Manager - NA4M
Manager Notes

Reviews For: MEGA-WATT S-400-12

Category: Power Supplies

eMail Subscription

Registered users are allowed to subscribe to specific review topics and receive eMail notifications when new reviews are posted.
Review Summary For : MEGA-WATT S-400-12
Reviews: 21MSRP: 65.00 + 5.00 SHIPPING
Product is in production
More Info: http://EBAY SELLER ELECTRONICS_5000
# last 180 days Avg. Rating last 180 days Total reviews Avg. overall rating
W5KVV Rating: 2023-04-08
Best supply for the money Time Owned: more than 12 months.
I own six M/W S-400 supplies. The oldest unit is 11 years old & it has been running 24/7/365 FOR 11 YEARS! I have one in my shack, five others powering repeaters. No problems with any of them making rated power, or cold starting with a full load.

The only thing I do to these supplies is swap the as-supplied fan for a commercial COMAIR-ROTRON unit. These are 100k/hr fans and will last the life of the supply. Fan size is 60mm x 25mm and they're easily sourced online.
K6BRN Rating: 2021-02-10
Strong supply - but needs some work before use Time Owned: 0 to 3 months.
These are my personal impressions of a MegaWatt S-400-12 power supply delivered in early February 2021 at a cost of $75.51 (with tax and shipping). It’s VERY similar to the MegaWatt S-350-12. I had to add about $20 of parts to make it usable. Your experience may be otherwise as manufacturing copies, personal parts bins and expectations often differ.


The silver colored, bead-blasted S-400-12 chassis is clean, well done and has no sharp edges. However, its “cage” form factor is usually found inside electronic gear, not on a bench, and so has none of the safety and utility features really needed for bench work. There are no “feet” and without them bottom ventilation is restricted if left flat on any surface, which it tends to slide on and scratch. Large fan and ventilation holes on the very top risk easy debris or spill entry, and since slots exist on all sides, rotating the case does not help much. Small AC and DC terminal lugs are at one end of the case with no attached wiring when delivered but do have a small flip-up insulating plastic protective lid to provide some short protection.

Some investment in time and parts is recommended for bench use. This includes: 1. Feet to improve cooling air flow to the bottom, prevent surface scratching and provide standoff room from conductive debris commonly found on work benches 2. Lugged power cords (AC and DC) for secure power connections – wrapping bare wires around the terminals is inviting pull-outs and shorts, 3. A 2.4 inch mix 31 toroid for RFI suppression, otherwise use on 160M or 80M is problematic, 4. PowerPoles to plug into ham gear, 5. An AC power switch and, 6. Kapton tape (or other means) placed on the terminal strip to secure the safety lid closed. MegaWatt helpfully provides a detached, switched AC power cord with the supply, filling some of these needs, but terminal lugs need to be added. Expect to spend the equivalent of about $20 on additional parts to make the power supply both bench-top and HF-band friendly.

S-400-12 output voltage is settable over a limited range by a small trimpot next to the DC/AC power terminal strip. Before use, this should be adjusted and secured with a dab of nail polish to ensure output voltage stability.

In testing, voltage regulation was excellent at less than 1.6% change from zero to 36 Amps load at 14 Volts. Power quality is good with traditional (linear supply) low frequency ripple nonexistent. However, under a 21 Amp load (typical for an HF transceiver), significant 800mV p-p high frequency switching transients could be seen every 36uS at the supply terminals. RFI emissions from the DC cord are pronounced and easily seen with a spectrum analyzer, starting at 27.768 KHz (and odd harmonics) up to just under the 160M band at 1.8 MHz, after which they begin to roll off significantly. Waving an AM radio close to the supply itself results in AM broadcast band “hash” that fades rapidly with distance from the supply.

