help with car amp

Discussion in 'General Electronics Chat' started by lokeycmos, Nov 25, 2011.

  1. lokeycmos

    Thread Starter Active Member

    Apr 3, 2009
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    i picked up a free pioneer GM-X702 car amp. it turns on for a few seconds then just shuts off. i can hear a click like a relay. just wondering where i should start troubleshooting. i know im not giving you much to work with, just looking for a starting point. i took it apart. no obvious visual signs or hot/burnt smell. i did find a schematic, which i attached. any advice would be appreciated. ty
     
    Last edited: Nov 25, 2011
  2. thatoneguy

    AAC Fanatic!

    Feb 19, 2009
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    Does it do the same thing with no speakers connected?

    Are any of the capacitors in the power supply area bulged or leaking?

    Can you post high resolution, clear photos of both front and back sides of the board, without flash bounce? Easiest way to do this is to force flash off and take photos outside. Otherwise, put a reflector (tin foil bent to shape and taped on) in front of your flash so the light is bounced from the ceiling and back down more diffused.
     
  3. bountyhunter

    Well-Known Member

    Sep 7, 2009
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    Almost certainly some kind of overcurrent sensor has activated and kicked out the relay. Can be caused by an overload (short) on the output or some kind of internal failure.
     
  4. Yako

    New Member

    Nov 24, 2011
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    You have a circuit schematic I see. At least that is something.

    To be honest with you -- doing repairs at component-level is often a case of being in the 'know', as in that particular industry (Pinoneer audio amp boards in this case) -- and knowing the trade secrets. Without this it can become a reverse engineering exercise sometimes.

    I don't see any measurement references on that schematic either. You need to know how to test for leaky transistors and capacitors out of circuit to begin with. But, if you were a technician working for Pinoeer then you would most likely just follow a certain protocol of remove and replace based on the symptoms of the unit.

    It becomes expensive to pay someone to sit at a desk for a few hours testing 3-cent parts. It is usually more economically viable where possible to just assume them to be the culprit (or at least related) -- and just replace them in that particular segment of the circuit. All circuits are broken down into building 'blocks', as depicted on the rather large and very complex looking electrical schematic in that pdf.
     
  5. sheldons

    Active Member

    Oct 26, 2011
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    first things id check is around the op stage-especially Q571,D557 and especially C567 and R601.....had failures here before.Any one of the transistors in the op stage may also be sc.One other thing to check is the fets and components to do with the dc to dc convertor.on switch on before shutdown is there any dc present on any output??
     
  6. Yako

    New Member

    Nov 24, 2011
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    I offer the following FREE advice based on my experience:

    • Purchase analouge meter for accurately testing components.
    • Audio signal generator
    • Cheap CRO ...
    The first thing that you should be doing when you are trying to repair a board is to measure the supply rails at all points. Things simply don't work without power. Then check for dry solder joints.

    A schematic with no voltage references for a working unit means that you're in for a rough ride. One of two way (both are time-consuming):

    1. Inject audio signal and trace it around the board using a CRO
    2. De-solder and test caps & transistors based on your educated guesses.
    Good luck!
     
  7. Yako

    New Member

    Nov 24, 2011
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    Well there you go, he is giving you a 'trade secret' like I said.
     
  8. Yako

    New Member

    Nov 24, 2011
    245
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    I have been given similar tips within the industries that I have worked too.

    Repair is harder than engineering in some cases.
     
  9. MrChips

    Moderator

    Oct 2, 2009
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    With the schematic and the board layout we should at least be able to pinpoint where is the problem. How much experience do you have in electronics? Do you have a DVM?

    You will need to trouble shoot this on your workbench (kitchen table if the missus allows?) using a 12V power source. Do you have one?

    Do not connect the speakers for trouble shooting at this point.
     
  10. lokeycmos

    Thread Starter Active Member

    Apr 3, 2009
    432
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    here are some pics i took. lmk if this helps. i have a good amount of electronics knowledge.im an electronics technician student. i have several digital meters;DVM, LCR, ESR meter, transistor tester. im good at building circuits from a schematic, but not much experience trouble shooting commercial electronics. i could start blindly unsoldering parts and testing them, but would rather not if possible. i noticed some brown spots on the bottom of the board, but it looks like someone else had it apart and removed the black square spacing stickers.
     
