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stewj 31st January 2012 04:14 AM

Bad resistors - how common?
I'm a relative beginner using 2 10k resistors from RadioShack (271-1126) in my chip amp and getting pretty bad distortion through one channel. The problem - one of them measures .9K ohms. They are both marked correctly (brown-black-orange), both gold band, one measures 10K, the other .9K. What gives - is this common? Should I measure them all before installing?

kaos 31st January 2012 04:55 AM

Mislabeled resistors happen sometimes. When itís really bad is when they put the stripe on the wrong end of rectifiers.

anatech 31st January 2012 05:24 AM

Hi stewj,
It can happen, depending on where the parts come from. The quality of the resistor also can matter. You can even have manufacturing defects like poor end cap contact(s).

Should I measure them all before installing?
Sure, yes. I would recommend that, especially if you aren't that skilled in fault locating. I measure almost every resistor I use, and most capacitors. I buy from approved distributors as well.

Components are variable in performance. It's easier to find them before installing them in a circuit.


Enzo 31st January 2012 07:14 AM

Stew, I have been soldering for close to 60 years, I was building amplifiers in the 1950s, and I have been running a pro audio repair facility for the last 25 years. And you know, even after all that time, I always measure a resistor when I pull it from the drawer.

Are you measuring that .9k with the part in your hand, or is it soldered into the circuit? If soldered in, unsolder one end and lift it from the circuit board. Now you can measure it without other parts confusing the readings. If that is indeed what you did, then as the guys said above, you got a mismarked part. It happens, even from good suppliers. A company like Mouser buys resistors by the millions, they don;t have a guy check each one. If they get a bunch of complaints from people over the same part number, they will check the bins, otherwise not. Radio Shack didn;t make those parts, they bought them as commodity parts somewhere.

indianajo 31st January 2012 04:19 PM

I have quit buying from RS, as a lot of parts I got from them were rejects. Their clip leads are okay, that is as far as I will go. Maybe their cellphone batteries are okay.
However, it means it the cheapest parts I get are $6.70, the cheapest freight from anywhere that stocks anything. I find better parts in televisions and computer monitors left out on the curb for the garbage man than I find at RS. I'm buying mostly dozens of metal film vishay or multicomp resistors from newark. Usually $.02 to $.10 each. They are 5% tolerance. Sometimes all the parts will be 4.99% off value, all exactly the same value.
If newark sells rejects, they have a little symbol for price reduction. Most of the resistors I bought with the bargain symbol measured okay. But the TO3P transistors with the 200mw rating, did in fact blow up if you put as much as 4 ma on them at 90 V. The regular parts had a 200 W rating. Read the selector chart, it tells you something useful. Other vendors like digikey don't have nearly as much information in the selector chart.

davidsrsb 31st January 2012 10:51 PM

Matching resistors left and right channel is often more important than getting extreme accuracy in most audio circuits, but this case looks like a binning mistake. What I have seen is red paint looking like orange, so a 1k brown black red looks like 10k brown black orange

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