Reading Resistance Bands - Going Blind?

Discussion in 'General Electronics Chat' started by thatoneguy, Jan 3, 2013.

  1. thatoneguy

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    Feb 19, 2009
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    VOODOO NOW, SKIP TO THIS POST


    Attached is a cropped picture of a motor control board I repaired about 2 years ago.

    I got one today where both of these resistors are cooked and burned to over 1 Meg Ohm and no remaining markings.

    It's out of an Elna Sewing Machine, though that information is very extraneous.

    I am going color blind because I can't match many colors to a chart. I got red, black, maybe purple, and those aren't right.

    If you can read the colors, that'd be great (I know that Bad Boys Rape Young Girls But Violet Gives Willingly, but it's no help when I can't tell what the colors are supposed to be). I have a terrible time reading the codes from the blue coated resistors and usually resort to a meter. Sadly, I no longer have this board in hand. :(

    [​IMG]
     
    Last edited: Jan 13, 2013
  2. BReeves

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    Nov 24, 2012
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    Look like 1meg to me
     
  3. thatoneguy

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    Feb 19, 2009
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    Both ended in K

    I'm going to write down all values of anything I touch from now on. I guess you never know when you'll see something again. I only do these for a friend who can't get it going.

    This one almost works with a 4.7k on left, and an 820k on right, speed control doesn't shut all the way off, so maybe the 820k on right should be 82k...

    The left connects to the cathode of an SCR (not the main TRIAC, SCR produces the pulses for TRIAC), the right one is downline from the gate.
     
    Last edited: Jan 3, 2013
  4. GopherT

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    Nov 23, 2012
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    One the right is (top to bottom)...
    Brown, black, green, red

    Left has five bands that appear to be (to to bottom)...
    Brown, brown, black, black, brown
     
  5. Dodgydave

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    Jun 22, 2012
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    Right-hand resistor looks like 1Meg 2%

    Left-hand is 1K 1% or 110 ohms 1%
     
  6. vrainom

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    I see a gray band at the bottom of the right one
     
  7. MrChips

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    Left one is 1.0k 1%
    Right one is 825Ω 1%
     
    Last edited: Jan 3, 2013
  8. R!f@@

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    Quite correct.
     
  9. BillO

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    I agree also.
     
  10. thatoneguy

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    I suppose I should have titled the thread "Monitor Calibration Test".

    Blue band on the right resistor looks teal/green to me. Is that the resistor color blending, or my display?

    The left one looks like a bunch of black or brown bands (1k). Time to change contrast and color ratios.
     
  11. k7elp60

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    Nov 4, 2008
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    I disagree. The one on the left is brown,brown, black, black, space brown. So it is 110 with the second black strip as a multiplier of 1 so it is 110 1%
    The second one is brown, black, green, red. or 1.0M 2%
     
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  12. GopherT

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    K7elp60

    I Agree.
    These are the only values from the E12 series that make sense.
     
  13. thatoneguy

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    I also see a faint white/grey band at the bottom of the right resistor, so reading bottom to top, 820 Ω sounds correct, and matches my notes, though I didn't make enough notes. I remember trying to get their value the last time to attempt a schematic, but gave up. That was an in-circuit measurement value, however.

    The left one, read from the bottom upward as well, shows 1k 1%.

    The combination of 5 bands and even spacing with the blue background using semi-translucent colors can be confusing (to me, at least). Is there a known standard way to know which end to start reading from at a glance?
     
  14. THE_RB

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    If the resistors "burned" with obvious signes of heat (ie overcurrent) it is unlikely either was 1 megohm. Heat damage on resistors only really happens on lowish values.

    If a Elna schematic is not available, and you can't contact an Elma repairman by phone etc, I would try to reverse engineer the part of the schematic and seeing what those resistors does will give you a lot of information.

    My GUESS would be the top brown band on both is the tolerance, and they would be auto insterted into the board by a machine so all other resistors would match.

    So to me the colour codes would most likely be;
    brown black black brown (brown) = 1k 1%
    grey red black black (brown) = 820 ohms 1% (where a black band is faded a bit due to heat)
     
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  15. thatoneguy

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    Ok, I am giving up on this one after putting in 4 hours.

    I've tried a few different combinations, and even went to the shotgun extreme of pulling all the components in the circuit with the resistors/around them/half the board and testing them out of circuit.

    The way it works, this circuit is for the "single stitch" button. There is a reed switch on the top rotor to tell the board when the needle is up, it is working. On switch press-release, a 22uF cap is charged to rectified line voltage, then it is fed through a 22V Zener, several BJTs, the main variable resistor for motor speed which eventually gets to these three components, around an SCR, that give pulses to the Triac to run the motor.

    The problem is, It WORKS, KIND OF. I haven't seen a failure quite like this one. The reed switch normally shorts the capacitor down to < 22V to stop the motor running. When you press the single stitch, it always moves for Exactly FOUR cycles, and always stops with the needle up, no matter the starting position or how fast you have it set. I got it not working at all. Now everything works, but stopping. It will always go four extra stitches, then stop with the needle up.

    In other words, it's acting like a counter set to 4 instead of 1, and it's all in analog. I can't find any components that fail the M3 analyzer tests, the reed switch is 0.03Ω when closed (it doesn't dump the current directly, it turns on a different transistor).

    The odd thing is from a crawl, where you hear the motor steps, it will grind away at the slowest speed for 10 seconds to get the needle up after 4 stitches. At full speed, it stops after 4 stitches very quickly. It is driving me insane, though I am amused by the operation. It also doesn't seem sensitive to the values of resistors shown above (that came in as greyish-black blobs). Currently 82Ω and 100Ω, had an 820k in there with a 1k, and most other combinations. Same behavior. It's VOODOO. :eek:
     
  16. GopherT

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    This new story is a lot more interesting and is now much different than your title. You might want to start a new thread (on the forum, not on the sewing machine).
     
  17. vrainom

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    That's interesting! Do you think the circuit isn't discharging the cap fast enough?
     
  18. thatoneguy

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    That's the only thing that would keep it running, if I pull the cap out of circuit, it only runs for the duration the button is held down.

    How in the world it has a variable time constant is beyond me. I did replace 2 SCRs (D100 and CR100) as well as the two resistors, and checked dissipation, value, and ESR on all electrolytics on the board. Speed control runs perfectly fine, until you want it to stop. The other complicating factor is it is AIR Controlled. The foot pedal pushes on a diaphragm that has two switches and a couple variable resistors internally. So I am unable to isolate the speed control only, since 7 wires need to be re-connected to that for it to even work. It's not remove-able or take-apart-able.

    If anybody has ANY idea, I'm willing to give it a try. I once was able to find the patent for the Air control with schematic of an early version (no single stitch button/circuit) by searching something like Air motor control sewing on patents, but my normal patent search place doesn't exist anymore. If somebody can find a good source for a PDF of it, that'd be great! IIRC, it was by Elna in the 70's or 80's.
     
  19. DerStrom8

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    Feb 20, 2011
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    I agree with the first one. Not sure how you get 825Ω for the second. I clearly see brown, black, green, red, which is 1MΩ 2%. How do you get 825Ω?
     
  20. THE_RB

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    To thatoneguy; it raises the question, if that resistor gets discoloured (or even fails) due to heat, it obviously gets some current through it. What other components in the circuit does that current run through? Maybe if you hypothesise failure modes and current paths that will lead you to a part that might test "ok" but is not operating perfectly?
     
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