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Identify a burnt resistor

Discussion in 'Troubleshooting and Repair' started by rico4295, Jun 28, 2018.

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  1. rico4295

    rico4295

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    Jun 28, 2018
    Anyone care to take a shot at identifying this crispy critter? It's an AC air blend door actuator.
     

    Attached Files:

  2. Bluejets

    Bluejets

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    Oct 5, 2014
    Try measuring it.
     
    rico4295 likes this.
  3. rico4295

    rico4295

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    Jun 28, 2018
    Honestly didn't think of that. Not sure if tinker'er is below hobbiest or not but that hot spot look had me thinking it would be open. I'll get it off the pcb and and try tomorrow.
     
  4. (*steve*)

    (*steve*) ¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥd Moderator

    25,418
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    Jan 21, 2010
    Try measuring it before you remove it.

    If it's fragile it may break as you remove it. With the aid of a measurement of resistance across 2 points before and after removal we can calculate the resistance if necessary.

    I recently replaced a resistor that was so fragile it broke *while* I was measuring it (before removing it).
     
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  5. rico4295

    rico4295

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    Jun 28, 2018
    also good, I will, thanks
    it bounced around 37.5 in pcb.
     
    Last edited: Jun 28, 2018
  6. KJ6EAD

    KJ6EAD

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    Aug 13, 2011
    39Ω (orange, white, black) is a standard value so that's a possibility. Is that a fried inductor just northeast of R1?
     
  7. 73's de Edd

    73's de Edd

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    Sir rico4295 . . .

    Observations and suspicions . . . .

    The unaffected R2 and R3 units are being of CARBON deposition / SPIRAL laser trimmed construction .
    The affected R1, I VERY highly suspect, to be of METAL film / SPIRAL laser trimmed construction.

    They hold up QUITE well under overload . . . despite the apparent coincidental cosmetic degradation.

    I have seen metal film units running at an orange glow ( viewed in the dark) state of overload and STILL holding their resistance value as well as a much larger power wirewound unit would do.

    39 ohms is still a good and valid estimate of its initial as well as present value.

    Seems like you need to make a resistance check on both sides of the resistor to see which is being
    closest to ground potential, thereby suggesting that side is towards the overloading direction , that is trailing towards a bad component, which is responsible for the overloading of that R1 resistor . . . . which is still hanging in there . . . . VERY-VERY begrudgingly, however.

    I am seeing no symbol annotations akin to L? or T? being made around any of the 3 of them.
    Plus . . . the liberal deposition of excess goober-loober in their immediate proximity, suggests of them being MECHANICAL attributes, not electrical / electronic, particularly the due EAST one, with its protruding shaft.

    73's de Edd
    .....
     
  8. rico4295

    rico4295

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    Jun 28, 2018
    Thanks for the responses good sirs. The resistor is the only burnt part. A really simple looking circuit with just the 3 resistors and a pot with gears, oh and a little cap. I took it out today at work and my better meter there read 34.5 ohm. Was thinking the colors may show better on the bottom but a lot of the coating is gone. I'm also thinking of placing the new part on the trace side of the PCB because one of the solder points is completely burnt off.
    Akin to your thinking '73, I'll almost bet it would still do its job if wasn't for the burnt off solder pad. The location and position of these actuators are known to have freezing and condensation issues

    Just another pic of it.
     

    Attached Files:

    Last edited: Jun 28, 2018
  9. (*steve*)

    (*steve*) ¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥd Moderator

    25,418
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    Jan 21, 2010
    My guess is that it was originally 39 ohms (orange white black) from the look of the remaining bands.

    However, this isn't the major problem. Something caused that resistor to overheat. If you replace it, it may just fail again.

    Also, assuming the original value is something like 39Ω, the small change in value probably wouldn't have stopped the unit working. Is be looking for something else, maybe a seized motor, or a shorted transistor?
     
  10. rico4295

    rico4295

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    Jun 28, 2018
    Update.. I got it soldered back in, albeit the underneath side along the traces lol. Being new to this I feel it was somewhat of a small accomplishment. It was a little tricky to get it act right. These small actuators' main gear operates on a 180° arc with 5 different positions. After a few attempts to line up the gear attached to the pot and not running it past its' traces I think it's working great. Thank you all for your ideas and support.
     
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