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Help identifying a resistor

Discussion in 'Troubleshooting and Repair' started by Bernardo SOUSA, Aug 6, 2016.

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  1. Bernardo SOUSA

    Bernardo SOUSA

    11
    2
    Aug 6, 2016
    Hello there!

    I need help identifying a faulty resistor that I pulled out from the primary circuit of a power supply board from an LG tv. The bands on this resistor are ambiguous to me. I can't decide if I read it from one side or the other, and I also can't decide if the 4th/2nd band is yellow, orange or gold.

    Here are some pictures: https://goo.gl/photos/GtbsCU6KAksiY7USA

    Any advice on identifying it is highly appreciated. Noob here! lol
     
  2. Heliman

    Heliman

    65
    13
    Feb 4, 2016
    155k 5%
     
    Bernardo SOUSA likes this.
  3. Heliman

    Heliman

    65
    13
    Feb 4, 2016
    Have you checked it with a ohm meter?
     
  4. Minder

    Minder

    3,103
    664
    Apr 24, 2015
    Could be 1.5meg.
    M.
     
  5. Alec_t

    Alec_t

    2,970
    805
    Jul 7, 2015
    hevans1944 likes this.
  6. Bernardo SOUSA

    Bernardo SOUSA

    11
    2
    Aug 6, 2016
    Thank you for your replies so far!

    So, it's brown on the left, and the last green band is temperature coefficient? I was reading it as a 0.5% tolerance.

    All I have to test it is a cheep DT-830B digital multimeter, which reads infinite resistance/no current flow. The other resistors from the board behave totally different. They all let current through, all within their tolerances (if I'm not misinterpreting the bands).

    So it is ambiguous! How to determine which resistor should I buy to replace it from the circuit board? Hopefully avoiding going to college to learn.
     
  7. Bernardo SOUSA

    Bernardo SOUSA

    11
    2
    Aug 6, 2016
    So far, three people looked at this thing and each said a different value. Am I doomed? lol

    Edit: thanks for sharing that article. I've read many similar articles in the last 2 days, but I'm still confused by this particular resistor.
     
  8. hevans1944

    hevans1944 Hop - AC8NS

    4,615
    2,154
    Jun 21, 2012
    This is an inexpensive meter based on a widely available integrated circuit. As long as it is assembled properly it is a fine meter for hobbyist use. The maximum resistance range is 2000 kΩ or 2 megohms, well within the range of a 1.5 MΩ resistor. You should test the meter to make sure it actually works on this range by measuring some known--good meg-ohm valued resistors.

    Your resistor is probably open, not an uncommon failure mode for high-resistance carbon film and metal film resistors. Why not purchase another 1.5 MΩ resistor and see if that solves your LG problem? It is unlikely that such a high resistance is going to cause any problems, no matter what the actual resistance, resistance tolerance, power rating, or temperature coefficient of the original resistor happened to be. Of course that may not be the original resistance, so the problem is still there.

    OTOH, if we are reading the color codes "backwards" and they should be read as "green, gold, green, green, brown" or maybe that's "green, YELLOW, green, green, brown" who knows what that means? i tend to go with @Alec_t's interpretation of "brown, green, green, ORANGE, green" meaning 155 kΩ with 0.5% tolerance.

    Notice that there is a similar resistor, R804, on the other side of the board with colors "brown, yellow, silver, gold, black" which means what? Maybe you could remove that resistor and measure its value to get a clue as to what the color codes mean. Of course the best bet is to read the value off a schematic, but I am gonna assume one is not available. You might try SAMS Photofact to purchase and download a service manual.
     
    Last edited: Aug 6, 2016
    Bernardo SOUSA likes this.
  9. Bernardo SOUSA

    Bernardo SOUSA

    11
    2
    Aug 6, 2016
    I'll do that. Be right back!
     
  10. Bernardo SOUSA

    Bernardo SOUSA

    11
    2
    Aug 6, 2016
    R804 reads 1.2 Ω
     
  11. Bernardo SOUSA

    Bernardo SOUSA

    11
    2
    Aug 6, 2016
    After measurements, it's still not clear. But if I ignore the last black stripe and consider it a 4 band resistor, it gives me 0.14 Ohms 5%

    I suspect I'm making a dumb scale mistake on my meter. When I set it to 200Ω, it reads 1.4 then quickly drops to 1.2. If that's right, 200 on the meter actually indicates x10 (?). Why?? Anyway, that doesn't disqualify the "dead" diagnostic of the first resistor, which by the way is a bit smaller. No current goes through that.
     
  12. 73's de Edd

    73's de Edd

    3,121
    1,315
    Aug 21, 2015
    Sir Bernardo SOUSA . . . . .

    I will need more time than I have available now, but in initially looking at the units use in the circuitry design.
    I am seeing that resistor as completing a resistive ground link for two peak limiting snubber capacitors for two power MOSFETS that drive the primary of your switch mode power transformer.
    A value of 1.55 meg would sound about right.

