Trying to fix a receiver

Discussion in 'General Electronics Chat' started by SilvrEclipse, Jul 16, 2008.

  1. SilvrEclipse

    Thread Starter Active Member

    Nov 21, 2007
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    0
    Ok guys I have a sony stereo that the output transistors blew out in. The unit used to just stay in protection mode but now doesn't work at all and last time I plugged it in the relays just clicked rapidly and I believe a transistor got smoked. Now I am not really good with electronic trouble shooting so this is kind of a learning experience in hope of getting it back working also. I know it is more cost efficient to just buy a new one but like I said its a learning experience.

    I found some heat spots on the board where it looks like a transistor burned and another piece I dont know what it is. The resistors look fine and so do the capacitors. I plan on removing most of the parts from the board and testing each one to see what I can find thats burned out. What are the chances of me fixing this stereo?

    Also it has 3 chips on it, do these normally fry. I dont think they could be replaced very easily.

    Here are some pics.. I marked the burned spots and what is that piece I put in the red box?

    http://i189.photobucket.com/albums/z173/SilvrEclipse/DSCF2652.jpg?t=1216245795
    http://i189.photobucket.com/albums/z173/SilvrEclipse/DSCF2651.jpg?t=1216245830
    http://i189.photobucket.com/albums/z173/SilvrEclipse/DSCF2650.jpg?t=1216245853

    Thanks guys..
     
  2. theamber

    Active Member

    Jun 13, 2008
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    You will need a schematics to speed up the troubleshooting and to learn about it.
    You probably have a direct coupled amplifier and has caused what is called the domino effect. You may have several transistors burned up. I should be checking each diode out of the circuit and suspect of that power IC next to the sand resistors to be a candidate for the troubles. If you replace the transistors but did not get to the root of the problem they will blow up again.
    Does your unit has an STK large IC? If so that goes out a lot too.
     
  3. blocco a spirale

    AAC Fanatic!

    Jun 18, 2008
    1,438
    368
    It might be worth considering replacing the whole of the power amplifier section with a ready-built module with similar power-supply requirements. You will need to remove components to isolate the old amp section from the input, power-supply and output terminals.
     
  4. tibbles

    Active Member

    Jun 27, 2008
    249
    3
    thats good idea blocco i'll have to remenber that one...
    you would need a schematic to pick up the l-r channel inputs.

    silver eclipses amp looks a bit tricky,looks like its using a stereo module as a driver for the output transisters?this would prevent him from working on the channels one at a time.
    also the output transisters or mosfets will be around a fiver each uk
    regards
    dougal
     
  5. SilvrEclipse

    Thread Starter Active Member

    Nov 21, 2007
    58
    0
    theamber - The STK IC is the thing I have boxed in the 3rd pic? I believe there are only 3 on the board. If you look on the underside of the board it looks like there is some discoloring on that part. Is there anyway to test it to see if it is still good?


    Blocco - That does sound like an option but like I said, this is more of a project/learning experience.
     
  6. alonso1999

    Member

    Jul 16, 2008
    11
    0
    I have not work on fixing audio amps in a long time, but what I remember is that the problem ussually was the output chips like the ones on the 3r picture that has paste on it against the heat sink(metal),
     
  7. SilvrEclipse

    Thread Starter Active Member

    Nov 21, 2007
    58
    0
    Yea the output transitor is fried on the right channel. But other things have gone out also. The amp use to work and just stay in protection mod but now the whole thing does not work at all.
     
  8. SgtWookie

    Expert

    Jul 17, 2007
    22,182
    1,728
    Gee, it's too bad you didn't fix it when it was just the output transistor.

    Now you can't even get it to turn on.

    Something is probably putting a dead short across the supply. Start disconnecting/unsoldering things until the short goes away.

    Otherwise if there's a marina close by, someone may need a boat anchor.
     
  9. theamber

    Active Member

    Jun 13, 2008
    318
    0
    I don't get exactly what is happening but I told you before if you do not get to the root of the problem more things will blow up as your amplifier is Direct Coupled. Just because the print circuit board is discolored under a part that does not mean the part is bad. I have seen burned up circuit boards areas caused by heat and the parts were good.
    At this point if you don't get power you are going to have to replace the STK IC. Or your final output transistors will blow up again. Some largers STK also control the power supply, check the schematic of your particular IC you could get it from an ECG crossreference book. I had similar problems and they are very hard to troubleshoot it may even be a glass diode. Isolate the area you are working by disconecting at the connectors CDs or tapes from the power supply.
     
  10. SilvrEclipse

    Thread Starter Active Member

    Nov 21, 2007
    58
    0
    Thanks for all your help theamber, Ill see what I can find out this week.
     
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