Fooled by idiosyncratic resistor network

Ken YapKen Yap wrote 09/29/2021 at 12:34 • 2 min read • Like

One of my projects uses a 4 unit resistor network for switch pullups. I picked one out of my pile of spare parts, some stripped from old boards. However the buttons behaved strangely, one worked, but another caused strange symptoms. I checked the circuit board for breaks in the trace to the faulty button, but seeing as it did something, that couldn't be the reason.

Finally it occurred to me to measure the resistor network. It turned out that contrary to nearly all resistor networks sold today where the common junction is at one end and indicated with a dot, or band, this one had the common junction in the middle. There was a filled circle in the middle of the text, thus: 10 ● kΩ. I thought it was just strange punctuation, but the circle was the position of the common pin. Once I replaced it with a regular unit, the circuit worked as I wanted. Lesson to me: check components when populating the board or face rework.

I couldn't even find a picture of such a resistor network; it must be rare. I know that manufacturers sometimes used resistor networks with unusual topology for boards, like A-D converters.