When capacitors in a computer monitor's power supply failed, the problem was obvious. Take a look inside of the AC mains power adapter.
Aluminum electrolytic capacitors fail. It's what they do. That's the bad news. The good news is that when aluminum electrolytic capacitors fail, they're easy to spot. That was the case when an AC mains power adapter used on a computer monitor failed.
A few weeks ago, the computer monitor on my wife's PC began having intermittent problems. The screen image would shift to the right, making it impossible to reach the slider bar. At first, the fix was to have the monitor readjust itself through a sequence of button pushes. I made a diagram documenting the sequence so she could fix it herself.
Then one day, the monitor failed. The "on" indicator LED flickered when it should have stayed on steady, and the screen was dark. Fortunately, the AC mains power supply was external. Looking at the adapter's LED, I saw it flickering as well.
There are many replacement 12-V, 4-A AC adapters on the market with prices ranging from $7 to $35, but finding the right one wasn't so easy. Why? Because some DC connectors had 2.5-mm inner barrel diameters, while others were 2.1 mm. Which one did I have? Lacking a caliper, I was able to determine that mine was 2.1 mm by inserting Allen wrenches into the barrel until I found one that fit. Unfortunately, I also needed a right-angle DC plug because whoever designed the monitor placed the DC power socket directly behind the monitor's stand support. Finding the right combination of output power, barrel diameter, and connector angle proved futile. Fixing the power adapter was a better solution and more fun.
Opening the power adapter revealed two 1,000-µF electrolytic capacitors (Figure 1). The tops of both caps were bowed outward. That was a clue. But as long as I had the power adapter out of its case, I had to take a look.
Figure 1: Two 1,000-µF, 16-V capacitors with bulging tops showed their age. (Click image to enlarge.)
I really hadn't expected to see so much shielding around the switching power supply. Maybe this one wasn't just another super-cheap AC power adapter. After all, it worked for 12 years. The manufacturer is Linearity 1 Electronics.
Figure 2 shows the underside of the power adapter, Note the second board over the main board where the shield attaches. The power adapter had more shielding than I expected. The shield connects to the main board's circuit ground through a single wire. I cut the shield along a line between the two arrows to separate the shield from the bottom board.
Figure 2. The board underneath the main circuit board connects to circuit ground through a wire and to the wraparound shield. (Click image to enlarge.)
Figure 3 shows the top side of the board and components, though there are some active components on the underside of the board as well. A transformer isolates the AC side from the DC side.
Figure 3. The AC power adapter has a clean layout with the AC input and DC output on opposite ends. (Click image to enlarge.)
I removed the wire, pulled the lower board up, and could then see the underside of the main board (Figure 4). Note the active and passive components on the board's underside. Given the thermal paste between the main board and the secondary board, it appears that this secondary board is used for both EMI shielding and for dissipating heat.
Figure 4. The underside of the main board has both active and passive components. (Click image to enlarge.)
Next, it was time to replace the bulging capacitors. That was easy. Figure 5 shows the underside of the main board with the capacitors removed (yellow circles). I did have to change soldering irons because my old one (an old Weller iron with a cork handle) didn’t provide enough heat. Replacement 1,000-µF, 16-V, 105°C caps were available at my local electronics store for $1.07 each. I found some for $0.32 online but didn’t want to wait for (or pay for) shipping. Add $1.35 for some thermal paste around the new capacitors and between the two boards.
Figure 5. Circles indicate where the failed capacitors used to reside.
One of the failed capacitors appears in Figure 6, while Figure 7 shows a new capacitor still in the package.
Figure 6. One of the two failed Samxon capacitors, removed from the AC power adapter board.
Figure 7. One of two new NTE capacitors was all that I needed. Even if they only last a few years, the monitor will have already lasted a long time.
After installing the new capacitors and adding some thermal paste, I applied AC power to the adapter with the DC connector open and measured the voltage. Sure enough, it close to 12 V and the power adapter's LED was on steady. Figure 8 shows the monitor working again. Total parts cost: around $3.50. All that was left was to reconnect the EMI shield by soldering it across the cut and inserting the power adapter back into the case.
Figure 8. The LCD monitor is back in service.
Perhaps I should have replaced the other aluminum electrolytic capacitors while I had the case open. The plastic case was held together with glue but easy to open. To keep the case closed, I used two cable ties. Management is happy, at least for now.