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Clarity SMA-120 Mixer-Amp: Suitable resistor replacements?

Discussion in 'Troubleshooting and Repair' started by adamnp, Apr 2, 2011.

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  1. adamnp

    adamnp

    5
    0
    Apr 2, 2011
    Hello folks, I'm looking for some help identifying two burnt resistors and suitable replacements for each. Both resistors have no identifiable markings on them. I have included pictures of each along with the resistance readings from my multimeter. The resistors were taken from a PA amplifier. There is another resistor similar to the larger (ceramic?) resistor on the circuit board, it reads: FA2, 4.7 ohm 5%

    Thanks for your help.
     

    Attached Files:

  2. Resqueline

    Resqueline

    2,848
    2
    Jul 31, 2009
    There is a distinct possibilty that both of those resistors are actually ok ,with 620 ohms and 4.7 ohms respectively. Nothing more can be said w/o a schematic diagram.
     
  3. adamnp

    adamnp

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    Apr 2, 2011
    Okay, but given their condition I would still like to replace them. If I can't find a diagram, does that mean I'm SOL?
     
  4. Resqueline

    Resqueline

    2,848
    2
    Jul 31, 2009
    If Googling brings no joy then post info about make, model, & part #'s - and see if someone has a similar good one and is willing to open it up and share/compare.
     
  5. adamnp

    adamnp

    5
    0
    Apr 2, 2011
    No luck with the Google. The amp is a Clarity SMA-120. The resistors in question are R101 & R228.

    Thanks for your help!
     
  6. (*steve*)

    (*steve*) ¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥd Moderator

    25,474
    2,820
    Jan 21, 2010
    In that case show us the circuit board, both sides, from directly above to allow us to trace where these devices are.

    It's the latter one I'm most interested in. It may be an inductor.

    If you can take a really sharp image of the first resistor, then enhance the saturation of the image you may be able to make out the colours of the bands more easily.

    It is highly unlikely that any fault that damaged these devices is *in* these devices. There is likely to be something else at fault (if indeed there is a fault)
     
  7. adamnp

    adamnp

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    0
    Apr 2, 2011
    I found someone on eBay selling the exact same model amp who was willing to open it up and tell me the markings on both resistors. Here is what they responded:

    R101 = from the front - blue red brown (space) gold
    R228 = not color coded - labeled CRC FA2 4.7 ohm

    It's now confirmed that resistor R227 is the exact same as R228. Please see the attached image of R227.

    Is more information still needed at this point, or can I start looking online to where I can purchase replacements for each?
     

    Attached Files:

  8. (*steve*)

    (*steve*) ¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥd Moderator

    25,474
    2,820
    Jan 21, 2010
    Well, you have confirmed that the resistors have the same resistance that they're supposed to.

    So why is it that you want to replace them again?

    If there is indeed a fault it is clearly not with the resistors. Perhaps you should determine whether there is a fault, and go about fixing that?

    It's pretty clear that this resistor is going to get hot. All the high power resistors are raised off the board. The discolouration may simply be due to an expected amount of heat.

    Is there a fault with the amplifier? Or are you looking to replace these because they look wrong?
     
  9. adamnp

    adamnp

    5
    0
    Apr 2, 2011
    Sorry, I should have explained myself better in my initial post. So the actual fault was a short which blew the fuse on the circuit board, and upon inspecting the board for other faults I noticed the two discolored resistors. I thought maybe they had also been damaged so I removed them from the board and tested them. The charred outer coating on the smaller of the two resistors literally crumbled when I touched it. That's when I decided I might as well replace them both, I also didn't want to risk them failing in the near future.

    What I'll probably end up doing is soldering the 4.7 ohm resistor back in place and finding a replacement for the 620 ohm.

    Thank you Steve and Resqueline for your assistance.
     
  10. (*steve*)

    (*steve*) ¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥd Moderator

    25,474
    2,820
    Jan 21, 2010
    Ah, that helps a lot.

    A resistor that has been overheated enough to crumble needs replacement (indeed, not being able to read the value is also a good enough reason.

    I've been umming and aaahing about whether or not to question if the 4R7 resistor is a fuseable resistor. I agree with your decision to put it back.

    Did you replace the fuse and try the amplifier prior to removing these resistors?

    Was the short applied across the output?

    I hope everything "just works" when you put it back together. If it doesn't, answering the questions above will help us help you further.

    Did the fuse blow immediately?
     
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