How to test capacitors in-circuit?

"phaeton"

** Generally, that is impossible. ** Only electros bulge. ** Film and tantalums caps sometimes do. ** Electros can be checked - in circuit - with an ESR meter.

But this does not find leaky ones.

Particular kinds of circuit misbehaviour indicate a faulty cap at a certain spot.

....... Phil

Are you trying to find something specific, or is this a general question?

One old trick is when you suspect a capacitor, jumper it with another capacitor of the same value. If things improve, then it may be the capacitor. But of course, this implies that the original capacitor is open or has lost capacitance, rather than shorted.

Michael

It is a general question.

(and not a homework question!)

-phaeton

For electrolytics in most circuits, an ESR meter works pretty well. In some circuits, the capacitor will still have to be removed for testing.

Richard

You have a look at either side of it with a 'scope and estimate what you should be seeing e.g. if it is a large coupling capacitor in an audio path then both sides should be the same size.

If it is some sort of first order filter then work out the capacitor's Xc and compare that to the "load" and if e.g. they are the same then half the signal should be dropped across a working cap.

If it is part of a UHF oscillator (that won't) then do as the other poster says i.e. bridge it (hold in place with end of plastic trim tool).

A worse case is where you have a large output transistor with four caps soldered to strip-line pcb layout at UHF and the output power is

10Watts instead of 20. This is straight off the production line and has never worked before. Then you have no option but to remove all four and measure them with a bridge.

Robin

If you're getting paid by the hour, it's probably cheaper to just swap out the cap.

I once worked repairing video game PCB's, and when we'd get a really knotty problem that took too much time to diagnose with the scope, we'd "shotgun" it - just replace every chip on the board, because it was cheaper to take an hour to replace 20 or so chips at $0.30 apiece than to spend four hours troubleshooting at $30.00/hr.

And I've been in the business since 1968, and have never seen a capacitor tested in-circuit, let alone done it.

Good Luck! Rich

I have a nice B&K tester that works very good with in circuit stuff. It's an LCR meter and it's been around for a bit but it's what I use for in circuit tests.

That model isn't have I have but it's close.

--
"I\'m never wrong, once i thought i was, but was mistaken"
Real Programmers Do things like this.
http://webpages.charter.net/jamie_5

I posted a URL of what I use which works very well, I can tell you a basic theory of how it works. The in circuit uses the resistive load in the circuit to offset the reading from the test cycle on the capacitor. It's not perfect but it's very close for doing debug work. what I have found is this. In many cases where a cap test resulting in much higher value than should be, may indicate a leaky cap. Of course, one that test much lower is most likely a weak cap.

As far as doing repairs, I haven't yet pulled a cap from a board using this LCR meter that showed a below normal reading to be false.

I have seen where at times, when it shows far to much Uf on a cap in circuit that it may be a false reading. But this hasn't happen to much.

Don't go for a cheap cap meter. Get one that is designed for in circuit tests.

--
"I\'m never wrong, once i thought i was, but was mistaken"
Real Programmers Do things like this.
http://webpages.charter.net/jamie_5