MFJ-4125P Fan Noise

•April 7, 2018 • Leave a Comment

The MFJ-4125 switching power supply  has some notoriety for fan noise. MFJ uses a 60x60x15 mm fan from Yate Loon.  The high noise level is a result of running the fan at full blast.  Any noise abatement comes from dialing back the voltage going to the fan at the cost of reduced airflow, which brings us to the black art of form vrs function.  The problem is to reduce fan noise without adversely affecting the power supply circuitry of the power supply.

How much cooling is enough? Chose a fan, watched the failure rate of the power supply and if acceptable from a business standpoint, good enough.

The fan noise level is described by MFJ as “whisper quiet”.  On the sound scale, “whisper quiet” is 30dB.   My cell phone sound level app does seem to confirm a +- 30 dB level of noise coming from the supply fan, an example of  the double edged sword nature of truth in advertising.

Noise increases with airflow. I plotted the data (see the Yate Loon product data sheet (2) for it’s fan product line further down ).  I plotted out their four data points below which shows the interesting relationship between Airflow and Noise.


To reduce noise, reduce airflow by reducing RPM.  The human ear detects sound intensity in increments of a decibel or two.  For every increase in one  cubic foot a minute  airflow, there is a one to two dB increase in noise.  It should also be kept in mind that the human ear is a very poor temperature sensor without a lot of help.

Reduce RPM by reducing voltage – 4 diodes in series

A number of hams reported using 60Ω to 100Ω power resistors to drop the voltage and reduce fan rpm.  I am trying diodes instead of resistors since I don’t have the needed power resistors.  4 diodes in series produce a 25% voltage drop from 12V to 9V.  Noise decreased noticeably; every diode drops .7 v drop and each results in a small drop in noise, so this might have been 4 to 8 dB.  At 9v, the fan still makes noise. The question is what’s the impact on airflow and necessary cooling.  E.g  a 25% drop in airflow from  22 CFM  would be 16 CFM …  is that enough to make a difference in the power supply temperature?   {9/1/18 after a few months, still seems to be fine with no apparent ill effects.  Using a noise app on my phone, the fan now elevates background noise in the shack from  30 dB to 35 dB with the mic a few feet away from my operating position.}

There are many comments out there of hams who have installed resistors. Nobody has reported any issues with dropping the fan speed using resistors.  We’ll just sort of see how it goes with diodes.

I put a thermistor on the heat sink to enable monitoring the temp on it and set up my arduino thermistor .  (let’s put a thermistor in there – now that we have a small inventory in stock !)  From April to September, the power supply ran with diodes in the fan circuit with no ill effects.

9/1/19.  Did some power supply temperature test with TS590sg  transmitting into a dummy load.  PS heatsink temp rose from 84º F to 103º F  100 watt cw.  That’s not too bad I think.  (the ts-590sg rig remains cold to touch).   Under digital condition the temperature of the heat sink however goes to 123º F.  For this we’d want to be able to switch on full power to the fan.

Temperature controlled fan driver (4)
(Taming the MFJ-4125 PSU Fan)
Below is a schematic from G0KLA who used this circuit for regulating span feed from temperature for his linear amp power supply.   He used the same circuit with an MFJ-4125 and posted it ( see 4 ).   It’s the same analog control concept as ( #6).

4/8/18  I built out the circuit for testing.


C17  – electrolytic,  filter/time constant ?   R31 – the gate resistor 10k seems to be typical to protect the gate from transient voltage at the expense of switching speed.  Switching speed for turning on a fan is hardly critical. (5)

Test Session #1 – Standby noise of about 35 dB and 109º up to 50 dB+  and temps above 118º on 9/13.



9/13/18 Testing temperature driven fan controller-  finally.   Removed the  diode breakout and inserted the test unit.    Insertion notes:  for testing I tapped tge 12V on the rear panel, and attached fan leads directly to fan + and  minus – .   There is a better way.  Make a power cable to get the 12V from the fan power socket on the board, and provide two header pins for the fans power plug. This allows the fan circuit insertion without cutting / soldering  on the MFJ4125.

