I’m currently repairing a device and I’m trying to understand what this circuit board does and how, and whether it is causing the malfunction. While doing that I stumbled upon this resistor whose color code does not agree with my multimeter. I’m measuring 152.1Ω, but the way I’m reading the colors it should be the very common value of 69.1MΩ. If I reverse the order, I get 1.51Ω/15.1GΩ/151GΩ, depending on whether the second color from the left is silver, grey or white. Black would give me 151Ω, but it definitely is not black.

The device this circuit board is from is pretty old. I don’t know how old, but there is exactly one IC on it, with a datasheet published in April 1974. The relay in the background has 1979 written on it; I’m not sure if that is supposed to be a year.

Any ideas? Am I reading the colors wrong? Do I trust the multimeter or the markings regarding the intended value of this resistor? Have you seen resistors whose color codes have changed over decades of use?

  • jeinzi@discuss.tchncs.deOPEnglish
    1·
    2 months ago

    I solved it, thanks for your help :)

    The relay winding is powered directly by the mains and current can only be interrupted by the main power switch and one of the two pressure switches. As there was electrolyte in the connecting gas line when I got the device, I’m suspecting the switch is damaged and doesn’t have enough hysteresis anymore, causing the rapid switching.

    • The heatsink is for a triac doing phase angle control. The second pressure switch is supposed to be set to a lower pressure and if it closes, it inhibits the phase angle control, stopping gas production. The problematic pressure switch seems to be just a redunandancy that removes power from the entire circuit using the relay.
    • As I found out, “manually affecting the circuit driving the relay” is just operating the main power switch, and that works without any issues.
    • There is no discrete transistor in the circuit :)

    My solution was to move the set points of the two pressure switches further apart (there are screws on them for that), so the switch inhibiting the triac triggers first again.