Hello!
don't measure after mounting, maybe its measuring over all resistor
Yes, I would guess that this is the problem. See attached picture.
Problem: measuring a resistor without unmounting it.
If you use your tester, then you will inject come current in resistor R1. But
as there may be other resistor or components, some current will also go in the
other parts and you will never be sure of the resistance you are currently measuring.
The solution is illustrated with one particular example. You want to measure R1, but
there are other resistors R2 and R3. The solution: you add an op-amp that copies the
voltage at point A to point M. In this case, if VA = VM, the current i1 will be 0
(Ohm's law). Therefore if you measure VA and know the current you are injecting,
you can calculate R1 accurately.
Important note: at first, you may think that copying voltage of point A to point M
could be easily done by using a plain wire. But this will not give you the right measurement
because in this case, I1 will not be 0, but IA/R3. Why is it different? Because the amplifier
brings the right amount of current that balances exactly the system so that nothing
is going through R2. Using a wire, you have indeed VA = VM, but the current is not 0
and therefore the resistor measurement is not right.
Dora.