In Receiver RFI testing with a Yaesu FT-991, no antenna connected, obnoxious and fairly wide but fixed buzz-saw-like spurs were heard periodically across the 160M band at harmonics of 27.768 KHz. Notably, these harmonics are frequency stable and do not wander annoyingly across the band as they do on most switching supplies. It’s likely that a ceramic resonator is being used to stabilize switching frequency - a very nice touch. Connecting a (poor) outdoor antenna raised the background noise floor to S3 which masked the RFI to a degree but also spread it evenly across the band – the harsh tone was unmistakable. Similar results were obtained on 80M, but at much smaller amplitude. Any 40M interference was imperceptible. Winding 10 turns of the DC supply cord through a Fair-Rite mix 31, 2.4-inch toroid placed near the chassis dramatically reduced RFI, making it imperceptible on 80M and audible only with receiver pre-amp 1 or 2 ON for 160M. This RFI suppression ferrite is likely essential for users of those bands. Note that mix 31 has low DC resistance and should be kept clear of bare power leads and terminals to avoid catastrophic shorts.

Power supply startup behavior at zero and maximum load was well controlled with minimal overshoot. Voltage rise-time was 8-10ms after power supply initialization with less than 1 Volt overshoot, although there was a very brief and harmless zero to +5V/-3V transient 15ms prior to ramp-up under no load conditions. Measurements were taken directly at the power supply terminal strip with DC power cord connected.

S-400-12 Ampere rating is 30A continuous, 36A intermittent (“90% duty cycle”). But this can be VERY misleading since it neglects maximum POWER rating. Power supplies of this type are normally rated in Watts as indicated in their model number, along with nominal voltage (i.e. S-400-12). Amperage rating varies with voltage while total wattage rating remains roughly constant. At 14 volts, where I set the supply to compensate for some voltage droop in the DC power cord, the S-400-12 can source 28.5A at 400 Watts.

In load vs. temperature testing (using a K-type thermocouple), the chassis rose to 70C at 13.8V/30A (414 Watts) at an ambient temperature of 20C, with the hottest point at the primary switching transistor mounting location (the chassis IS the heat-sink) behind the indent on the left side. This is pretty hot. At 36A it rose even further and I reduced load after a few minutes when the scent of hot (but not burning) electronics became obvious. At 21A, the typical load of a 100-Watt HF transceiver in TX mode, the supply stabilized at 53C maximum chassis temperature, which is just fine. These measurements suggest the MegaWatt S-400-12 is best used as a 350 Watt (continuous maximum) power supply for longer life. Operated at or below that point will guard against premature failure due to heat stress while leaving plenty of headroom for transient loads. I’ve labeled my supply “25 Amps @ 14VDC” for that reason. There is plenty of capacity to power a single 100 Watt HF transceiver, in any mode.

Note that I did NOT thoroughly test the S-400-12 for upset due to RF intrusion INTO the supply due to amateur radio operations. This has not been an issue in other reviews I’ve seen of MegaWatt supplies, but has been in other brands.


Overall, MegaWatt S-400-12 “real” cost was about $95 including added parts, was easy to order and promptly delivered. This is a bare “cage-type” supply and significant work is needed to make it useful, RFI quiet and reasonably bench-top safe. Capacity and regulation are excellent, but the supply generates significant RFI on the 160M and 80M bands, requiring a large DC line toroid to suppress. Care must be taken not to let conductive debris drop into the many open slots in the case top or terminal strip. With those reservations and some prep work, this supply is more than adequate to power a typical 100-watt output HF transceiver, in any mode.


MegaWatt S-400-12 capacity appears very close to the ever popular Astron RS-35A linear supply at about 1/10th the weight and 40% the cost (~$220) new. But the Aston is completely RFI silent and is ready to use right out of the box. And they are readily available used.

The popular PowerWerx SS-30DV switcher, which runs about $140 with tax and shipping, is physically smaller than the MegaWatt S-400-12, already has dual PowerPole connectors and ¼ inch binding posts built in and is about a pound heavier, with no basic prep work required before use. Its compact case, solid top and thoughtfully designed ventilation openings resist debris entry, making it safer for use on a bench. Casual testing indicates the SS-30DV can easily support a 21A/14V continuous load with periodic excursions to its case-marked maximum 25A/14V rating, where it gets pretty warm. This is adequate for a single 100W HF transceiver, even running FT8, but with significantly less headroom than the S-400-12 or RS-35A. Switching noise amplitude of the SS-30DV is 100 mV p-p on the DC line with a 21 Amp load compared to 800 mV p-p for the S-400-12, but SS-30DV noise is broadly distributed while S-400-12 spikes are periodic. SS-30DV RFI on 160M is present at low amplitude - but it WANDERS. On 80M it’s LOUD and WANDERS. With both supplies, RFI is imperceptible at 40M. In the end, BOTH switching supplies generate significant RFI and need supplementary suppression for users of 80M and 160M bands.


