Firstly, thank you to everyone who joined in on this whodunnit. It has been fascinating to read so many different approaches to solving the same problem. Some of the responses were quite detailed and a lot of thought went into them. While I posted this just as a fun challenge, I've also learnt a lot from the many knowledgable contributors, especially about the inner workings of a DVR to explain why the video went so weird.
And the winner is … Mark Jones! More about Mark’s answer in a moment.
What was the cause of the problem?
The cameras were all supplied power by a single AC/DC power adapter which was connected to a UPS. The AC/DC power adapter died with only a few seconds of warning so all the cameras stopped working. The green LED on the power adapter was no longer lit so this made it easier to confirm the fault.
The DVR had its own power supply so it kept working and simply recorded black video.
The monitor had its own power supply and was working despite the customer saying that it had gone black and wasn’t working.
What was the solution?
The solution was simply to replace the inexpensive AC/DC power adapter.
It’s difficult to say why the power adapter died. It cannot have helped that the electrician, who originally wired power to the cameras, mistakenly thought they used AC power. There is a green LED on the power adapter and it switched off when camera power connections were connected with the wrong polarity. It’s possible this shortened the life of the power adapter although I was more worried about the impact on the cameras at the time! It was this same LED which helped confirm that the power adapter was dead.
The Winner
Not only was Mark Jones the first member to identify the failure of the power supply to the cameras as the likely cause of the problem but Mark’s detective work was elegant in its logic and simplicity. He identified the components making up the CCTV system, and systematically eliminated all that he could from being a cause of the problem. He narrowed this down to one likely candidate and he was right. Congratulations Mark!
Challenges in solving the problem
In retrospect the problem and solution are simple enough but there were a few challenges to finding and confirming the problem.
- The customer site was remote from me so I could not just drop in, check out the equipment and find the problem myself.
- The customer was non-technical so my troubleshooting questions needed to be straightforward with no jargon.
- The customer did not speak the same language. I suspect the reference to the monitor going black may not have been an accurate translation. While I always take note of what customers tell me, I also try to find ways to verify the information they give me without causing offence! In this case, I was able to use Skype video to see the DVR interface showing on the attached monitor but all camera feeds were black.
- It was unclear if lightning was involved in affecting the CCTV system as the customer was asleep at the time. I checked the camera recordings for the 20 minutes before the camera blackout and found no evidence of illumination by lightning so I ruled it out as a probable cause.
- I also wondered whether there had been a general power outage but as the customer had been asleep, no outage was noticed. By taking remote control of a computer at the customer’s site, I was able to access the admin interface to the customer’s ADSL router/modem and check it’s “up time”. The admin interface showed the modem had been connected since well before the camera blackout event and was still connected. Given the ADSL router/modem had been working the whole time even though it was not protected by the UPS, I ruled out a power outage or power spike as a probable cause.
By ruling out some nasty scenarios, it quickly seemed likely that the cause was limited to internal problems and not caused by outside influences. Asking the customer to check the green LED on the cameras’ power supply rapidly confirmed the simple cause of the problem.
I get a kick out of solving technical problems when they happen and I hope you’ve found this fun too!