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Reviews For: MFJ-4125

Category: Power Supplies

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Review Summary For : MFJ-4125
Reviews: 71MSRP: 100.
MFJ-4125 13.8 volts 25 amp switching power supply
Product is in production
More Info:
# last 180 days Avg. Rating last 180 days Total reviews Avg. overall rating
N7AG Rating: 2018-09-07
Good, but noisy Time Owned: 0 to 3 months.
This 4125P, cute little bugger, too bad any the fan whine. With longer as or dc cords, could be hidden in a quieter location.
N1XOI Rating: 2018-04-03
Good value, no frills, noisy fan Time Owned: 3 to 6 months.
Got one last December, to go with my new IC-7300.

- Small and light
- Inexpensive
- Push-on 5A connectors in the back
- Decent quality PCB, assembly and component
- No detectable RF hash

- Fan noise
- Not detachable cord
- Voltage drop at high load

Reading reviews I was aware of the fan noise, but I had underestimated how annoying it can be over time... I wanted to wait for the warranty to expire before doing something about it, but after almost four months I could not stand it anymore.

To keep it simple I just added a resistor to the positive wire supplying the fan, as countless others have done. I built a simple extension to the original fan cable, with the negative being a pass-through and the positive having a few resistors in parallel for a total of 78 Ohm. That way the mod is easily reversible.

I tried with about 100 Ohm first. While I could barely hear the fan, the airflow seemed reduced too much. I found 78 Ohm to be a good trade off for me. Prior to the mod I measured to noise with a Sound Meter app. While not necessarily accurate in absolute terms, the relative difference between levels should be. After the mod I measured a reduction of more than 10dB at one foot from the front panel vs before the mod.

In my operating position I have the supply on a shelf underneath the table where my radios are. My ears are about three feet or so from the supply with the surface of the table in between but the shelf is open. At that distance now it sounds about the same as a desktop computer fan. Much better!

After the mod I tested the supply with an Agilent electronic load we have at work. Loaded it up to 20A for five minutes and the fan exhaust got warm but never really hot. The top of the case was also warm to the touch, but not even close to be alarming. I usually operate Digital Modes at QRP or low power level, so I do not expect any issues with the reduced airflow.

One thing I noticed is that under load the output voltage sags noticeably, although it remains stable at any given load. The set output was lowish to begin with, just below 13.7V, even at no load. Got down to about 13.4V at 20A. Others have reported only about 100mV drop at 20A or so. It might just be my unit with a bit worse load regulation. At any rate, nothing a typical transceiver cannot handle. For the price it seems a more than decent power supply. As I had the supply opened anyway, I adjusted up the supply voltage to 13.85V at no load. There is a trim pot in one of the upper corner of the vertical daughter board affixed to the main PCB.

Be aware that the schematic on the MFJ product page is not the right one. It is close enough but it is for one of the adjustable models, with Ammeter and Voltmeter on the front panel.

All in all a good value, wish the fan was thermostatically controlled with variable speed. Maybe this will be my next mod when I have more time.

We'll see about durability. I had a Samlex SEC 1223 smoke on me (bad capacitors) and those are at least 50% more expensive than the MFJ-4125. Hope to have better luck with this.
K3VA Rating: 2017-01-13
Compact/Reliable Time Owned: 3 to 6 months.
Used one time on a Dx/Vacation trip and
worked fine.
Fan is a bit noisy and the power cord is
not detachable for packing/travel use.

Also, the 5amp fuse is Internal and not
on the rear panel as the manual states.
WA0ZZG Rating: 2015-07-21
Quiet RF wise Time Owned: 0 to 3 months.
Picked up one at an estate sale, still in the box and plastic, for $70. It is very small and light. My meter shows 13.79V. No RFI problem. Who ever built this for MFJ uses the same design for Yaesu switchers. Plan on bypassing the 120/220 switch and using it for a Hi/Lo fan switch along with a 68 ohm resistor.
K0DOG Rating: 2015-04-29
Noisy Time Owned: more than 12 months.
In a weak moment (cheap moment) this power supply appealed to my cheap side.
Construction is robust enough.
Noise level is objectionable.
Absolutely would not buy one of these things again
K1EAR Rating: 2015-01-18
Hash-free as advertised Time Owned: 0 to 3 months.
Had and returned two other switching supplies (Jetstream and Radio Shack), which had awful RF hash, making weak signal use impossible. After a month of heavy use this one seems, as advertised, entirely hash-free. Will probably install resistor some day to quiet the noisy fan, but for now, quite satisfied...
W4KVW Rating: 2014-11-13
Does the job well in a small space Time Owned: 3 to 6 months.
Must say I was quiet surprised at how well it works for it's size.Got a Great deal on mine used for $25 & it does just what it's suppose to do & takes up such a small space I forget it's even there.The fan runs all of the time but is very quiet unlike many other supplies out there.For my main gear I run an ASTRON RS-70M but the little MFJ handles the jobs I have given it without any problems.I'd say it's worth what they cost even at New prices & for sure at the price I paid for this one.MFJ has a Winner in the MFJ-4125 Switching Power Supply & it's really a Mighty Mite! {:>)
KK5DR Rating: 2014-09-22
Good supply Time Owned: more than 12 months.
Had mine for several years. Never miss a beat. Fan noise doesn't bother me, as the unit is positioned behind my gear. No need to have it up front, nothing to look at on it anyway.
K6SDW Rating: 2014-04-15
Works FB Time Owned: 0 to 3 months.
Perfect power supply for portable as advertised!
KD5FLN Rating: 2014-03-29
Whisper quiet? Time Owned: 0 to 3 months.
Well that is what the advertisment says, "whisper quite fan". I wouldn't say its whisper quite but its not annoying either. I've always liked MFJ products althought they do have this "home made" look & feel about them I never had one fail on me. Currently I'm using the 4125 to pwr my Icom IC-2300h at my QTH and I can run up to full 65 watts full pwr no problem with it... Maybe later I'll be brave enough to do the resistor mod and quiet down the fan a bit, that is my only complaint...