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A bit colour blind..

Discussion in 'General Electronics Discussion' started by snowington, Oct 10, 2013.

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  1. snowington

    snowington

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    Oct 10, 2013
    Hi everyone,

    I've found a blown resistor on the pcb of my PA amp. It's got a lot of burn around it and, luckily, doesn't seem to have effected the other components..

    ..trouble is, I'm a bit colour blind and I'm having trouble identifying it??

    There's a photo for anybody who can help.

    Thanks in advance,

    Snowy
     

    Attached Files:

  2. BobK

    BobK

    7,682
    1,688
    Jan 5, 2010
    3.9K

    Bob
     
  3. snowington

    snowington

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    Oct 10, 2013
    Thanks Bob,

    Snowy
     
  4. snowington

    snowington

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    Oct 10, 2013
    Actually not sure about that as 3.9 has a gold band and mine is black. So it goes, as far as my non colour blind flatmate says he thinks it's orange white red black?
     
  5. BobK

    BobK

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    1,688
    Jan 5, 2010
    The fourth band is the tolerance, but black does not make sense. It looks gray in the picture, is it maybe silver, which would be 10%?

    3.9K is orange white red. It might have a gold band for tolerance, which would be 5%


    Bob
     
    Last edited: Oct 10, 2013
  6. (*steve*)

    (*steve*) ¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥd Moderator

    25,491
    2,833
    Jan 21, 2010
    The resistor itself doesn't actually look damaged. If you replace it, replace it with a higher wattage resistor and mount it off the board (i.e. leave enough lead length so it doesn't rest hard on the board).

    My suspicion is that it gets hot normally and the board you have is sensitive to heat.

    A higher wattage resistor will be larger so the surface temperature will be lower and mounting it further from the board will allow airflow which will also cool the resistor and keep heat away from the board.

    If you take the resistor out you can measure the resistance. You may then be able to measure the value as a double-check. If the resistor is not badly damaged, the resistance is likely to still be reasonably close to its nominal value.
     
  7. snowington

    snowington

    11
    0
    Oct 10, 2013
    Thanks for the replies..

    I think the old one's pretty well had it. As you can see by the photos attached, not sure what will happen when I replace, whatever it is? Yoiks..
    S
     

    Attached Files:

  8. snowington

    snowington

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    Oct 10, 2013
    ...maybe a higher wattage would do the trick? What do you reckon?
     
  9. snowington

    snowington

    11
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    Oct 10, 2013
    ..also could only find a wire wound resistor of 3.9k would that be ok? what with the old one being carbon. The rating is 3w,

    Snowy
     
  10. davenn

    davenn Moderator

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    Sep 5, 2009
    well it looks like the 3.9k was way off the mark!!

    its 270 something, prob 270 Ohms ( I say that cuz normally it would be a low value hi watt resistor that woulde be getting hot like that)
    ... do you have a multimeter
    Have you measured the resistor yet ?
    Its obviously got VERY hot and has discoloured the colour bands
    so the colours you now see are nothing like the original

    Dave
     
  11. snowington

    snowington

    11
    0
    Oct 10, 2013
    Hiya,

    Just took a reading and I got 11.00 on 20ohms setting. Wondered if it could be damaged because of the burn and give me a false reading?

    ..so we have 3.9 v 270 v 11? err???

    Cheers

    S
     
  12. BobK

    BobK

    7,682
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    Jan 5, 2010
    The colors in the picture are quite readable, I cannot believe they could be a burnt versionn of 270 which would be red violet brown

    Bob
     
  13. snowington

    snowington

    11
    0
    Oct 10, 2013
    Hi Bob,
    Just had another good look at it and have been working it out on a resistor calculator. It seems that the marks are orange white and red, and taking a multimeter reading comes up as 11?.. And there's a black band on the tolerance (other end of the resistor.
    Checked out 11ohm comparison and the colour chart is nothing like??

    ..think I'll go back to knitting..

    S
     
  14. (*steve*)

    (*steve*) ¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥd Moderator

    25,491
    2,833
    Jan 21, 2010
    Looking at the size of your fingers, either you have HUGE fingers or that's a pretty small resistor, possibly 1/4W or 1/2W.

    Does it show any signs of being replaced? Since you seem to have clipped it out, can we see the soldered joints on the other side.

    The board clearly says 270, so my best guess would be to replace it with a 270 ohm 1/2W resistor and just see what happens.

    The board is pretty toasted under it. This could cause you problems too. What does it look like on the other side?
     
  15. snowington

    snowington

    11
    0
    Oct 10, 2013
    My hands are pretty big, but the image is magnified. From pole to pole, the resistor measures around 11mm, so not that small. There wass no sign of it being replaced before, You're right about the 270, and it's worth getting one of those in, I reckon.
    Tried to get a schematic from the manufacturers, but no luck yet..
    What do you think of upping the wattage a bit? It seems it may have burnt out originally as it was too low.

    S
     
  16. (*steve*)

    (*steve*) ¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥd Moderator

    25,491
    2,833
    Jan 21, 2010
    Try a 1W resistor in its place.

    Although if the wrong value was inserted, that may have been the cause for excessive heating.

    More pictures of both sides of the board, and some description of what this device is would help. If it was used for switching or driving something, some indication of the specs of this device and also of the one being switched or driven might yield some clues.
     
  17. snowington

    snowington

    11
    0
    Oct 10, 2013
    Well, the device itself is a Johnson 8 Channel PA, running to 2/300w per channel.
    Very confused though, as a rating of 270 would have a violet band and there isn't one, yet it says on the board 270 and I get a reading of 11ohm resistance??

    Most of the other resistors on the board are wire wound, Don't know if that helps?
    I'll take a photo of the full board if you want?

    Here are the specs:
    SPECIFICATIONS:
    Power: 300Watts @ 4 ohms
    180Watts @ 8 ohms
    Frequency Response: +2dB @ 20-20kHz
    Dynamic Range: 90dB
    Total Harmonic Distortion: -0.2% THD
    7 Band Graphic Equalizer: (63, 160, 400, 1k, 6.3k, 16k) Hz + 12dB
    Channel EQ: Low (+15dB @ 80Hz), Mid (+12dB @ 2.5kHz), High (+15dB @ 12kHz)
    Channel Effects: Reverb (JPM-8R), Digital Delay (JPM-8D)
    Input Jack: 1/4” Digital Delay Footswitch (On/Off) JPM-8D (Master Section)
    Balanced XLR Inputs: 8
    Unbalanced 1/4” TRS Inputs: 8
    RCA Inputs/Outputs: 2 (Master Section)
    Speaker Outputs: (2) 1/4” output jacks (Back Panel). A total of two 8 ohm speakers can be connected,
    one on each jack.
    Note: More than two speakers, or using 4 ohm speakers will put an overload into
    the mixer shutting it down and possibly damaging the unit.
    Chassis: Heavy Gauge Metal
    Dimensions: 18 1/2” x 10 3/4” x 11 3/4” (47cm x 27.3cm x 29.8cm) W x H x D
    Weight: 35.2lbs (16kgs)
     
  18. davenn

    davenn Moderator

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    Sep 5, 2009
    Bob,
    the colours can change quite dramatically after a resistor gets ad hot as that one obviously has
    have experienced this problem often over the years fixing CRT TV's and amplifier gear

    D
     
  19. davenn

    davenn Moderator

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    Sep 5, 2009
  20. snowington

    snowington

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    Oct 10, 2013
    Yup Dave.. the very same one.

    S
     
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