Which is the best choice, MegaWatt, PowerWerx or Astron? The industrial-looking Astron RS-35A is RFI quiet, weighs a ton and costs the most (new) by far. The sleek-looking PowerWerx SS-30DV is tiny, costs much less than the Astron and has dual PowerPole outputs plus binding terminals (very handy), but generates wandering RFI on 160M and 80M. The erector-set-like, featherweight MegaWatt S-400-12 requires work to make ready, generates loud but fixed RFI spurs on 160M and 80M, easily matches the Astron in capacity and costs the least. So – which is most important: cost, convenience, capacity, RFI, size, appearance? Since every ham has different priorities, only you can decide what’s best.


For testing, insulated lugs were crimped to the (detached) AC cord/switch provided by MegaWatt and screwed securely to the terminal strip on the supply. A four foot, 12-Ga DC power cord with “45 Amp” PowerPole connectors at the end was then fabricated. It was necessary to fan out the power supply end of the DC cord to three “+” and three “-“ crimp lugs to attach to corresponding screw terminals on the power supply. This ensures minimum voltage droop and avoids potential PC board trace overloading.

The PC board which the power supply screw terminals are connected to flexes alarmingly when stress is placed on the AC or DC cords. To relieve stress a loop in each cord was created close to the supply and zip-tied to the chassis via handy slots near the terminals. Later, the mix 21 ferrite was zip-tied to the DC loop as well.

Power supply bottom vent holes were covered when placed flat on a bench for testing. To enable better cooling, 3M stick-on rubber feet were attached to each bottom corner, providing clearance for air flow.

Load testing was conducted using a Rigol DL3031A 350 Watt electronic load and 90-350 Watt passive range extender, a Rigol DSA815TG spectrum analyzer, Rigol DS1104Z oscilloscope, Fluke 87V Max DMM, ancient Radio Shack pocket AM radio and Yaesu FT991 HF transceiver for receiver testing.
KA9HJZ Rating: 2017-07-12
Works like they said it would. Time Owned: more than 12 months.
After I received this power supply , I got a little worried, it was small and light weight, I thought well I will give it a try and hope for the best. I have been using it with my old Ten Tec Paragon and so far have never had a problem. The reason for the 4.5 rating is because I only transmit around 70 watts. I have run it without ever turning it off for as long as I had it, except for maintenance on my radio and a few tests on my other rig. I also used it with my Kenwood 590 S to see how it worked. The cooling fan is as quiet as it was when I bought it. Maybe I was just lucky and if it does finally go Kaput. it cost me no more than 60 bucks.

Earlier 4-star review posted by KA9HJZ on 2016-09-21

I have used this PS intermittently for over a year, I was surprised how small it was, I use it for a Kenwood TS 590 and My old Ten Tec paragon, I never use more than 70 W power so I don't know how it would perform powering 100 watts or Teletype for any length of time, it does have a on/off switch on the power cord, which is a good idea. bought it from the company on line, which makes it a little cheaper in cost. Be careful when buying from Ebay because there is a look a like that is not as good, being sold at a lesser price The fan does not bother me as far as noise goes, I keep the PS. several feet from the radio.
KD9WXB Rating: 2017-04-05
Awsome Time Owned: more than 12 months.
The megawatt has been an awesome power supply for my shack. I have used it constantly for two years without issue. The owner of megawatt, John, has been so helpful with all my annoying emails about setup and changes I wanted to make my system. I've now bought a second one I used to power my two pill. A super good quality, free of RFI and I love customer service.

