Measuring soldered parts

I have a board that I know something on it is bad.. I went about testing each component; but what I realized is while the parts are soldered in place, I am not testing the part, but rather, the part in a circuit.

Is there a trick, technique, ability to, for example, read the value of a resistor that is in a circuit? Or is the only way to test it to remove it, test it, and return it to the board?

I just read the value... Resistors are normally printed with the value unless tiny SMT (or
coloured bands if through-hole)

Of course if it is unmarked and you don't know about the surrounding circuitry you have to
de-solder and measure out of circuit.

Defective circuits are usually diagnosed when powered up, by checking the voltages are
as expected (assumes you know what the circuit is).

MarkT:
Defective circuits are usually diagnosed when powered up, by checking the voltages are
as expected (assumes you know what the circuit is).

This should be followed with the following:

  1. When testing a live circuit - power it down, make your connections with the test equipment, then power it back up.

  2. Don't move any of the leads connecting the circuit to the test equipment while it is powered.

If you must move a lead (aka - probing the circuit) - do so with a single lead only.

Power off the circuit, then hook the "ground" lead to a nearby ground (or other common connection - you'll need to make your own judgement here, depending on the circuit). Then power up the circuit, and probe with the other lead.

Do this with -only- one hand (leave the other in your lap - keep it away from the board), and pay attention as you probe. Doing all of this is to prevent shorts, as well as to keep you healthy (on low voltage circuits, this may not be an issue - but on mixed mains/logic voltage circuits, or on HV circuits - it could save your life).

Using a single probe (and hand) will also minimize any body resistance or other issues (capacitance and such - especially when working with RF circuits or similar) from playing havok with your measurements, and it will also help prevent the "nudging" of parts that can lead to looseness, shorts, or such (this is especially true when probing a breadboarded circuit).

Finally - the "one hand in pocket" rule is played here - learn it, and love it - get used to it on low voltage boards, so it is a habit when you deal with higher voltages.

jAssing:
Is there a trick, technique, ability to, for example, read the value of a resistor that is in a circuit? Or is the only way to test it to remove it, test it, and return it to the board?

The “trick” is absolutely to know what the circuit is. Unless you have that understanding, you will generally get nowhere. And generally as others have said, you need to test the circuit in operation for voltages and signals.

Now digital multimeters tend to test resistance using a voltage which is less than the conduction threshold of a diode, so resistance can be checked directly in simpler parts of the circuit. Unfortunately, unlike in the “old days” of valve radios, resistors are most unlikely to be at fault unless they are burnt to a cinder.

Thanks folks.
I got the gist of the board & what it does. (controls 2 sets of 3 digit 7 segment LED)
You can't really measure the resistors, which is what I was after -- when "in a circuit" other forces, even powered down / off , come into play.

As near as I can tell, everything checks out, except 2 220uf capacitors which seem to hold about .400nf.

Some resistors measure close their value, others way off, but tracing around I can see how I could get two or three (or more) resistors being measured at once.

I don't have a schematic, so it's a bit harder (especially not being an EE person) to decode.

Thanks for the tip on resistors, That really leaves two potentiometers (one doesn't seem to have a full swing to 0) and the two diodes. I have not tried to check the 2 IC chips.

A few transistors I would have liked to been able to test in-circuit. I have failed at getting results I think I should be seeing, so I may just replace them (I have new ones) & the two capacitors; if that doesn't do it; I will try to get #'s off the potentiometers & order up duplicates & see what they should be doing.

I was hoping a magic wand of some sort to allow me to plug on resistors & transistors (etc) to test them "as if they were not in the circuit". Having test points would have been great, but there A) doesn't seem to be any obvious ones & B) no schematic or guide to tell me what proper values should be.

Thanks. I guess I'm on the right track with figuring it out (or not).