9/10/18 Results.   The circuit works well.  Maximum temperatures are about 118º F with full power digital operation.   I set the trigger point to occur at something close to 105º F by trial and error .  At this setting with transceivers powered up, the fan idles with heat sink temps around 105º and 110º and fan noise is barely audible with total ambient noise of about 35 dB.    On transmit, the fan speed revs up, and with a lot of transmitting activity like with a digital mode, the fan goes to full speed as the MOSFET switches on to deliver the full input voltage to the fan. The fan at full power adds another 10 to 15 dB of background noise.


The spec we’re most interested in is the junction temp of the power transistors attached to the heat sink .   Then the heat sink temperature might tell us if the cooling is enough.  We’re leaving this to another time.

9/10/18 Production Insertion Notes.   Came up with the better idea for how to make the insertion. Soldered two header pins to plug the fan end connector in.   Used a two pin socket to plug into the fan power supply socket on the rear of the 4125 main board.

4)  – heat sensitive ran speed reduction circuit.

Fan Specifications 

The fan used OEM is a 60x60x15 mm 2 wire YateLoon S60SM-12 run at 12V with the fan load, 13.8v no load.   It’s

The specs and replacement information are included here because the originals bearings are on borrowed time and multiple re-lube operations.

Below: product sheet from Yate Loon (2).  YL has data on four different models of 60x60x25 fans with different motors.  Airfow and noise increase with RPM

fan specs

OEM Source (3) 60x60x20

D60SM-12 Technical data:
Dimensions: 60x60x20mm
Nominal voltage: 12V
Nominal speed: 2700rpm
Airflow: 18CFM
Noise level: 30 dB(A)
Power consumption: 1,2W
With 3Pin Molex plug, 60cm cable and rpm signal

Instead of a genuine YL, here’s what I bought below.  It’s now plugged into the power supply and seems to be running fine.


12/13/2018  So far so good.  With only receivers on, the power supply fan is off.  At some point the fan my idle as the load increases.  On transmit, the heat rises in the PS, the fan rpm rises rapidly and continues until the heat drops.  If we stop transmitting the fan idles and eventually turns off.





Shipping Lithium batteries

•December 15, 2017 • Leave a Comment

After buying lithium cells for years from Battery Junction, Tenergy, and others, in Dec 2017,  Battery Junction will not ship batteries to Hawaii because their carriers all use air freight.

Today I ordered 4 CF123A rechargeable cells from Green Batteries.  Green batteries worked with me to find a solution.  The USPS allows air shipment of two batteries per package.  Green Batteries sent four CR123A rechargeable lithium ion batteries in two small packages which arrived in a few days.

So the future of the lithium battery segment of the industry is to find surface shipment qualified for hazmat shipping.  Nimh may be a different story still.  The air travel restrictions apply only to lithium, but demonstrably safe packing measures – factory packing if possible, taped terminals, individual plastic bags or containers are the minimum requirement.  Boxed and sealed to avoid contact with any object prevents short circuits. Most important, they must be carried on and cannot be checked.

June 28, 2016

Lithium Ion Batteries: (a.k.a.: rechargeable lithium, lithium polymer, LIPO, secondary lithium). Passengers may carry all consumer-sized lithium ion batteries (up to 100 watt hours per battery). This size covers AA, AAA, cell phone, PDA, camera, camcorder, handheld game, tablet, portable drill, and standard laptop computer batteries. External chargers are also considered to be a battery. With airline approval, devices can contain larger lithium ion batteries (101-160 watt hours per battery), but spares of this size are limited to two batteries in carry-on baggage only.  … 

This criteria allows qualifies a 10 AH x 12V = 120 watt hours LiFePo4 battery for travel.  Two of these in parallel is 20 AH – the maximum allowed.

Carrying a small shipment of several lithium metal cells could be problematic.  This is where securing an online source is just better.

