Quieting the MFJ 4245MV Power Supply - ModsAndRepairs Forum at eHam.net

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eHam.net Forum : ModsAndRepairs : Quieting the MFJ 4245MV Power Supply

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Quieting the MFJ 4245MV Power Supply	Reply
by KC2NYU on August 16, 2009	Mail this to a friend!
Has anyone found a way to quiet down the noisy, always running cooling fan on the this power supply? I am happy with the the 4245 performance except for the fan noise. 73 Paul

RE: Quieting the MFJ 4245MV Power Supply	Reply
by KA7GKN on August 24, 2009	Mail this to a friend!
If you are hearing anything but the noise of air movement then the power supply may have another problem. A simple way to eliminate fan noise from being picked up by your microphone is to locate the power supply further away from the radio. Eliminate near field noise pick up. I have a computer table with a lower shelf at about knee level. All my power supplies are mounted there. I also have a 3" piece of solid Styrofoam under the power supply to eliminate mechanical coupling of power supply noise to the desk. I built some remote LCD metering to monitor the voltage and current so I never need to be concerned about the power supply. I also blow out the dust inside once a year. Martin KA7GKN

RE: Quieting the MFJ 4245MV Power Supply	Reply
by WB6THE on August 24, 2009	Mail this to a friend!
There are two 12VDC cooling fans in the power supply. I installed a 45 Ohm 5 Watt resistor in series with each fan. Also installed a DPST miniature toggle switch on the rear panel to short the resistors in case I want full fan speed. The resistors are mounted on a lug terminal strip. There is not a lot of space to work inside the cabinet but it's do-able. No ill effects noted in the several months I've been running the fans at low speed. I am only powering some QRP rigs with the supply, normally drawing only about 5A maximum. I have no idea how fast the fans need to run at much heavier loads. But that's what I did to mine and the fans are now very nearly inaudible. Alan WB6THE

RE: Quieting the MFJ 4245MV Power Supply	Reply
by WB6THE on August 24, 2009	Mail this to a friend!
Oh... I forgot to mention that those were wire-wound resistors. Carbon or film types might not be the best to use. Why 45 Ohm 5 Watt ? Because I had some. The resistors only get very slightly warm so a lesser wattage value might be OK and would be physically smaller and easier to work with in the limited space inside the power supply. Suggest that you clip the positive side of the fan wires and extend their length as need be to reach the resistors because the circuit boards in the power supply are not the best. The front panel LED in my power supply does not light with the fans running at lowered speed. The schematic provided with the power supply is not 100% accurate but leave everything else alone and you should do fine. As a very general WAG (wildassguess) if the large heat sinks become too warm to keep your fingers on without gritting your teeth against the pain then you need more air. Alan WB6THE

RE: Quieting the MFJ 4245MV Power Supply	Reply
by K8AC on August 26, 2009	Mail this to a friend!
With the advent of "home theater" PCs and cases, there are a lot of new 12V fans available that are substantially quieter than the older ones. Find one of the websites dedicated to "quiet" PCs and you'll find quite a few fans - and they're fairly inexpensive. If the problem is noise from the rushing air, there are a number of homebrewable circuits available using a thermistor to turn the fan on/off or vary the speed depending on the temperature of the thermistor (mounted on the heat sink).

RE: Quieting the MFJ 4245MV Power Supply	Reply
by KC2NYU on October 26, 2009	Mail this to a friend!
Alan- even though the noise really buggs me I am just getting around to implementing your fix. I have the wire wound 45 ohm resistors and the DPST switch, but just want to make sure I understand how you installed. I see the two fans each with a red and black wire leading to the fan. - where did you put the lug terminal strip? Srew to back wall of case? - "installed a 45 ohm resistor in series with each fan" my first take is that you cut the red wire going to each fan and soldered in the resistor. But then saying resistors are mounted on lug strip, confuses me, can't picture that. - how did you wire the switch. - could you provide a few more details Please be patient with a history major, I tend to be overly cautious working inside gear. 73 Paul KC2NYU

RE: Quieting the MFJ 4245MV Power Supply	Reply
by WB6THE on October 26, 2009	Mail this to a friend!
Hi, Paul... Harder to explain than to show but... What I did was to get a terminal strip with 5 solder lugs on it. The center lug has the mounting to the chassis. The remaining four terminals do not connect to the chassis. Solder each end of the resistors to those terminals. Do a lot of planning as to where things will fit so that the resistors don't contact the chassis, interfere with each other or contact any other components, etc. Just come up with some method that fits! Cut the red (positive) wire leading to each fan. You'll need to extend the wires so they'll reach the resistors. Use heat shrink tubing to insulate those connections. Connect one resistor in series with each fan. I wanted to be able to run the fans at normal speed if need be so I connected a DPST miniature switch across each resistor to short them which would apply full power to the fans. I put the switch on the other end of the rear panel. Mark and drill your holes for the switch and the terminal lug and mount them. End of obnoxious fan noise! If you want I'll shoot a digital picture and email it to you, not all that hard to open the MFJ-4245MV. I've still had no heat problems with the power supply. r+a+d+i+o+6+t+h+e+@+c+o+x.n+e+t Alan WB6THE

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