Resistors invisibly failing

Aug 12, 2023 Last reply: 2 years ago 7 Replies
C

Gentlemen,



Whilst trying to discover why there was no audio output from a 4W amp board, I noticed there was only 0.6V on the supply pin of the audio output IC concerned. There should have been 13.6V. This pin was fed via a 1 ohm 1/4W resistor. DC voltage measurements showed there was



13.6V on one side of this resistor and 0.6V on the other! Clearly something amiss here, I thought. So it must have gone open circuit. However, there's not the slghtest sign of any phsyical damage even under high magnification, whatsoever to it: none at all. Prior experience has always taught me such resistors burn out in an obvious way which is dead easy to spot. Not this one. I'm just wondering how unusual this is and if anyone else has encountered such an issue with a resistor. Here's a photo. The resistor in question is the one right next to the largest electrolytic. There it is, looking all innocent like butter wouldn't melt, yet it's caused me a massively disproportionate amount of head-scratching, the little s*it.

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When I get that out of circuit.... just you wait....


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L

Micro crack in the solder?

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C

It'd have to be a pico crack given the hi-mag examination I gave it and found nothing suspicious.

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T

**Many years of experience has taught me to suspect low value (<47 Ohms) resistors and high value (>100k) resistors. Both types can fail with little or no visible evidence. Cracked carbon types are the ones that are problematic, as metal film types seem to be far more reliable.
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D

Possibly a bad wave-soldering job? I've heard of cases where bad/incorrect solder or flux caused a mostly-failed joint... the solder flowed up over the component lead, formed a decent-looking meniscus, but didn't actually bond to the lead properly. The lead could eventually crack away from the solder _inside_ the joint, leading to an open circuit which is invisible to the eye.

You might want to try solder-sucking away the existing joints, re-fluxing, and re-soldering, and see if that fixes it... although I'm not sure if this would teach you anything more than you'd learn by just unsoldering the resistor and measuring it out of circuit.

An internal crack in the resistor is probably more likely, though.

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L

If it were a large heavy component, I would look for circular cracks in the rings of solder around the terminals; the solder blob in two parts with one attached to the pin and the other as a circle surrounding it. This used to happen mainly where the dip-soldering conveyor was vibrated to shake off solder splashes and mass of the large component stressed the cooling solder blob. The centre of the blob was cooled by the pin, the periphery was cooled by the track and the last bit to solidify was stressed into a ring of 'dry joint', which later failed almost invisibly.

I can't imagine that happening to anything as small as a surface-mount resistor, but include this historic information for the benefit of anyone repairing through-hole boards with large heavy components.

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C

No, this is through-hole and about 35 years old. As a mattter of fact, it also suffers from the issue you mentioned above, in the PSU section, where the heat from a couple of 5W resistors exacerbates the problem. In fact the original PSU only lasted about 10 years before requiring a rebuild as a result of this particular design flaw.

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L

Googling F2887103 gives you all the info you'll ever want about R30.

RL

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