Cubic Communications CDR-3250 VLF-HF Communications Receiver Fan Noise Solution
Cubic Communications CDR-3250 VLF-HF Communications Receiver Fan Noise Solution
I recently had the opportunity to acquire a Cubic Communications CDR-3250 VLF-HF Communications Receiver (two, in fact), fully functional, in very good condition. This is a very high quality, mil-spec receiver using DSP for the final IF, giving a large selection (51 possible, depending upon the mode) IF filters. It is stable, selective and has one of the best menu systems I have used in a receiver. Also the the audio quality and audio recovery is the best I have heard this side of a tube radio like the R390A or SP600JX. Oh yes, stereo headphones work (mono in both ears) properly from the headphone jack.
The receiver incorporates a small box fan in the rear of the chassis to pull air through cooling the modules and power supply (also at the rear). Unfortunately, due to space limitations, the Cubic Engineers were forced to use a 52mm x 52mm x 12mm , 4500 rpm, 12v fan. This fan is very, very noisy: I measured the noise level of 69dB at 1 foot distance. The high rpm and the fact that the fan blades are very close to the rear exhaust slots (see image) generate a lot of noise. This fan in free air is reasonably quiet as I tested it. Installed and running, it reminds me of the fan running on a loaded Pentium 4 Extreme Edition “back in the day”.
My solution to this problem involved fitting a readily available 60mm x 60mm x 25mm, 3000 rpm, 12v fan to the outside of the case, using a short duct to adapt and mount the larger 60mm fan. This moves the fan blades away from the case and improves the air flow by removing the fan body from the front of the exhaust grille slots. In fact the replacement fan moves more air and is 20dB (measured in the same way) quieter than the original. Since the receiver is mounted in a rack with plenty of room, the protrusion of the replacement fan from the rear of this radio chassis is not an issue. The retrofit is completely reversible and no wiring was cut or altered; the replacement fan plugs into the original fan plug inside the radio.
The duct is made from a naturally occurring, fiber composite material (wood), carefully fitted to allow access to the adjacent BNC and DB-25 jacks. I was originally going to use 3D printing and polymer to design and fab the duct, but I decided that wood is better suited to accepting and holding fasteners.
Shaun Merrigan
