YS Tech fan used in the MFJ-4225MV power supply
Before putting the MFJ-4225MV back in the box and storing it away (see the Fake Fuses blog:
https://wb9kzy.blogspot.com/2023/07/fake-fuses.html
I thought I'd look at the fan, it is marked (in hard to read white on yellow ;):
FD1281257S-1N
Doing a Google search found this page:
https://icrontic.com/images/mm/Articles/FanGuide/Sorted_Data_html/80x80x25xcfm.htm
Looking at the last line here:
The fan is spec'd at 2.76 watts, 2900 RPM, 33.5 dba, 43.7 CFM
Also, the S in the part number is for sleeve bearings. I remember the techs at GTE didn't like the sleeve bearing fans and threw them out when replacing them. I did try taking one of the discards apart and cleaning the bearing (just a cylinder over a post) and it sorta worked but intuitively, ball bearings seemed better to me, too.
So if I do summon the courage to replace the fan, the main thing is the CFM should be similar - sure people put in quiet fans but are they simply just moving less air ?
This Noctua fan was mentioned in a couple of youtube videos (just search on MFJ-225MV fan replacement):
https://noctua.at/en/nf-a8-flx
The Noctua has three airflow specs (and noise levels) which are in cubic meters per hour not cubic feet per minute.
Bing says .59 is the conversion factor:
So, 43.7 CFM of the stock fan is 74 cubic meters per hour. This means that the Noctua nf-a8-flx is only putting out about 2/3 of the air that the stock YS Tech fan produces, probably part of the reason why it is quieter :) And comparing the RPM of the two fans backs this up, the replacement fan is 2000 RPM, the stock fan is 2900 RPM.
I noticed two sources of noise from the YS Tech fan, first was the airflow itself moving through the supply, as many have noted on the internet, taking off the cover really reduces the noise level. The second noise is what I'd call bearing noise, not as loud as the air moving but still irritating.
I also looked at a possible simple way to quiet the fan, when run at lower voltages the fan is slower and less noisy. Maybe a simple switch could be installed to reduce the fan voltage during low currents (the FT-450d uses 1A in receive) but then speed up at higher currents.
The ammeter itself is seeing about 1 mV for every 1A of current used. There are two resistors in series with the ammeter, the voltage at the first one reads 1.6 mV at 1A, the voltage at the 2nd resistor is 3.2 mV at 1A. Unfortunately these voltages are too low to use directly in a simple circuit. Some kind of amplification will be needed to increase the voltage without affecting the ammeter reading.
Maybe I will try this first: add a resistor in series with the fan to bring the fan voltage down to the 9 volt level (or a voltage that allows enough airflow to keep the temperature down at a 1A load). Then have a transmit switch that can short the resistor and bring the fan up to full speed.
Here is a page suggested by one of the youtube videos for calculating a series resistor value:
https://www.blackfiveservices.co.uk/fanspeed.shtml
Next thing tried was to measure the rpm of the MFJ-4225MV fan at low, medium and high voltages. To do this I used two green LEDs, one as an emitter and one as a detector. At first I was going to use a bright mini Maglite LED flashlight but noticed this while testing it:
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| this accounts for the strobe effect outside at night with dust or snow in the wind |
The light from the mini-Maglite is a square wave at over 120 Hz, so that won't work for measuring RPM although it's nice and bright compared to the green LED.
So the emitter LED was placed on one side of the fan and the detector LED was on the other side. I took scope pictures at the lowest voltage, 9V and 13.8 V. The results were:
lowest voltage 7.0 ms period which equates to 143 Hz (or blips per second)
9V 5.2 ms period which equates to 192 Hz
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| started riding the 60 Hz wave ? |
13.8V 3.3 ms period which equates to 303 Hz
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| 60 Hz is even worse here |
but knowing that there are 7 blips per rev (due to the 7 fan blades) and 60 seconds in a minute the conversion factor from blips/sec to RPM is 60/7 or 8.57
lowest voltage: (143 blips / sec) x (1 rev / 7 blips) x (60 sec / min) = 1225 rpm
9V (192/7)*60 = 1645 rpm
13.8V (303/7)*60 = 2597 rpm
So that's enough for this entry, I'll think more about it - anyway, the MFJ-4225MV gets a reprieve from being boxed up !
Best Regards,
Chuck, WB9KZY
http://wb9kzy.com/ham.htm
