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APC Back-UPS 700VA 230V outputting noisy electric wave


APC Back-UPS 700VA 230V outputting noisy electric wave

This was originally posted on APC forums on 3/31/2021


I bought an APC Back-UPS 700VA 230V almost a month ago, online. I connected a recently built pc to the UPS and I started the PC. It got stuck on boot before the operating system, because I could see by the ethernet switch light that the network card was only operating at 100 Mbps (instead of 1000 Mbps). The reset button didn't work, so I had to shut it down holding the power button. When I started the pc again, it wouldn't boot. After I clicked the button, it stayed on for less than 1 sec and then it immediately stopped. I tried connecting it directly to the wall (bypassing the APC Back-UPS 700VA 230V) and the problem was the same: only stayed on for less than 1 sec.

I tried changing all the components except for the CPU and the motherboard and the problem persisted, so I concluded one of these or both were at fault. I also concluded that the fact that they failed when I connected the UPS was a coincidence. I also performed a reset CMOS and the problem persisted.

A week later, and before I took the motherboard to the store where I bought it, I decided to test it again, just to make sure I wouldn't seem ridiculous at the store, going there with a working motherboard and presenting it as faulty. The motherboard also didn't work when connected without the UPS, a week later. Then, I took the motherboard to the store and it worked fine there, on the exact same day. So, my attempt to not seem ridiculous seems to have been useless.

When I got back home, I tested the motherboard without the UPS and it worked fine, just as it worked on the store. I then connected it using the UPS and it got stuck on boot again, but the behavior was different than last time. I think (not sure) the CPU debugging light stayed on (suggesting a fault on the CPU). I shut it down, by holding the power button, booted again and it seems to have got stuck. I repeated the process multiple times and it got stuck some of the times and others not. I connected it directly to the wall (without the UPS) and it worked fine everytime.

So, it seems that the cause I thought most unlikely is the real cause: the UPS. However, I would have never thought a UPS could cause "complex problems" on a computer. I thought it could cause it to either (not power on at all) or (power on properly), but it seems like that is not true.

So I decided to measure the UPS' outputs with a multimeter. The voltage seemed just fine: 230V AC. However, when I switched the multimeter to measure the frequency, things got really nasty: the meter showed that the frequency changed 3 times per second and would sometimes go from 40 to 110 Hz in just one second (when it should be at a constant 50 Hz, the standard frequency in my country). It is possible that the frequency changed more than 3 times per second, but the polling rate of my multimeter does not detect that. I don't have an oscilloscope to see exactly how the signal changed over time. I tried measuring the frequency at another power socket (that was on the same power strip, connected in parallel with the UPS) and the frequency was also changing, but less than on the outputs of the UPS. I disconnected the UPS and the frequency stopped changing in the power outlets.

So, it seems that this UPS is altering the frequency and it, somehow, caused the motherboard to fail. It also caused it to fail even after disconnected from the UPS. Apparently, and somehow, connecting it to a power socket at about 10 miles away, made the motherboard come back to life. Connecting it again to the UPS make it act oddly, but did not manage to kill it this time and it didn't need to be connected to that 10 miles away magical socket to come back to life.

I have two APC Back-UPS Pro models and I've never had problems with them. Since now I wanted to backup a less important system, I decided to go with a cheaper APC Back-UPS (not Pro), but it went terrible wrong and I could have damaged the motherboard, which could die any second.
Is it common for non Pro Back-UPS models to show problems? I'm going to RMA the UPS, but do you think I should trust a model that comes as a replacement or should I get a Back-UPS Pro? Is there a signifficant difference in quality between the two?

Thanks in advance.

Best Regards,

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