Resistor value changed in circuit?

Tidying up minor issues in an smr-4, caused because I ran out of correct value resistors. New ones arrived and I decided in changing them over, lacking a hot air pencil and teh board being tight, that I’d cut off the parts and tack solder on the right values. success and all up and tuned etc, except for the toggle for 2-4 pole. The hardwrae and acble and leds are correct at both ends, and the only thing mucked with are the two 10k resistors between the silfscreened 4/2 and bracketed on the otehr side by cap 18 and cap 22. They both had tested out at 5k in circuit so I tacked on two 10ks. Previous the toggle worked. now it doesn’t and the new resistors read 3.89k and 5k. Weird. Should have left it alone, but my newbie question is what causes a resistor in circuit to have a much lower value? sometimes elksewhere I’ve seen 50% once its in circuit. Puzzled.

I’m just going to undo the tack job, clean and redo.

i guess there is another rewsistance in parallel in the circuit

You don’t have to be puzzled. The meter is sourcing a small constant current and measures the voltage between the two probes. It then uses ohm’s law to figure out the resistance (measured voltage divided by small current sent). If the current sourced by the meter goes through only one resistor, you read the value of this resistor. But in a complex circuit, the current will flow everywhere, not just through the resistor of interest!

Let’s put things another way: You put two 10k resistors in parallel. You put the meter probes on both ends. Would you want the meter to show 10k? The meter only “sees” a 5k resistor - the equivalent resistance of two 10k resistors in parallel.

I should note that in 2.5 years of doing electronics I have yet to see a “defective” resistor.

10k/2 =5k because of the circuit? Its now cleaned up with the new parts and done correctly. Im I still getting 5k measured across each one in circuit and 10k out of circuit prior to install and 10k on the just removed parts that read 5k in circuit and test for 10k when removed. If that is normal I shouldn’t have replaced them in the first place but most other resistors showed true value in circuit. New Mouser parts. Funny business.

I don’t think they are defective, been measuring and I agree, I am just puzzled at why I see that difference, maybe the battery is goiung in the VOM. Mystery.

In summary, you can’t measure a resistor’s value while it is in the circuit. You have to disconnect one end from it from the circuit.

If you have two identical boards but one has a fault then it can be possible to compare measurements at the same points.

so it has to do with the circuit, which makes sense as its about choice of filter mode. and there are two 10k in parallel.

Nothing puzzling. The meter is measuring resistance of whatever connects the two points. Out of the circuit you have one resistor. On the SMR4 board you have, well, a whole populated SMR4 board :slight_smile:

let’s say you have a throat that can take 1/2 gallon of beer / minute. it has a specific resistance to the beer, so it lets through only 1/2 gallon per minute. if there is a second throat, with the same specifications, both together will take a whole gallon of beer per minute. so the overall resistance to the beer has been halved, because they can both take the same amount at the same time. that’s also why beer runs more easily and quickly when enjoyed in huge groups of throats.
the measurement can be distorted however if there are throats that can only take an ounce of beer per minute.

… and measuring a resistor in circuit is like dropping a keg at your friend’s house and trying to deduce his “resistance to beer” by coming back 1 hour later to see how much is left. What? Nothing left? Maybe he’s a drunkard or maybe he just has roommates :smiley:

@rosch That’s a classic! Now I finally understand. That paragraph should be in every beginner’s textbook.

Let’s just not get into explanations of what it would be if the resistors were in series please.