  11. lokeycmos

    Thread Starter Active Member

    Apr 3, 2009
    432
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    couple more
     
  12. thatoneguy

    AAC Fanatic!

    Feb 19, 2009
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    Can't see much. Something is wrong with the Right Channel, unless you've been soldering on it.

    High Resolution is 3+MegaPixels, or at least 2000x1050, so we can read out the resistor and diode names on the top side of the board, and follow traces on the bottom side.

    On the bottom side, several areas have gotten hot enough to boil the flux back out to the edges, and a surface mount device (MOSFET, Diode, etc) appears to have "slid" diagonally on the heatsink pad.

    Can't tell from the photo, but are there places on the solder mask that are "darker green" than the rest of the green on the bottom side? That is a positive indicator of high heat as well.

    Focus was good, lighting was good, just need much higher resolution before we can ask for measurements.
     
  13. MrChips

    Moderator

    Oct 2, 2009
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    We will assume that the relay that you hear clicking is the speaker relay. This is happening because there is a DC voltage fault. So the first thing we have to do is check all supply voltages.

    DO NOT CONNECT ANY SPEAKERS.

    Connect 12V DC to POWER +B. Connect 0V to GND. Look at the board layout shown and locate pins 11 and 12 at CN121.


    [​IMG]



    Pin 12 should be +15V and pin 11 should be -15V, with respect to GND.

    Then we'll take it from there.
     
  14. lokeycmos

    Thread Starter Active Member

    Apr 3, 2009
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    im getting -1.7 volts on pin 12 and -15 on pin 11. if you look at the pics i just noticed today that someone previously did a solder job to bridge some open traces. according to my ohm meter there are no shorts even tho they did a terrible solder job.
     
  15. thatoneguy

    AAC Fanatic!

    Feb 19, 2009
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    Get a toothbrush and some 90% isopropyl alcohol, and a spray bottle.

    Scrub away all that flux so what is there can be seen very clearly. Your last to photos are the resolution needed, but of the top side as well, try to go bottom/top, bottom/top in 2" chunks if you must go get the entire photo.

    First, clean off all the flux now that we know the source of it. Keep in mind though, that the traces which blew were probably related to the previous failure, so we'll want to look at all power components in that path and test them. There may be a shorted component that was missed.
     
  16. MrChips

    Moderator

    Oct 2, 2009
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    So either the +15V supply is shot or something is killing the supply. Run the amp for 5-30 sec and feel if anything is running hot.

    Measure the voltage at three points labelled B, C and E with respect to GND as shown below.


    [​IMG]
     
    Last edited: Nov 27, 2011
  17. kam-22

    New Member

    Nov 21, 2011
    19
    0
    Did you check the supply voltage at the collector's on the power transistors?

    most car amp's work on switching supply's..followed by +/- dc balanced rail's to the amp stage such as +/12 or 15 volts to the opamps then there's say 25+/- dc rails at the collector's...oh, there's also protection circuit that will 'check' if all is well so to speak.

    most car amp suffer from the following...blown o/p stage..due to being pushed to it's limit and more!...next the power supply goes belly up.. switch mode ic..and switching fet's..

    you can use 12v 2amp psu to work on it...here's a tip...use a working car amp to cross ref voltages.. such as o/p..power..etc.
     
  18. lokeycmos

    Thread Starter Active Member

    Apr 3, 2009
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    ran it for about 30 seconds. nothing got even remotely warm.
    B= -58mv
    c= -.88v
    e= -.88v

    working on the better pics
     
  19. MrChips

    Moderator

    Oct 2, 2009
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    I have to go out. We'll continue later or someone else may like to pick up the thread.
     
  20. lokeycmos

    Thread Starter Active Member

    Apr 3, 2009
    432
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    heres more pics. they were too high resolution and size to post on the forum so i posted them here: http://imageshack.us/g/208/20111127171119.jpg/
    im kinda frustrated because the origional picture files, when opened within windows, can be zoomed in on very clearly and the individual traces can be seen. but you cant zoom in on that site.
     
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