Measuring capacitors in circuit

[Prime]

I'm struggling to measure the capacitors in an old Philips scope in circuit. I've tried a bunch of multimeters but they aren't reading correctly (2 Flukes and a Gossen).

I'm not sure if I am being silly and you can reliably measure caps in circuit; or if they are dead and I am ignoring the evidence.

Could I safely use an LCR meter with kelvin probes or do I run the risk of smoking the LCR meter?

Thoughts please?

[tautech]

As a rule any measurement of power rail caps in circuit will be affected by all other caps on that rail so for accurate measurement you'll need to isolate the DUT in question.
This can be accomplished an various ways including unplugging other PCB's, lifting a leg, cutting tracks and resoldering them after.

Just think of it as good soldering practice. 

[wraper]

I'd say that more or less reliably for in circuit measurements will work only ESR meters that output ~100 kHz sine wave and then measure the amplitude of it. All other kinds of ESR meters often will show some kind of impossible BS. Just checking capacitance with multimeter is not nearly enough anyway. Using proper LCR meter for in-circuit measurements will be a lot of hassle and don't work good enough too in my experience.

[kripton2035]

you can get esr meters to build or buy on my repository (web link below)
you will not blow up an lcr meter if the DUT is unplugged and the capacitors discharged.
some esr meters have a protection against this, but it's always safer ti discharge the main capacitors. (and high voltages one in a tv or scope !)

[MosherIV]

Hi

"I'm not sure if I am being silly"
Sorry but yes you are. The way most dmm measure capacitors is to measure the time constant by applying a charging voltage and measuring the time it takes to charge. If you do it in circuit, the measurement will be affected by the other components in the circuit.

The other responders have already mentioned that you can use Equivalent Series Resistance (ESR) meters in circuit. Bear in mind the value they give is not in any way accurate but just an indication. Basically, most caps should show a very low resistance when they are ok, if they go above 1ohm then chances are it is going bad.

Bad capacitor show the correct value when using the time constant method. A LCR bridge should show caps are bad because they measure the AC impedance. Most hobbiest do not need a LCR bridge, they can be expensive and unless there is a need to measure caps or inductor parameters there is little use for them. LCR bridges also cannot be used in circuit .

[ez24]

By now you know you need an ESR

Here is a respected one if you are looking to buy one:

https://www.amazon.com/-5000-Handheld-LCR-Meter-accessories/dp/B00S298KJO/ref=sr_1_1?ie=UTF8&qid=1467447234&sr=8-1&keywords=de-5000+lcr

[wraper]

By now you know you need an ESR

Here is a respected one if you are looking to buy one:

https://www.amazon.com/-5000-Handheld-LCR-Meter-accessories/dp/B00S298KJO/ref=sr_1_1?ie=UTF8&qid=1467447234&sr=8-1&keywords=de-5000+lcr

I have this LCR meter and it completely sucks for in circuit measurments but is decent thing otherwise.

[kripton2035]

I don't understand why you say this meter sucks for in circuit measurments ?
I have one too, and use it very often for in circuit esr measurments...
it does not work for high value capacitors, or many parallels capacitors you have to force Rs measurments.

[Shock]

The issues when measuring in-circuit are:

If the circuit is powered or has capacitors that remain charged and this exceeds the input voltage threshold then you may damage your LCR/ESR meter. Only test when the circuit is fully isolated from the power source and you have confirmed your capacitors are discharged.

Parallel capacitance and resistances will affect the measurement, so it's not always possible to measure in-circuit.

The LCR/ESR meter may turn on semi conductors depending on the voltage it applies to the device under test, this can additionally throw out measurements.

When checking the ESR health of a capacitor Kelvin measurements aren't so useful. Testing ESR is more a ballpark test. It is dependent on the test frequency and should be ideally compared to the datasheet or a known good capacitor measurement, same series and value etc.

For accurate milliohms or microohms Kelvin measurements are more useful. You should verify by testing out of circuit anyway if you find suspicious components.

Some other tips:

For something like a 30 year old piece of test equipment, if you found some marginal components and you were restoring it for your own use then you would be wise to go through and replace at a minimum all the same values and series that are reused throughout the device.

If the capacitors show signs of electrolyte leaking then you risk damaging the PCB leaving them or the residue for a prolonged period on the board. Capacitors can fail open or short or anywhere in-between and can cause damage to other components. They also can become reduced in capacitance or leak capacitance, so keep in mind it's more than just ESR.

If you are working on a circuit where you can determine the failures are design, heat or stress related then you can opt for just replacing the affected capacitors rather than go for the full restoration approach.