[Milsurplus] Fan noise and the message

[CL in NC]

The note on the mounting panel of the control box is interesting to me in that the mount would have been installed in the aircraft first, so perhaps when the woman installed the control to the mount, she wrote it.  I actually put my callsign and date somewhere hidden in everything I have fixed for folks, or things I have bought to repair and resell.  

Which brings up this topic from the comments on noisy switching supplies, a DC brushless fan.  I bought two 4" elcheapo fans from Amazon, they came in a package of 2 for 10 bucks or so.   When I put one in the amp I was adding cooling to, which also had a receive preamp, it made a buzzing noise in the RX.  I had hooked the fan to the DC line for the internal relays that was noting more than the half wave rectified filament with a 1000uf cap.  I tried the usual things on the fan wires, tight twisting the wire, caps, wrapping around a toroid, shielding, made zero difference.  I disconnected the fan and hooked it to a separate 12 volt PS, quiet as a church.  Then I tried a pair of 100 ohm resistors in parallel with a 100uf cap to ground to feed the fan from the amps DC line, and it was then totally quiet in the receiver.   The preamp may have exacerbated the problem as it also got its DC from that same rectified filament supply, but the preamp could be turned off and the noise was still in the receiver.  This won't help with the switching supply problem but if you ever get a DC brushless fan it may. 

Got it all solved and was putting the fan in but I dropped it, maybe 3 inches, onto the amps power transformer and a blade snapped off.  No forgiveness there, so if you buy one of the cheap fans, treat the blades nice.  Had to use the other one.

And my last comment, I think I have mentioned here before.  I use a 30amp Mean Well switching supply for my BC375.  While it would start the dynamotor, it would get it to spin, but the supply would shutdown on over current, come back up, and spin the motor faster, and did this about 3 times before dynamotor start current got below 30 amps.  Brian, KN4R gave me the suggestion to put a resistor in line and then bypass it when the motor was turning up.  Well, the BC375/PE73 dynamotor made this easy.  I added a 1 ohm resistor across the NO contact of the PE73, when I turn on the Mean Well, it draws 26 amps and gets the motor spinning, and when I turn on the BC375 power switch within a second, the contactor pulls in and bypasses the 1 ohm.  Bill Lear used this same technique in his UT-6 transmitter back in the 1930's to start the dynamotor, but his was hooked to a PTT relay and dynamotor start relay that did it almost instantly.  Mean Well supplies seem to be very radio quiet, you can find them many times on Ebay inexpensively.  The 24 volt units are cheaper than the 28 for some reason, and I use the 24 volt model and raise the voltage to 25.5.  The airborne military gear was designed for 24 to 28 volts for engines not running and thus no generator and just battery and mine seem just fine on 25.5.  When I had a BC191, I had a Mean Well 50 amp 12 volt supply and it worked great too, again with the added 1 ohm resistor. 

Charlie, W4MEC in NC