Checking capacitors in circuit

[Crikey12000]

I understand that to get absolutely correct readings, a capacitor must be taken out of circuit.

Currently I'm more interested in just "dead or not dead" without removing from the board [testing a 5-6 year old motherboard]

My method is to set my old analogue multimeter to Ohms, probe the points of the capacitor on the bottom of the board, and see if resistance goes to 0 and increases.

If a cap goes to 0 and stays, it that is it? (officially shorted/dead) Or could it be affected by other circuitry

[mariush]

You can't.

On motherboards, capacitors will often be connected in parallel with other capacitors. Not necessarily other electrolytic capacitors, often the electrolytic capacitors are also in parallel with ceramic capacitors of various capacitances (usually 10nF, 100nF , ~470nF etc, to reduce EMI, for decoupling etc)

The meter will measure the result of all those capacitors, not just that one you want, so the results will not be accurate. Unless you can SEE the traces the capacitor is soldered to, and what's around the capacitor to be sure nothing will affect measurement, it's safer to desolder a lead (or both) and test the capacitor.

[David Hess]

If a cap goes to 0 and stays, it that is it? (officially shorted/dead) Or could it be affected by other circuitry.

Aluminum electrolytic capacitors usually fail open.  Solid tantalum capacitors usually fail short.

If you have some idea of the ripple current through the capacitor from the design, then a ripple voltage measurement while operating may be better than an in-circuit ESR measurement.