    Edd's . . . . . .Timely Tech Tip . . . .# . . . .Two thousand, four hunnert and 'lebenteen


    Put a jumper lead from one end of the resistor to the other.
    Set your meter to OHMS function and in a range that will display a 2 meg reading
    Hold one ohmmeter lead in constant pressed contact with one resistor lead end.
    Hold your other ohms probe lead against a bare metal single edge razor blade.
    Temporarily touch that blade to the other ohms lead to confirm a short at that instant of touch.
    Go to the very center of the resistor and very-very-very-very lightly and gradually, and laterally scrape away the paint coating with the sharp blade and be expecting a resistance reading to be showing up, upon contact, at one point.
    Twice that resistance should be the ~ value of that initially good resistor.
    ( Check again . . .eliminating your skin resistance from your fingers being in circuit. )

    Failure of this resistor WOULD NOT be a principal cause for a dead power supply.
    It just gets HAMMERED to death, with 100K's per second of high voltage pulses over a period of time.
    Look at the browning of the PCB under the power transformer and you can see that this unit has had some run time on it.

    F.Y.I. .

    A metal or carbon film resistor has a laser vaporized spiral groove around the periphery of its body.
    On a HIGHER value resistor it will be using s...e...v...e...r...a...l side by side spirals.
    On a LOW value resistor, it will be using but a few wide spirals.

    73's de Edd
     
    Last edited by a moderator: Aug 7, 2016
    Bernardo SOUSA likes this.
  13. Bernardo SOUSA

    Bernardo SOUSA

    11
    2
    Aug 6, 2016
    What would it be then?

    Also, the power supply is not completely dead. It gives out the correct voltage at 3.5 and 12 volts pins. Only the two 24 volts pins read zero.

    I could buy a brand new board to replace it, although I'd consider that almost a failure. It'd be so much cooler to find the bad component (if there's any other than this resistor) and replace it too!

    Either way, thank you for the cool tip on how to discover the true value of that resistor. I'll do that this afternoon.
     
  14. Bernardo SOUSA

    Bernardo SOUSA

    11
    2
    Aug 6, 2016
    Apparently I'm a total spoon. That resistor is good. I was reading it on the wrong scale. On 2000k scale, it reads 1485. I'm assuming it means 1.485 MΩ. I'm back to troubleshooting the board.
     
  15. 73's de Edd

    73's de Edd

    3,121
    1,315
    Aug 21, 2015
    .

    Sir Bernardo SOUSA . . . . .

    After a "reading" of your board I have transferred markups onto the schematic snippet below.

    Looks like the sets 24 volt supplies A and B are being derived from the dual anode power Schottky rectifiers D205 and D206.
    Grab ye olde voltmeter probes and place the main instrument in DC voltage mode and of a range to display +24VDC.
    Use one of the two STAR grounds below and stab your negative probe into it.
    The main metering + probe will then be probed to test the presence of +24VDC at one and then the other center tabs of D205 and D206 cathode tabs which tie into my marked 24A RED and 24B BLUE power busses that they connect to.
    If you find that the +24 is at both points then the fact that you are NOT also reading those voltages at the output connector,is because those voltages are being switched by the Q201 and Q202 Mosfets which I have marked with RED and BLUE voltage paths.
    The A and B are the voltage inputs and the AA and BB are the voltage outputs of those transistors, IF there is gate drive directed to those Mosfets .

    OBSERVATION:

    VERY VERY poor solder work on YELLOW squared areas, and also see if YELLOW ? connection might have a floating ring solder connection.

    Its testing time
    . . . . . .


    FOT-O-GRAF:

    LG PS PCB.jpg



    73's de Edd


    .




    .
     
  16. john dougherty

    john dougherty

    9
    1
    Feb 11, 2013
     
  17. john dougherty

    john dougherty

    9
    1
    Feb 11, 2013
    Generally speaking the last color is gold or silver which is the tolerance of 5 % so if you hold the resistor in your hand and read from left to right you should be able to identify the color code which will give the value of the resistor . color code is b b r o y g b p g w . hope this will help you .regards des use Spirirg solder wick to remove the solder from the printed circuit board . I buy from fred sharpe electronics and its good stuff .
     
  18. Bernardo SOUSA

    Bernardo SOUSA

    11
    2
    Aug 6, 2016
    Spoon count: 2

    The board is in order. Same mistake. Learning to use the multimeter. DC voltage scale was in 20. It displayed 1 (out of scale) when I touched the 24 v pins. When I set the scale to 200, it displays 23.9 volts. Something else is preventing the TV from turning on. I feel lost. How do I test the other components? Any generic tutorials out there?

    Also, sorry for the time I made you loose with that silly mistake. And thank you for trying to help me!
     
    73's de Edd likes this.
  19. john dougherty

    john dougherty

    9
    1
    Feb 11, 2013
    What make of tv is it ?? when you press the on /off button doe you see any lights come on indicator lights green or blue . Is the HT fuse and the mains fuse ok . do you have a reading in the power supply . do you know how to use a Multi meter and if so does your multi meter have OHMs volts and amps on it ?? I would suggest you feed in to google the make and model of your T>V and see what happens ??
     
  20. Bernardo SOUSA

    Bernardo SOUSA

    11
    2
    Aug 6, 2016
    Hey, john dougherty. Thanks for the suggestion. Yes, I have a multimeter bought recently to help me try and fix this TV. I am currently learning how to use, as you can tell by my two big mistakes described above. I think now I know enough to do some basic testing. Yes, the power board seams to be working properly. I powered it feeding 3.5v from one of the pins to both "switch on" and "power on" pins, and when I do so, all other pins behave as expected.

    Now, here's something new: it does turn on, but only for a few seconds (20 to 30). Then it turns off and doesn't turn back on when I press the power button. That's why I though it couldn't turn on. Interestingly, if I wait for half an hour, it will be able to be turned on again, but once more only for a few seconds.

    It's an LG, 39LB5600
     
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