Novice seeking help - info on transistor please

Thread Starter

ColoradoRobert

Joined Jan 22, 2016
155
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.
 

dl324

Joined Mar 30, 2015
11,848
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 ?
That component is a resistor and the value is as you described.
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.
For a 5% tolerance resistor, the bands would be brown-gray-brown-gold. If it's carbon composition, there may be some extra bands.
 

Thread Starter

ColoradoRobert

Joined Jan 22, 2016
155
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.
 

shteii01

Joined Feb 19, 2010
4,644
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.
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.

 

dl324

Joined Mar 30, 2015
11,848
After taking a picture of these the lines look more clear. Any guesses on values?
The top one looks like (red or brown)-(??)-brown-gold. Bottom brown-black-orange-silver
In the daylight, it looks like the stripe on R20 has a gold stripe and R21 has a silver stripe.
R21 = 5%, R21 = 10%
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.
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.
 

Thread Starter

ColoradoRobert

Joined Jan 22, 2016
155
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?
 

dl324

Joined Mar 30, 2015
11,848
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?
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.
 
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.
 

Thread Starter

ColoradoRobert

Joined Jan 22, 2016
155
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?
 
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