[R-390] DC Fan Noise

[n4buq at knology.net]

Esteemed colleagues,

I've been working on a project that will power a small fan on the IF deck.  
I'm using the 26.2VAC supply through a single diode to a 24VDC fan.  This 
yields about 11.5VDC (at least that's what my Fluke meter shows).  I 
realize this is half-wave DC so 11.5 is an "averaging" value.

When I hooked it up last night, my Grundig Majestic nearby picked up noise 
from the fan motor.  It was most evident when the [rather long] ground wire 
connection was somewhat close to the Grundig.  If I moved the wire away 
from the Grundig, the noise faded, but I could still hear it.  Oddly, I 
couldn't detect it in the R390A.

Do DC motors normally generate hash like this?  Is it because I'm feeding 
it a half-wave rippled voltage?  I realize the 26.2VAC peaks are over the 
24VDC rating but figured the fan would be okay as long as the "average" 
voltage is well below 24VDC.  Should this be a problem, given the "average" 
value?

I tried a small cap (0.1 or 0.01 (not sure)) across the fan, but that 
didn't help.  Placing a "large" filter cap briefly across it causes the 
rectified voltage to go to well over 30V or more (approximately 1.414 * 
26.2 (?)).  As the fan speeds up, so does the noise.  I realize the fan 
can't be run for long at this higher voltage, but I was wondering if it was 
more a case of ripple to the fan motor or whether the fan itself was 
generating the noise and it appears the fan is generating the noise.

Any comments?  Is it possibe the Grundig is just not well shielded against 
the noise?  I figure the extra long leads I was working with were acting as 
an antenna of sorts so the noise was being propagated in the shack, but I 
also don't want a little noise generator sitting on the IF deck that could 
wreak havoc on other gear in the shack either.

Thanks!

Barry - N4BUQ