Yup, some of them sure do make noise. Had this happen on a Henry UHF repeater amplifier which are thermostatically controlled. Only got the buzzing when the fans came on, and stopping them with my fingers did NOT cure the noise, so it's not due to mechanical rotation. Henry kindly sent me some 120VAC fans and the problem went away.
Yet I have another continuously running 12VDC fan mounted inside the cabinet and I have no noise from it whatsoever. Some brands are worse than others. Line filtering usually does not cure it; it's radiated from the motor windings and/or electronics. Bob M. ====== --- On Mon, 4/13/09, John Harrington <[email protected]> wrote: > From: John Harrington <[email protected]> > Subject: [Repeater-Builder] Funny noise on repeat audio > To: [email protected] > Date: Monday, April 13, 2009, 10:59 PM > Here's a problem that caused me much > gnashing of teeth and tearing of > hair before I nailed it. Some of you may find it old > news, but here it is: > Mastr II station with NHRC controller. Mobile PA, > with fans added so > the PA would live through nets and long transmissions. > Problems with a > "humming" noise on repeated audio, low deviation, but > annoying. Station > mike clean, but hum (sorta like PL tone) on any repeated > audio. Lived > with the problem for a while, but an overhaul on the bench > on other > problems (whiskers in the rx front end, etc.) gave me some > time to play > with it. Turns out it was the fans! I had used > 4" 12V brushless muffin > fans. The fans were running from the 12V station > power supply. When I > disconnected them in desperation, the noise went > away! I built a little > filter from a toroid core and a few caps in a pi-net, but > the noise was > still noticeable. I installed two 120VAC fans and the > problem was solved. > > My theory is that the 3-phase oscillator in the fans > produced square > waves to run the little fan motor. Square waves have > lots of harmonics > and was radiating them into the rx audio line into the > controller. Word > to the wise-- be careful of 12V brushless fans around > susceptible equipment. > > John W5EME
