Silencing a whining SMPS (NEC LCD 1990FXp)

I've always been pretty satisfied with my previous monitor, an LCD 1990FXp from NEC. The only thing that buggered me was an annoyingly noisy, whining and howling power supply, especially when in StandBy. I have lived with it for ten years now, but only because of a master-slave switcher that cut the power when not needed ;) . Since it was about time for something new, I’ve got myself an EIZO now. I’d have picked another NEC in a heartbeat, but most of the reviews I’ve read were not that promising with regards to noisy power supplies.

Wanting to put the NEC back into use after almost a year, the noisy supply started to p!ss me off again. Since I didn't have another master-slave switcher handy, I tried to tackle the real problem. To locate the component(s) that create that awful noise, I used a microphone with a long tube as a ''sound probe''. Connect that to a spectrum analyser or a headphone amp and I’ll find the culprit within seconds. At least so I thought. Turned out it is not that easy! It actually helped me to exclude most of the parts, like the transformers or anything else consisting of wound-up wire. Some of the capacitors looked promising, although there seemed to be some more sound coming from next to the capacitors instead of from the capacitors themselves. That seemed rather strange to me at first, but actually it was a good hint.

Of course I don't have an original schematic, but this part should fit 99% of the SMPS‘ around.

So I've asked Dr. Google, and he told me that MLCC singing is actually not unheard of! Several capacitor manufacturers will happily tell you about it and even Davey Jones of EEVBlog has a video about it. In a nutshell: some of the ceramics used as dielectric are actually ferroelectric, so that any applied AC will produce a magnetic field and set the capacitor in motion (piezoelectric effect). Since both ends of the cap are firmly soldered to the PCB, the cap will flex itself and the circuit board, too, which makes the board act like a loudspeaker!

There it is, miniscule but a pain in the ass, really!

Since the noises were most annoying when the monitor was in StandBy, I've had a look at the primary side first. There were not that many capacitors, so my pick was this one: C103, the snubber cap in parallel to the primary winding. Due to the notoriously bad marking on SMT caps I unsoldered it and tested it at around 4,7nF. Since I didn’t have anything else handy, I had to use two big 3KV ceramic discs in parallel with 2,2nF each. Tossed them in and – beautiful silence! :) Using an unpopulated hole and drilling a second one, I was able to move the duo to the top side.

The ''Héroes del Silencio'' in position. Looks almost like stock for a PSU made in China...

The PSU is not totally dead quiet, since there are some more SMT capacitors contributing to the concert. Packed up in the monitor case and a usual viewing distance away from your ears, you are hard pressed to hear anything at all, though. You’ll have to get really close with your ears, which makes the monitor ''practically quiet''. It is imperceptible now and certainly no longer annoying to the ears.

 

Stand der Dinge: 24.12.2017. Keine Skripte, keine Kekse. mailto:lasse@preamp.org (PGP-Key)