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FAN MOTOR NOISE IN MY RECEIVE

unit_399

EL CAPO
Jun 17, 2008
2,087
2,888
273
ALEJANDRIA, COLOMBIA SA
I installed a small cooling fan inside my base radio to help keep the driver/final cooler. Trouble is, now I get a "whirring" noise in my receive whenever the fan is on. The fan runs off of the base's onboard power supply. What's the secret to getting rid of the noise ?? Any and all suggestions welcome.

- 399
 
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Our fix has been a "common mode" choke made on a T50-2 toroid. Each wire is wound in the opposite direction on one side only of the core. As many turns as will fit, equal number of turns on each side.

Usually works if the problem is RF getting into the radio.

And if the fan is simply injecting audio-frequency noise into the power supply, a small dropping resistor and an electrolytic cap will usually help with that. The choice of a ground point for the fan's negative lead can make a big difference. As a rule, the closer to the main filter cap on the circuit board the better.

More than one way for a brushless fan to sneak into a radio.

73
 
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Along with all those great hint's...

Some little ones...

Not sure of how long that run of wire is, but the makers using such schemes also twirl the wires powering the fan to help act as a choke. They also try to route wire to the fan in regions that are not sensitive to noise so just try moving the wires to another route on the board along the chassis - much like how Cobra and Uniden used to do to their looms on their discrete boards.

Also if the fans case is simple plastic then this is moot but usually the assembly has been known to be "anti static" which means the case itself is not true insulated - it's designed to reduce static when the blades blow air so to reduce that possible CMOS killing potential - the blades and case are conductive - but as a high ohmic resistor not dead shorts - so if the rotating part of the fan is towards the sensitive receive section - try reversing the fan to use it's stationary side of it's housing to provide shielding and lessen it's influence in the proximity of the receive. The leaky fan - might have to be mounted closer to the power leads - the power supply itself with it's big caps and larger case - it may be better to use the power supplies own mounting case and location and just push air around - but if the need arises - then the fan really should be mounted to the heat sink the final and driver use - and understand that location of the fan by the tap to get the receive side - you'll have to make some tough choices - if you can spare some disc caps - the common mode method would be to attach one leg of the cap to the wire then the other to the case - on both leads

Some fans also have a speed monitor - 3 or 4-wire models to help the user or CPU monitor the fan speed - so as the temperature rises it can increase the power to raise the speed of the fan during those peak moments of "Meltdown" - these unused wires need somewhere to go - so they don't act like antennas radiating the emf the induction makes.

Other tips are - if mounting to the chassis - again, the rotating cap of the fan - too close to the side panel - can induce the rotation into the case - turning the fan to stationary side 180 degrees - might help.

Some of the fans have a grommeted front cover that if you remove that foil - which is part of its shield - will expose the bearing - you can try to use a conductive carbon - like graphite - in a small quantity - applied directly to the bearing - might help reduce the noise - that foil can then be reapplied.
 
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These fans are cheap. I've repurposed very many "computer muffin fans" as radio equip. coolers. Never had a noise problem. I think it's an issue with this particular fan you have. Replace it with a different one.
 
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Had a quirky problem with 12-Volt brushless DC-motor fans 35 years ago or so. We serviced a business computer called the "Osborne 1". It ran hot, so a 12-Volt fan would be added to bring down the temperature. Just one problem. AC ripple coming out of the fan motor would make the CRT image "shimmy" left to right. The CRT display just didn't contain enough power-supply filtering. The fix was to install a common automotive "alternator whine" choke between the fan and the power supply. Fixed it right up every time.

73
 

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