistors also causing it to blow or die. this action is also happening to the parallel MOSFETs gate.

    am not sure if you have a multimeter with a diode test range, but if you have, this will have about 4v across the DUT. this voltage is enough to partially turn on most MOSFETs. but if your meter has an open circuit test voltage of about 2v or less, it wont do for this kind of test.

    TEST STEPS:

    1. connect the negative probe to the source pin (caution: hold the device by the case or tab, not the pins)

    2. touch the positive probe to the gate pin

    3. then move the positive probe to the drain. see your meter and it should read low. by doing this, you have just charged the gate capacitance and the DUT has been turned on.

    4. now while the (+) probe is still connected to the drain pin, use your finger to touch the source pin and gate pin, this way the gate will discharge thru you. check your meter and it should read high w/c now indicates a non-conducting DUT.

    the above test simply measures the pinch-off voltage, meaning the highest voltage that can be applied to the gate without having the device to start conducting. above test result indicates a good mosfet.

    the above test is not a fool-proof method but it can suffice. there is another way to make it fool-proof but that entail making a test equiptment just for that purpose.

    hope this helps.

    moz
     
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