BK Precision power supply making strange noise at startup - Page 1
Forum section: electronics / testgear

author: Martin781
author_rank: Newbie
timestamp: April 15, 2025, 06:59:24 pm
content: Hello all,I have a second hand BK Precision power supply, and it always makes a very loud noise for the first 45 seconds after being switched on. The PS is cooled by a large fan on one side and three little fans on the other side. I think the noise comes from them. I am wondering if this is expected or not. I have attached the audio along with this post (see startup_noise.mp3 below).

author: Wrenches of Death
author_rank: Regular Contributor
timestamp: April 16, 2025, 12:06:31 pm
content: Sounds like a dry bearing in the fan.WoD

author: Martin781
author_rank: Newbie
timestamp: April 16, 2025, 12:31:20 pm
content: Some relubrification is probably needed then.

author: Wrenches of Death
author_rank: Regular Contributor
timestamp: April 16, 2025, 02:18:46 pm
content: Most of the stuff today is made to NOT be serviced. A number of the small fans like that that I've had were able to be lubed after either removing the label on the fan hub or cutting a split in the label with an Xactro knife. Take a picture or write down the fan info before removing the label. If it is one that can be lubed, make sure you cover the lube point when you are done or it will become a dust magnet.As new as your PS is, the fans are probably common spec. and easy to find.Keep your finders crossed.WoD

author: Martin781
author_rank: Newbie
timestamp: April 22, 2025, 10:03:46 pm
content: The fans are made by Chiefly. The three small ones are the CC 4020S24M (https://www.chiefly-choice.com/fans/uploads/1229392650lyg44f.pdf).

author: Martin781
author_rank: Newbie
timestamp: April 22, 2025, 10:36:13 pm
content: They are rated as 24V / 0.07A / 7000RPM / 8.60CFM. Would a fan rated for 24V / 0.07A / 8190 RPM / 9CFM be a safe replacement (https://www.digikey.com/en/products/detail/orion-fans/OD4020-24HB/2621123)?

author: thm_w
author_rank: Super Contributor
timestamp: April 22, 2025, 10:39:18 pm
content: Yeah anything about the same power should be fine.I would be tempted to replace with a slower, larger, quieter fan. But if you had no problem with the fan noise before, going with the same spec is good.

author: Martin781
author_rank: Newbie
timestamp: April 23, 2025, 07:39:01 pm
content: Does it matter if the amperage rating of the new fan for the power supply is lower (eg. 0.045A instead of 0.070A)?

author: thm_w
author_rank: Super Contributor
timestamp: April 23, 2025, 10:03:25 pm
content: Quote from: Martin781 on April 23, 2025, 07:39:01 pmIt would just imply that the fan is less powerful, eg not moving as much air, so potentially could overheat (though really unlikely here under normal use).

author: tunk
author_rank: Super Contributor
timestamp: April 24, 2025, 12:51:59 pm
content: Quote from: Martin781 on April 22, 2025, 10:36:13 pmI guess it depends on the CFM ratings.But I would think it's designed for higher temperaturesthat's normal in the UK, so they probably will be fine.

author: Martin781
author_rank: Newbie
timestamp: April 24, 2025, 06:33:29 pm
content: Thanks for your answers. I checked the small fans voltage and each of them is fed with 27.35V instead of 24V. It is a constant voltage, not just for the startup of the power supply. The PS does not seem to adjust the voltage of the small fans, and they are driven with a voltage above their rating...