My wife works at a surgery center where they had a 36” flat panel TV screen in the waiting room. One day as I picked her up from work she said she had the flat panel in a box.
Sometime during the day the TV began to “scream”; a loud high-frequency buzzing coming from the speakers. They called the local cable company to come out and fix it (thinking it had something to do with the cable box). When the cable tech couldn’t fix it they called in a TV place to fix it. That person told them the panel couldn’t be fixed. Given that it is a waiting room they said replace it and put the old one aside to go in the trash.
My wife (of 24 years) immediately thought “Mike can fix it” and asked if she could take it home. Her boss said “sure, but they said it can’t be fixed.”
O ye of little faith…
As soon as my wife told me the issue I had a pretty good idea what was wrong, but I had to open it up to be sure. I’ll give you the symptoms and if you’ve been around any piece of electronic equipment in the last 10 years you’ll most likely get it right away.
The panel came on, the CCFLs lit and all controls worked. As it came on you could hear the “squealing” start and ramp up very fast to a high pitch. It didn’t matter what was selected as an input, it was still there. Turning the volume control up and down changed the volume of the squeal, but not the frequency. Is it?
- A Giant Hissing Cockroach has taken up residency and gets angry when the lights are on.
- The panel mimicking the machine that goes “PING”, poorly.
- A switch-mode power supply acting up.
- A vacuum leak.
- None of the above.
If you thought switching power supply you’re right on.
Look to the cap
Having worked on SMPS and seen similar problems before I knew one of the power supplies had to be the culprit, and more to the point a filter cap had to be the source of the problem.
I opened the thing up (about 20 screws) and honestly was surprised at what I saw. There were three different SMPS in there as well as a couple of “power” panels. A quick look around the boards and I found the culprit; two of the electrolytics on one of the supplies were bulging.
I pulled the supply out and removed the caps. If I remember the supply was +40VDC (or something) and the cap rating was the same! I have a bunch of old electronic units lying around (who doesn’t?) and found the right value but higher voltage.
I replaced them but had to leave them a little cock-eyed in the PCB, but there was plenty of headroom so I didn’t care. I turned the panel on and presto, no noise. So, a panel that had been deemed “unrepairable” was now mine for the cost of about 40 minutes of my time and a couple of replaced caps.
I admit I smirked that whomever had “designed” the supply had pulled a real “slick” move in spec’ing the caps the same voltage as the supply, but had they not, I wouldn’t have a “free” 36” flat screen. That was about a year and half ago and we’ve used the screen every day ever since.
(Mike Tripoli is an electronics/mechanical engineer. He’s worked on products that have needed a blinking LED as well as heart defibrillators. He really believes that blue electrons taste better than orange ones because the blue ones have a softer shell.