Novice seeking help - info on transistor please

Discussion in 'General Electronics Chat' started by ColoradoRobert, Jun 5, 2016.

  1. ColoradoRobert

    Thread Starter New Member

    Jan 22, 2016
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    So I've decided to try an fix a few guitar amps by mostly doing a visual inspection on them. I do a little with my cheap multimeter too but where I find hot marks on the PCB, it's those things I usually just change out with great success (so far). Of course the easiest parts to change out are the ones with part numbers printed right on them but it's these pesky resistors that are giving me problems especially this one who's stripe colors are not identifiable and wattage/voltage in the area has me wondering too. So I have a very beginners question for you tech guys. First, I was able to locate the schematic for the board and track down the resistor location = written in the footprint for the resistor is '180,' then the circuit line continues out from there and is written 3W. So does this mean that this transistor is 180 ohm and 3 watt ? * Please take a look at the section I cropped and give me your opinion. Thank you so much. * Secondly, I am lost on the stripe problem because the colors on this thing are so indistinguishable, that I don't know what to do to match it exactly. Opinions? Thanks.[​IMG]
     
  2. dl324

    Distinguished Member

    Mar 30, 2015
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    That component is a resistor and the value is as you described.
    For a 5% tolerance resistor, the bands would be brown-gray-brown-gold. If it's carbon composition, there may be some extra bands.
     
  3. bertus

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  4. MaxHeadRoom

    Expert

    Jul 18, 2013
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    For resistors, you can generally get an indication of the wattage by the size that would confirm the 3w rating.
    Max.
     
  5. shteii01

    AAC Fanatic!

    Feb 19, 2010
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    Looks like R20 is identical. If R20 is intact, use it as a model for the R21.
     
  6. dl324

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    You saw the post before I corrected it from 18 to 180 :rolleyes:
     
  7. ColoradoRobert

    Thread Starter New Member

    Jan 22, 2016
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    eh? After taking a picture of these the lines look more clear. Any guesses on values?

    I know Mouser has a place on their website where you can plug in the stripes and it will then
    give you a list of results.... but on these resistors they are of a sprayed on material, I even
    imagine calling them a fuzzy type coating; not the high gloss defined type.

    In the daylight, it looks like the stripe on R20 has a gold stripe and R21 has a silver stripe.

    Talking about intact.... I just did a continuity check across both of these and they check out
    unbroken. This is pretty much all I do and know how to do. As I said earlier, I change out
    anything that looks overheated. Around the legs of R21 it looks overheated and the Motorola
    chip Reg 2 also. That chip is on order but this resistor is troubling me trying to find an
    exact match since I am not sure what it is.

    BTW, I do not understand tolerence and why (or if).... it has to be exact for everything to
    work properly (novice speaking here). Can you explain why or why not in easy terms tolerence
    must be exact or everything will go up in flames. Thanks for your sharing. [​IMG]
     
  8. shteii01

    AAC Fanatic!

    Feb 19, 2010
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    A simple explanation of tolerance.

    World is not perfect. When we, the people, make stuff, the stuff we make is not perfect. So. The question. How IMPERFECT is the stuff we make? Tolerance tells you how imperfect the item is. Take the gold band on the resistor. That is 5% tolerance. If I have 100 Ohm resistor with gold band (5% tolerance), what does it mean? It means that my 100 Ohm resistor will have value between 995 Ohm and 105 Ohm.

    The silver band is actually 10% tolerance. Meaning 100 Ohm resistor will have value between 990 Ohm and 110 Ohm, more variation in value of the resistor. This is actually not as good as the gold band resistor, but, theoretically, it should have been cheaper to buy. Makes you wonder why in one portion of the circuit they used a tighter tolerance (gold band) resistor and in another part of the circuit they used a sloppier tolerance (silver band) resistor.

    [​IMG]
     
  9. dl324

    Distinguished Member

    Mar 30, 2015
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    The top one looks like (red or brown)-(??)-brown-gold. Bottom brown-black-orange-silver
    R21 = 5%, R21 = 10%
    In general, using a tighter tolerance resistor won't cause any problems. The opposite isn't true.

    In general, substituting a resistor with a higher power rating wouldn't cause problems, while substituting one with a lower power rating could.
     
  10. ColoradoRobert

    Thread Starter New Member

    Jan 22, 2016
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    Does everyone see the brownish marks on the board. I used alcohol over this area with a swab and it is a heat mark burned onto the board, in my opinion. Therefore I want to change both the chip (as I mentioned earlier) and this silver striped resistor.

    If what is printed on the board can be relied upon then I think I need to search for a 180ohm 3W resistor with either a 4 stripe silver or gold band on it.

    But further still.... (and Thank you for the chart), it is similar to what I saw on Mouser. Confession time: I do not understand how the lines are used. Any simple explanation and example for these and must they be exact to what I am removing?
     
  11. dl324

    Distinguished Member

    Mar 30, 2015
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    On a 4 band resistor, the first 2 bands are significant digits, 3rd is the multiplier, and 4th is tolerance. Note that gold and silver are not valid in the first band, so you can determine the direction to read the bands.

    brown-gray-brown-gold = 18 x 10^1, 5% tolerance
    brown gray-black-gold = 18 x 10^0, 5% tolerance

    As noted earlier, wattage is determined by size. But be aware that manufacturers make 1/4W resistors that are the size of what used to be 1/8W exclusively; ditto for 1/2W and likely others. I guess they're doing anything they can to make things more confusing.
     
  12. bertus

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  13. KeepItSimpleStupid

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    Mar 4, 2014
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    One thing that wasn't mentioned is the type of resistor or the material they are made from. Some examples are:
    Carbon composition
    Carbon film
    Metal film
    Metal oxide
    Wire wound

    The resistors you showed look like metal oxide.. Sometimes these are known as flameproof resistors. They tend to totally disintegrate when severely overloaded and are sometimes used as a "fuse".

    Carbon composition tends to go up in value and actually crack. They have a fair amount of inherent electrical noise.

    Wire wound is just that a piece of resistance wire.

    Metal film is used a fair bit on precision resistors and they have a lot less inherent electrical noise.
     
  14. ColoradoRobert

    Thread Starter New Member

    Jan 22, 2016
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    Everything I've looked at around the net would lead me to believe these are fire proof wire round. Still trying to figure out those colors and does the 180 mean 180 or 180k? And should I go with what is on the PCB 3w or can I go anything up to that?
     
  15. dl324

    Distinguished Member

    Mar 30, 2015
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    Brown-gray-black is 180, 180K would be brown-gray-yellow.
    3W or larger.
     
  16. KeepItSimpleStupid

    Well-Known Member

    Mar 4, 2014
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    It's a 180 ohm 3W metal oxide resistor. e.g. http://www.mouser.com/ProductDetail/KOA-Speer/MOS3CT631R181J

    180K makes no sense in the circuit. The resistor is basically used as a fuse.

    Reading them can get confusing. There are different "sets" of values as well. The 1% values are not the same as the 5% values available.
     
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