Resistor ID

Discussion in 'General Electronics Chat' started by rasteri, Feb 5, 2009.

  1. rasteri

    Thread Starter New Member

    Feb 5, 2009
    4
    0
    Hey all,

    I'm trying to repair a turntable (Numark TTX) that I can't find a service manual for. I'm a bit of an electronics newbie, so don't judge me harshly if these are stupid questions!

    One of the obvious things wrong with it is a burned-out resistor - I think one of the power transistors for the motor has shorted and the resistor bore most of the damage. I'm having a hard time figuring out what value it is - possibly the heat from the short has burned off some of the markings, but see what you think. I'm thinking maybe 9Ω (black-white-black). Looking at the PCB, it seems to be going from the emitters of the transistors to ground.
    (sorry for the awful pics)

    Also, I need to replace the power transistors. The current ones are ROHM 2SD1889 & 2SB1340, which are discontinued. I was thinking replacing them with NTE2343 & NTE2344, which are (apparantly) basically identical. Sound reasonable?
    Datasheets :
    ROHM - http://www.datasheetcatalog.org/datasheets/50/240459_DS.pdf
    NTE - http://www.datasheetcatalog.org/datasheet/nte/NTE2343.pdf
     
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  2. mik3

    Senior Member

    Feb 4, 2008
    4,846
    63
    Can you figure what circuit the transistors make?
     
  3. rasteri

    Thread Starter New Member

    Feb 5, 2009
    4
    0
    I'm not sure. A lot of the circuit is under the motor. what uses 3xNPN and 3xPNP power transistors, and drives a 3-coil stepper motor? (well, 12 coils, but I think they're wired together in groups of 4)
     
  4. mik3

    Senior Member

    Feb 4, 2008
    4,846
    63
    First, black-white-black is not 9 Ohms (its invalid).

    if each transistor has one of these resistors on it then look for the value of the good ones.
     
  5. rasteri

    Thread Starter New Member

    Feb 5, 2009
    4
    0
    Nah, there's only one. Here's how I think it's wired up -
     
  6. mik3

    Senior Member

    Feb 4, 2008
    4,846
    63
    I think this resistor is used as a current sense to provide a feedback to the control circuit and keep the current at safe values. If it is the case it has to have a low resistance, somewhere between 1 and 10 ohms. Try a 10 ohm first to see what happens.
     
  7. bertus

    Administrator

    Apr 5, 2008
    15,649
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  8. beenthere

    Retired Moderator

    Apr 20, 2004
    15,815
    282
    That resistor is toast. In my experience, red stripes generally burn black. It was probably a low value, though.

    Do check all those transistors. The NTE's will probably do for replacements.
     
  9. eblc1388

    Senior Member

    Nov 28, 2008
    1,542
    102
    Carefully scrap the insulating material off along a straight line on the resistor body using a pen knife until you can see the spiral track of the resistive material.

    Chances are you can still measure some resistance, using either end of the connection lead and a pointed pin, pricking at the spiral track.

    Starts from one end and advance on each spiral track until you have found an open circuit. Record the resistance before this happens. Do the other end using the same method.

    Add the resistance found in both cases and adds 10% more. This will give you the approximate resistance of the original resistor.

    This method often works because resistor burns open in one spot only.
     
  10. rasteri

    Thread Starter New Member

    Feb 5, 2009
    4
    0
    Thanks all for the great tips. Especially eblc1388 - what an inspired idea.

    Unfortunately I accidently ordered 12 NTE2343s, not 6 of each as I thought, so looks like I have to wait another few days to see if this works.
     
  11. italo

    New Member

    Nov 20, 2005
    205
    1
    Is it burned out open or just discolored? did you try to measure it? the darlington probably do use a low value . and why bother with the resistor if the darklington went sour?
     
  12. SgtWookie

    Expert

    Jul 17, 2007
    22,182
    1,728
    Did you even bother to read what anyone else wrote?
    The Darlingtons were likely damaged when the motor shorted, and the resistor got fried.

    Did you bother to look at the pictures of the resistor? One lead looks like it was burned right off.

    If he doesn't replace the resistor with an equivalent value to the original, the motor will receive too much or too little current.
     
  13. alim

    Senior Member

    Dec 27, 2005
    113
    1
    Hi , if you are sure that the second band is white, the first band has to orange, i.e. 39, if the third band is black it is 39 ohms
     
  14. cabdoctor

    New Member

    Jun 5, 2010
    1
    0
    Did you ever find out what value this resistor was? I just had mine burn up and need to replace it as well.
     
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