MFJ4125 25A Power Supply Fan Bearing Noise –

•November 23, 2017 • 3 Comments


D6-SH-12 Fan Making bearing noise, clicking, ticking. Thought it could even becoming from the radio audio.


Pulled the fan, removed sticker and pried out a plastic cap covering the bearing.  Have to pry it out gently – it’s a tight fit.  Couple drops of light bearing oil on the fan bearing.
Replaced plastic cap and sticker.
hardware : (8 x 3mm button head screws to remove the cover, use #2 phillips for screws and for the 4 x large self tapping fan screws)

Fan is no quieter but there’s no ticking noise.

2/4/18  rattle noise returned after 4 months.  Relubed.  It’s better but Pete grt is right of course.  The fan bearing is failing. 4/1/18 – relube fixed rattle – still good.

Performed a mod to insert 4 diodes in series


Noticeably quieter.  Installed a thermistor on the heat sink to allow heat monitoring.  At 100W CW temp goes up to 103 so this is effective cooling still.  Idle temperature is 96 or about 13deg above ambient with the ft-857 powered up (9/2/18).

9/2/18 perfomed preventive maintenance, lubricated, inspected.  8Xzfm6iK9MoQ_200.jpg



Noise Drop Nov 2017

•November 16, 2017 • Leave a Comment

November 2017 – noticed a significant drop in raw noise level to from S9+5 or more to S5 on 40 meters.  That’s 40dB of noise drop!  The mfj-1026 noise canceling signal enhancer is still useful on 40m to audibly improve signal to noise reception

Example 40 meter settings -best day to day settings vary

  • PreAmp – out
  • Freq – in
  • Phase – out
  • Aux Gain – 10
  • Phase – 9.6
  • Main Gain – 3

Antenna: 40m dipole

updated 1/3/18.   80m end loading in effect in Nov 2017 has been discontinued as ineffective with little change in operating conditions.

Kenwood TS-590SG restarts

•October 21, 2017 • Leave a Comment

Status update: resolved
Last night the TS590sg started doing something wierd while I was doing FT-8.  On transmitting at high power levels, it would lose power and the start “Hello” screen would appear.

Apparently, loose Power Pole connection to the power supply.  Reseat, it seems to be working.

{ the longer story

At exactly the same time, there was a power spike/brownout which affected the whole house momentarily, but all devices stayed up, because the dropout was a fraction of a second.  However, the radio’s power supply (MFJ 4125P) is plugged into a SurgeSentry power box that instantly cut power to the rig.

The Internet “Online” link on the router also dropped.  This as it turns out was caused by the regional power glitch, but I didn’t know it at the time.

I was running a QRO full 100W transmit in FT8 at the time because of conditions of the ongoing QSO.

So I didn’t know whether the RF took out the Internet ( never happened before but if the finals blew, a local spike might have occurred ? ).  Later testing seemed to make it unlikely the finals were out, but the power loss was puzzling.

This is the difficulty of contra-indications and multiple stuff going on at the same time.

This AM AH6IO had suggested that it sounded like a power supply and suggested I try another.  I unplugged, came up on battery, and all worked.  Then I plugged back into the 4125P and all worked.

I think what happened was that I was also working on the HTX-100 which is wired for power poles.  Unplugging that loosened the PP connection on the radio, so a high power draw through the high resistance power pole caused the radio to power supply to drop momentarily and the power supply reset … I can’t explain the exact mechanism so in a way the event is still unsolved.

Looks OK now after reseating the power pole!


Nov 24 17- no recurrence of the problem.  Loose power pole.

SDR Issue- very wide LO signal on waterfall

•July 26, 2017 • Leave a Comment


LiFePO recharge note

•July 9, 2017 • Leave a Comment

7/8/17 Recharged LiFePo after a days light usage on the TS590

Put the charger on a Kill-A-Watt to record the charge.  ~ 3.5 hrs .  Recorded .09 khw or 90 watt hrs.  The battery has a rated capacity of 128 wh so we depleted about 70% total charge with 30% left.


Resting open terminal battery voltage : 13.7    (down from about 14 just after charge)