I had concerns regarding this board and was looking for any suggestions/help someone could provide. The schematic focuses on the surrounding components to VR2 (10k trimpot). This pot is reading 3.6k on board and will no adjust any further but does adjust to 10k off board. I measured all resistors and the only off values I found were R35 (4.75k) at 180 Ohms, R33 (60.4k) at 7.41k ohms, and R51 (33.2k) at 2k ohms. They all measure there actual values when disconnected. Is this how the board should function? If not, I feel like it may have to do with something being in parallel.

 

hi tyco,
You cannot measure a pot's value correctly when installed, as it has other resistors in parallel on the circuit.
E

 

measuring any component that is in circuit will provide skewed results because surrounding components will have an effect on the measurement - essentially, rest of the circuit is in parallel with component that you are measuring.

in case of resistance checks, measured value will appear lower than actual (anything between 0 and marked value). for example if some resistor is 10k, it will measure about 10k out of circuit but in circuit, measured value can be anything between 0 Ohm and 10k. and if you swap the probes, value is likely to be different because many parts of the circuit are semiconductors (ICs, transistors,...) and they tend to have leakage current that is larger with certain polarity, specially if they contain diodes (real or parasitic).

in case od R35, one of the nearby components is R49 which is only 100 Ohm. While R49 is not directly in parallel with R35, it connects to one of the power rails and that means bunch of other parts. the more of them, the lower their equivalent resistance is. and that combined resistance plus R49 is what is in parallel with R35.

 

As others have already mentioned, if outside the circuit the pot can be adjusted between 0 and 10k, the pot is fine.

 

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I had concerns regarding this board and was looking for any suggestions/help someone could provide. The schematic focuses on the surrounding components to VR2 (10k trimpot). This pot is reading 3.6k on board and will no adjust any further but does adjust to 10k off board. I measured all resistors and the only off values I found were R35 (4.75k) at 180 Ohms, R33 (60.4k) at 7.41k ohms, and R51 (33.2k) at 2k ohms. They all measure there actual values when disconnected. Is this how the board should function? If not, I feel like it may have to do with something being in parallel.

Hi,

Yes, it's a shame that we can't measure some parts that are already in a circuit because they still interact with other parts. That means an Ohmmeter will measure not only the potentiometer (resistance) but it will also measure other parts of the circuit of which would be hard to determine. That means you can get all sorts of measurements and they may even change with the settings of other pots in the device.

A better way to test some of the pots when they are acting as voltage dividers is to power up the circuit and carefully check the voltage at the arm of the pot. If the voltage varies as it should, then you can figure it as being Ok. You may have to do a little circuit analysis too though to figure out what the allowable voltage adjustment range should be.

It looks like you have other parts circled too so not sure what you want to know about those also.