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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:

enter image description here

And here, a real photograph:

enter image description here

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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
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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
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1 Answer

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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:

Go to www.piclist.com
Join.
Ask your question with [EE] subject tag.
Tell me I sent you :-).

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Thanks for your answer. I found resistance between S2 contacts is quite higher than between S1's and every other switch. Measuring 4A and 4B pin resistance to ground on the IC side gives the same value. But when measuring in the reverse side (the side in the photograph), 4B resistance is much higher. – grieih Oct 8 '11 at 16:31
May be switch. May be IC. I assume you mean lower. try toothbrush or similar cleaning. Try cut track (can then check switch in isolation). ... – Russell McMahon Oct 8 '11 at 16:49
Yeah, you're right. Little mistake when measuring. 4B to S2 GND resistance is lower than 4A to S1 GND. – grieih Oct 8 '11 at 17:16
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@RussellMcMahon, I thought your advice was quite apt. Maybe more details about to accomplish those steps for someone whom is not used to this kind of thing. I thought grieih might want some feedback on the measurement he took. – Kortuk♦ Oct 9 '11 at 4:09
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@grieih - Pullup was 10k plus a high impedance pullup from IC. Whatever "gunk" was on the pad formed a low resistance but not zero resistance. As the same current (almost) flows through the pullup R and the gunk, tThe voltages are in the ratio of the resistances as V=IR and I is the same. So (5-0.8)/10 k = 0.8/Rgunk or Rgunk =~ 2k. Changing R2 to say 1K as I suggested (just solder scross existing) would have raised voltage to about 3.3V which would have been a large clue. – Russell McMahon Oct 9 '11 at 16:55
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