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MFJ-4275MV 75 Amps Switching Power Supply Review


Ham Radio Nerd
Apr 14, 2002
When I saw that MFJ had a new 75 amp switching power supply available for $269, I knew I wanted to try it out. I immediately saw the opportunity to replace an existing 100 amp linear supply with something that was much smaller with more diverse connection options.

Here is a picture of the face of the 75 amp power supply:


As you can see, the MFJ-4275MV comes with multiple connector types that you can make use of. This was one of the selling points for me. I really like that I can use a cigarette lighter style, Anderson, banana plugs, ring terminals, and just regular wire clips. The adjustable voltage with voltage and current meters on front is also nice. The power supply comes with quite a few wire terminals that you can attach to your power cords to use these various connectors.

The back of the supply also features trickle charge terminals for charging an external battery:


This is an awesome feature for the ECOM folks that want to ensure that a battery is always charged in the event of power failure. You can also the dual cooling fans in this pic.

I know that everyone likes to see the inside of equipment, so I took the covers off to expose the guts. Here is a shot from the top:


Here's another from a different angle that gives good perspective on the internal heat sinks that help keep it cool with the help of the fans:


Here's another inside picture on the 110AC input side. Note the large electrolytic caps. I didn't study the schematic at all, but I suspect these are intended to help the supply under peak voltage conditions:


Notice how all of the leads to the various DC connections have ferrite cores attached to help cut out RFI:


An overhead shot of the transformers in use:


This is the other side of the battery charger connection. Note the large resistors to maintain the proper charge voltage and the diodes to protect from a reverse polarity situation:


Here's what the supply looks like under my radio desk:


Next up, some thoughts on the performance of this 75 amp power supply.
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I have been looking at those supplies. IHo do you like that ALS-1300?? I love my ALS-600 and would like top see one of those 1300's in action and I think Ide dive into one ;)
Power Supply Performance

It's difficult to say much about a power supply - it either works or it doesn't. It either causes RF hash, or it doesn't. So far, I've had no trouble with this supply after using it for several weeks. I power almost all my radio equipment (except KW linear amps) using it and am happy with the results. I've read the eHam reviews where a few people have complained about RF hash, but I've experienced none after listening nearly every day on every band between 160m and 73cm. Perhaps those that experienced RF hash got a supply that was defective? If they did, I know that MFJ would issue a refund or exchange. (edit: see this follow-up post on the subject of RF hash)

It's very nice to have such a compact and light weight 75 amp supply after dealing with a large 100 amp linear supply. The old supply weighted 115 pounds; I'm not sure what this new MFJ-4275 weighs, but it's inconsequential. I really like all of the various power connection styles that are offered on the face because this gives me multiple options for all of the radios I want to power off of it.

The largest transmitter I've used on the supply to date is a Mirage B-0320-G 200 watt 2 meter amplifier in conjunction with a 50 watt radio. Of course, the supply handles this load with no problem. As a test, I wanted to give the power supply a tougher workout, so I left my 100 watt HF transceiver keying at full power into a dummy load for a while during an extended QSO on 2m using the amplifier. This put a load of about 60 amps on the power supply, which it had no trouble sustaining.

So far, I'm very happy with this power supply and the price is right. I have a few other thoughts on the noise generated by it that I'll comment on next.
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I have the smaller MFJ-4225MV it's 25A I wish I bought a regular power supply, without the Damm fan. I have had coments that people can hear it over my microphone. It's a real pain. I have tried moveing it around but can't get away from the noise completely.I am thinking about opening this thing up and findina a quiet fan.
It looks like a nice power supply... I might need to get one to repace an old Tripplite that just died.
Why are all connections on the front panel instead of the backplane?
Convenience i'm sure but...?

The inside looks clean.

Maybe the Engineers thought it would be used on a bench/table.... why bother buying power supply with meters and just putting it on the floor?:bdh:
It is a very nice looking supply, but looks can always be deceiving. I work in an industry were we use Switch Mode Power Supplies (SMPS) on a regular basis. The design of the supply is key to the amount of hash and RFI you get from these things. Knowing a bit of theory on how these things work is key to understanding why they emit hash and RFI. The firtst stage of these supplies is an oscillator, and therefore, you have to be quite design conscious when it comes to filtering out harmonics. A properly designed SMPS will cost you a bundle. Looking at the pictures of this thing, I can tell from experience that there was not a lot of extra effort put into filtering.

The type of filter capacitors in the last section of a SMPS are critical and must be of the a Low Equivalent Series Resistance (ESR) to function properly. These type of capacitors have a tendency to breakdown under load, and start increasing their ESR. This new resistance added to the critical time constants of the output circuit will add additional noise that probably will not get filtered. These capacitors are difficult for the consumer to locate and are quiet expensive.

Loading one of these supplies up to about 80 - 85% of its Peak load specification and have a look at the output with a 'scope. With AC coupling in the vertical channel, you should see ripple and noise that is less than 100 mV. Anything greater, including momentary spikes, will be detrimental.

While you may not hear anything objectionable in the speaker, it is quite possible that it will affect the transmitted signal, or the decoding of a digital signal such as PSK, JT65 etc.

As far as I am concerned, for the value, a linear type of supply, while not being as efficient as a SMPS, is a much better value in the ham shack.
Why are all connections on the front panel instead of the backplane?
Convenience i'm sure but...?


Ouch! all conections on the frount. And the switch on the back.:oops:

For my large power supply, I actually prefer the connections on the front. Because I do enough testing and playing with various equipment, I need a supply that is easy to connect and disconnect without pulling everything out. The supply stays on all the time, so having the switch on the back is fine with me.

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