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Smoking car amplifier, I need to replace 5 transistors

Discussion in 'Troubleshooting and Repair' started by olyanders, Jan 20, 2015.

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  1. olyanders

    olyanders

    2
    0
    Jan 20, 2015
    Hello, newb here.

    I have a surface level education in electronics, I haven't dove much into repairing. My friends car stereo amp broke, and I told him I may be able to fix it since I was bored at the time and wanted to take something apart.

    I'm stuck on ordering a new part, because of all of the numbers and letters on the transistors. What's really throwing me off is one out of the five is ever so slightly different.

    What part of this code is relevant to ordering a new part? If not all?

    Two of the five burnt transistors read as follows (some letters burned off on the bottom right hand corner):

    IRF3205
    1R 712P
    4N

    Two of the five have more letters burnt off and read the same as above, except the 712P

    The last one reads as follows (also partially missing due to burn damage):

    IRF3205
    1R
    4R

    All the others read as such:


    IRF3205
    1R 712P
    4N 1R

    And


    IRF3205
    1R 712P
    4R C1


    Any help is much appreciated, as I am as anxious to fix this as he is to have some bump in the trunk.

    Thanks!
     
  2. Gryd3

    Gryd3

    4,098
    875
    Jun 25, 2014
    Typically the transistors used for driving the output are all the same part number. The part numbers for the Left channel output may be different from the part numbers used for driving the Right channel output. Additionally, there will be a power supply circuit that may very well use yet another different part number.
    So... which ones are damaged? (Please note that these components run very hot and are usually clamped or screwed to the heat-sink along the outer edge of the board.)
    Pictures help a TON!
    Also please note that if the exact transistor can't be found, that you can swap out for a compatible transistor but it is recommended to swap out ALL the transistors if they are grouped together. (ie, if one of the transistors for the Left channel burned out, you should replace ALL of the transistors for the Left channel.)
    Balance here is key, they transistors much have almost identical characteristics otherwise some may end up working harder than others which will result in another failure.

    So... You should get us some really nice pictures so we can take a look.
     
  3. olyanders

    olyanders

    2
    0
    Jan 20, 2015
    So here are some photos. Hopefully these help

    amp 2.jpg amp 1.jpg SAM_0753.JPG
     
  4. Gryd3

    Gryd3

    4,098
    875
    Jun 25, 2014
    Thank you for the pictures. Because of the location, that certainly seems to me that it is responsible for the power supply in the Amp.
    The output transistors appear to be along the bottom right edge.
    I would still suggest replacing them all as a group, but my concern here is that perhaps there was an additional fault in the device that caused the damage. (Or it was stuffed under a cushion and ran way too hot)
    The power supply for an audio amp is usually a split rail. So you will be making a higher positive voltage, and a higher negative voltage in that amp when you provide it 12V. It makes this as fast as it can and stores it in the capacitors to dump into the speaker with the other transistors.
    The best approach right now is to get some info on those transistors. A price, and get you some soldering experience so you can prepare to remove and replace the damaged groups. Once we ID them, and find replacements, you can find a supplier and an estimated parts cost. We may also be able to test the remaining transistors to see if they do indeed require replacement for testing purposes.
     
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