Transistor replacement soldering question

Discussion in 'DIY' started by kjello, May 31, 2017.

  1. kjello

    kjello AK Subscriber Subscriber

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    Hi all gurus or other helpful souls of this beloved forum

    I have a question regarding pcb mounted transistors without heatsink.
    As you can see in this picture there are some heat damage on the board, (amplifier probably took some abuse) my questions are; would it be better to leave the "legs" a bit longer to get more air between the transistor and the board? or is it better to just mount a heatsink down at the board?
    Will the transistor legs "bend" by the heat if leaved too long, and what would be too long?
    [​IMG]

    And

    This resistor is a 4,7 ohm 1/4 watt, 5% I have 8 of them damaged (pic)
    Now I found some new ones on the www which claims to be the same specs, but they do not have the BLACK stripe, other stripes are the same, Im a bit confused by this.
    Can they still be 4,7 ohms?
    [​IMG]

    Best regards
    KO
     
    Last edited: May 31, 2017
  2. BinaryMike

    BinaryMike Pelagic EE Subscriber

    Transistors mounted without heatsinks must dissipate heat through their leads, so shorter is better in that respect, but they're more likely to get damaged by soldering heat. Pick your poison.

    The black band doesn't signify resistance value. It's meaning might not be standardized at all, but there's bound to be someone here who knows more on that topic.
     
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  3. petehall347

    petehall347 the brandy coffee man

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    i guess we are looking at resistors here not transistors ?
    resistors can be mounted above the board no problem .
    insulator rings on the legs are often used .
     
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  4. Alan0354

    Alan0354 Super Member

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    I would mount a small heatsink on the transistor and be done with it.

    This is the first time I see a black band on resistor like this. You might want to go with 1/2W if you have multiple failure with the 1/4W.
     
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  5. avionic

    avionic " Black Knights" Subscriber

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    Shorted ? Are you measuring in circuit or out of circuit ? Those resistors usually "open" or go real high ohms. 4.7Ω may just look shorted to your meter because they are low resistance.
     
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  6. Alan0354

    Alan0354 Super Member

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    Agree. They usually open, not shorted. All the ones I burned, they look burned also.....with smell and all. They usually don't die quietly.
     
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  7. kjello

    kjello AK Subscriber Subscriber

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    Sorry guys, I mean open resistors of course...
    I should not be writing in my sleep :) I have corrected my questions.
    these bad resistors are in circuit with power transistors that are also broken
    Now the good ones read 4,7 ohms on my meter (the ones from the working amplifier channel)
     
    Last edited: May 31, 2017
  8. kjello

    kjello AK Subscriber Subscriber

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    Im sorry for my confusing writing
    First question was regarding transistors, second question resistors.
     
  9. kjello

    kjello AK Subscriber Subscriber

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    OK
    So a 1/2 watt will behave like a 1/4 watt in circuit ?
    My guess in this case, is that the resistors was overloaded when the power transistors died.
     
    Last edited: Jun 1, 2017
  10. arts

    arts Super Member

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    Are you sure those are actually resistors? Might just be a small value inductor....
     
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  11. kjello

    kjello AK Subscriber Subscriber

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    This is from the schematics
    Im afraid I may not know the difference
    [​IMG]
     
  12. kjello

    kjello AK Subscriber Subscriber

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    They are reading Mega ohms in- and out of circuit.
     
  13. arts

    arts Super Member

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    Well,the schematic indicates a 4.7 ohm resistor. I'm thinking the ''F'' indicates either fusible or flameproof.

    EDIT:
    Also,the exclamation point indicates a component with a ''safety'' requirement. If you are measuring very high resistance (megohms) then these have failed under overload.
     
    Last edited: May 31, 2017
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  14. Alan0354

    Alan0354 Super Member

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    If it is a fusable resistor, you need to find out why it burn.

    You have the schematic, post the whole thing!!! With the schematic, we can tell you a lot better what you should and should not do.

    Indicate what transistor on the schematic that is getting too hot.
     
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  15. kjello

    kjello AK Subscriber Subscriber

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    Link to schematic: https://www.hifiengine.com/manual_library/nad/218-thx.shtml
    So far I found:
    Q326(C3788), Q318(A1478) shorted
    Q338(D1975), Q342(D1975), Q344(B1317) shorted
    Q332((A1837) shorted
    R358, R360, R366, R368, R374, R376, R382, R384 all 4R7F reading mega ohms out of circuit.
    R370 3W 0,22 ohm cement resistor reading mega ohms out of circuit

    All this in the right channel power amplifier board
    According to previous owner, damage probably caused by shorted speaker output.
     
  16. Alan0354

    Alan0354 Super Member

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    I need to login, I don't have account, can you just post the schematic?
     
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  17. avionic

    avionic " Black Knights" Subscriber

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    So there not shorted. They are for all practical purposes "open". A short would be like zero Ω's.
    1/4 watt most likely.
     
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  18. avionic

    avionic " Black Knights" Subscriber

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    It only takes 5 min. or less and is free.:idea:
     
  19. kjello

    kjello AK Subscriber Subscriber

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    Is it possible to post a PDF file?
     
  20. Bob

    Bob AK Subscriber Subscriber

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    black is usually no tolerance and sometimes 20%. lift one leg (or remove) to measure it. if it's
    still inside the range then it's OK.
     
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