How to Spot Faulty Capacitors

My wife’s Samsung Syncmaster 226BW LCD monitor started failing by flickering, flashing, and blinking at power up. Similarly, my friend’s Samsung 206BW monitor wouldn’t even turn on anymore. The root cause is a power supply failure due to bad caps.

There are lots of great web pages that include instructions on how to replace the aluminum electrolytic capacitors to repair the monitor. I was skeptical that it would be as easy as desoldering the blown capacitors with new parts, but in one evening I fixed two LCDs! So, don’t be dissuaded from trying it yourself.

I took some pictures that I thought might be helpful to others in determining questionable capacitors that are installed on a circuit board, regardless of the type of device. Sometimes it’s really obvious when a capacitor has gone bad, but sometimes it is a bit more subtle. Here are the actual capacitors in the broken LCD monitors.

Bad electrolytic capacitors with residue bulging and lift

Notice the brown crusty discharge at the top of the capacitor? That’s the electrolyte that is supposed to facilitate charge transfer across the storage plates. Capacitor C110 is bulging at the top, which means the electrolyte is trying to burst out of the vent.

The overheated, dried, and crystallized electrolyte will also try to force its way out the bottom. It pushes against the rubber seal on the bottom and lifts the capacitor case off the board. Here is a better view of that.

Faulty capacitors that are crusty and lifted

Contrast the damaged capacitors with a good one on the same board. Notice that the top vent on the capacitor is flat and clean. The good capacitor sits flush against the PCB.

Healthy capacitor with flat clean top and flush to board

But, what about that yellowish white gunk on the side of the capacitor? That’s adhesive. It prevents the capacitor from becoming damaged or detached due to vibration, such as during shipping. If you see goop on parts that looks like industrial-strength hot glue, then that’s okay!

Quiz time -- spot the bad capacitors.

Spot the bad capacitors

If you said “all three of them”, then you’re right! The tops are slightly domed and two of them are obviously lifted off the board. The adhesive seems to be somewhat holding them down, but one of the capacitors is clearly tilted underneath.

Even though none of those capacitors show discharge, they’re all bad and need to be replaced. Good electrolytic capacitors have flat tops.

By the way, did you know that the lines in the shape of a plus sign on top of the capacitor can are intentional? That’s an engineered seam to safely split and vent any pressure build up, rather than exploding.

Replacement Capacitors

For my monitors, all of the bad capacitors were CapXon brand. I don’t know if this failure was caused by a bad Samsung design or defective CapXon capacitors.

I decided to replace the capacitors with the best electrolytic brand I know -- Nichicon. But, then I overdid it by ordering the variant with the lowest resistance and longest rated life. This meant that the new capacitors were larger than the bad capacitors, and they didn’t fit in the monitor.

So, I hacked them in. Rather than being upright, I tilted them down and placed heat shrink tubing on the leads. This is not ideal, but it worked. Please don’t tell my friend that this is what it looks like inside his repaired monitor.

Replacement capacitors made to fit

Measured Capacitance and Resistance

After removing the bad capacitors from the board, I measured them with a DE-5000 LCR meter. These types of meters are similar to a multimeter, except they are specialized to accurately measure other aspects a capacitor, such as its resistance.

Generally speaking, an ideal capacitor would have no resistance. It could charge and discharge without wasting energy as heat. And, it would charge and discharge instantly. But, realistically, every part has some resistance, and capacitor resistance is usually low enough to not be a critical factor in common circuits.

Anyway, here are the measured values of the bad CapXon capacitors from the bad Samsung monitors, in comparison to the good replacement parts.

	Rated Capacitance in µF	Measured Capacitance in µF	Resistance @ 120 Hz in Ω
Samsung 206BW
C261	1000	65	15.5
C263	1000	100	6.0
C265	470	19	50.0
Samsung 226BW
C110	820	82	15.5
C111	820	82	17.2
C112	330	24	58.0
New Parts
C110/C111	820	757	0.1
C112	330	307	0.1
C261/C263	1000	985	0.0
C265	470	464	0.0

A couple of notes:

• The capacitors were measured at 120 Hz. This means the meter charged and discharged the capacitors 120 times per second, which matches the specified value in the datasheet for the new parts. If you measure the capacitors at a different frequency, they will show different values. Different capacitors are designed for different tasks.
• The bad capacitors have a much lower capacitance than their specified value. Well, their electrolyte dried up and leaked, so what do you expect? Obviously the circuit was designed for approximately the full value, so it isn’t going to work correctly at 1/10 the capacitance.
• Surprisingly, the new capacitors have a slightly lower capacitance than their specified value. That’s okay. Most bulk electrolytic capacitors can vary by up to 20% (high or low).
• The bad capacitors have extremely high resistances. 58 ohms? That’s going to restrict current and consume power. In comparison, the new capacitors have such a low resistance that they are almost unmeasurable by this meter.

As you can imagine, a bad capacitor is going to have other measurable characteristics that are suboptimal. Nevertheless, I was surprised to see any capacitance whatsoever. I guess that’s why an electronic device can slowly fail or still “sort of” work.