Can a resistor short?

Discussion in 'General Electronics Chat' started by uriahsky, May 31, 2009.

  1. uriahsky

    Thread Starter Active Member

    Apr 27, 2009
    I have a few questions about some test gear I am trying to fix. There are a couple of one-watt carbon resistors that I am unsure about. One has what looks like a overheating burn on it. It is color coded Beige?,brn,blk and gold. It measures at 8.0 ohms out of circuit. Is that correct? Can a resistor go up or down in value by just a little bit? Can a carbon resistor ever short? The other resistor is marked Red, Red, Gold, Gold, and the last band I can't make out. It measures 3.7ohms. I think it should be 2.2?? Could it possibly have gone from 2.2 to 3.7ohms from overheating? Or did I just read the bands wrong. Is that enough to worry about?

    And: This unit is microprosser controled. When it first turns on it boots fine but then it keeps rebooting. I can get it to work by pusing some buttons and kind of distracting it and it will be ok, but if something is rebooting what would you check? It is using a NSC800N-1 40 pin uP and has a 27C64 ROM chip as memory. Can I get some clues on the rebooting by checking pins on these chips. The voltage is stable at 5VDC, what else would cause the uP to reboot? Is it looking for a high or low that might not be there?
    Thanks for the help.
  2. SgtWookie


    Jul 17, 2007
    Well, "beige" isn't one of the standard colors.
    Black, brown, red, orange, yellow, green, blue, violet, grey, white; then silver, gold.
    Might've been red or orange? 21 or 31 Ohms, respectively?

    The old cylindrical carbon resistors (dark brown package) can change quite a bit over time, even if unused. I have some unused mil-spec (1% tolerance) resistors that were made in the late 1960's and early 1970's that have varied greatly in value (some more than double their marked value) since they were made.

    It might be possible for the old-style resistors to decrease in value if the package itself got "baked" and shrunk, but it's likely that it would burn up before it would short.

    Red red gold gold should be 2.2 Ohms, like you said.

    Check the board very carefully for cracked/broken solder joints. If there are tantalum or electrolytic caps on the board, replace them with new stock. Check the bypass caps across the Vcc/gnd pins of all the logic ICs.

    If the board has been subjected to lots of temperature cycles, you may have metal fatigue setting in. This can be mighty tough to find and fix.