# Voltage problem in printed circuit with switches

I've got a simple circuit consisting of a 74157 MUX, whose entries come from switches. These entries have also 10k pull-up resistors, so when the switches are open (default), inputs are at 5V. When pressed, inputs are at 0V.

However, one of the inputs (the one connected to R2 pull-up resistor) is having 0.8V when open instead of the correct value of 5V. I've checked and replaced the resistor but the problem remains.

Any hint of what should I check to find out what's wrong?

UPDATE: The problem seemed to be S2 switch pads. I brushed them with solvent, and now, the circuit is working OK. What I'd like to know is why the voltage was 0.8V instead of 0 assuming there was some kind of short in S2. Thanks!

Here's a simplified schematic (only 2 switches represented) of the affected parts:

And here, a real photograph:

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Have you checked the resistance of the resistor in question? –  Majenko Oct 8 '11 at 15:29
Yes, I did. Its resistance is ok. I also replaced it with a new one. @Heller I never said this question was about electronic design. –  grieih Oct 8 '11 at 15:33
Best guess: partial short at S2, or 74157 is dead. –  Majenko Oct 8 '11 at 15:46
@Majenko I'll first suppose 74157 is ok as all other inputs are behaving as expected. How should I check S2 short, cutting its track? –  grieih Oct 8 '11 at 15:53
See posted answer below about cleaning it with a toothbrush and solvent –  Majenko Oct 8 '11 at 15:55

Electronic design section

Try a lower value of R2 - say 1k or lower. Resistor is pulled down by switch which should/may tolerate higher currents. If this works, summat aglae. If this dinna work, summat aglae.

Other:

IC may be damage - quite possible

Switch pad may be contaminated with coffee beer blood crud ... - quite possible

Measure pin to ground resistances. Same?

Scrub pads with toothbrush etc with suitable solvent. Be ware of unsuitable solvents :-).

Scratch along spaces between tracks with scriber or equivalent.

Shouldn't be needed, but cut PCB track from S2 - easily repaired again. Better? Different?

If none of the above:

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