Earlier 5-star review posted by KD9WXB on 2015-10-25

I have had my S-400-12 running 24 hours a day for 2 years now. You can't get any better for the price. It truly is one of the few switching power supplies with no RFI.
Earlier 5-star review posted by KD9WXB on 2014-08-22

I have had this running for a year now and its awesome. I just thought I would check back in and report. Definitely well worth what I paid for it.
KE0EYJ Rating: 2017-03-31
Died after 3 months Time Owned: 0 to 3 months.
Worked fine for three months, but now powers on with no power output, and a high-pitch noise from fan. Will have to contact for warranty work. Sadly, I live overseas now, and will have to try next summer when I return home. I may still be under warranty. These units are really extremely light. Too light. Like the ultra cheapest of the cheap computer power supplies are. I simply turned it off, and it would not turn back on a few hours later. By the way, I really wish Eham would quit editing my review of this. The owner of Megawatt sent me a threatening e-mail, after seeing my negative review of his power supply. He threw a bunch of legalize at me, about what reasons warranty work wouldn't be free for, and then strongly hinted that I would need to update my review, if I expected warranty work. I have never been treated like this by a seller -- ever.

Going to buy a heavy Diamond brand power supply next, and hope they replace this. I will use that for a backup or something.

Earlier 1-star review posted by KE0EYJ on 2015-10-10

Worked fine for three months, but now powers on with no power output, and a high-pitch noise from fan. Will have to contact for warranty work. Sadly, I live overseas now, and will have to try next summer when I return home. I may still be under warranty. These units are really extremely light. Too light. Like the ultra cheapest of the cheap computer power supplies are. I simply turned it off, and it would not turn back on a few hours later.

Going to buy a heavy Diamond brand power supply next, and hope they replace this. I will use that for a backup or something.
W3ATT Rating: 2016-10-09
Good Power Supply Time Owned: more than 12 months.
If you are careful with your installation, you won't have any birdies or hash with this supply. I once moved my shack around and put the supply under a different desk which produced hash about every 50-60 hHz... Moved the supply back under my computer desk and all hash is gone again. I don't really know why this is, but all that aside, this supply has been running EVERY single day for the past 3 years with no issues. It never bogs down for power when running full 100 watts, CW. And, unlike supplies with big transformers, this thing has absolutely NO AC HUM! The little cooling fan makes a littls bit of noise, but nothing compared to the annoying 60Hz HUM that I was getting from an Astron RS35M (HUGE hum coming from transformer that got very loud on every dit and dah (under load at 80+ watts..)) In summary, I went from this switching supply to the Astron and went back to the Mega-Watt less than 24 hrs later. Just my experience.
N8WGM Rating: 2016-01-30
excellent choice so far Time Owned: 3 to 6 months.
I purchased my S 400-12 off E-bay. It seems to work well so far.

Yes, the fan is rather noisy. I fixed that by installing an external higher quality fan which was much quieter and yet gave more air.

So far it has powered my Kenwood TS480HX rather well along with my Alinco DM-330MV.

I would say that for the price, you cannot beat this power supply. Just be prepared to modify it for better service.

Earlier 4-star review posted by N8WGM on 2015-10-31

So far I recommend this power supply. The fan is a bit noisy but that can be fixed with a better fan.

It does put out the power I need and has no hash on any frequencies I am on. A pretty good deal for around $70.

A lot smaller and lighter than the Astrons that I have used in the past. The Astrons are tanks compared to this but I like small and light.

Will it last as long as my 35 amp Astron? Not sure but the switching technology is taking over.

I recommend this power supply so far. For the price.....well it seems to do the job it was designed to do.
W5JV Rating: 2015-07-17
Support As Listed Time Owned: more than 12 months.
One of my Mega-Watt S-400-12 supplies failed after about 18 months. I contacted the seller (still on eBay) who promptly replied, verified it had a 24 month warranty and invited me to send it in for repair. The supply was quickly replaced with a new one for $10 shipping cost. The new supply was installed, tested and ran great. I was told "the TL494 was damaged by a voltage spike". The TL494 is a 16 pin regulator IC that is apparently the heart of the power supply's control circuitry and not immune to AC line transients. I thought my station was immune to this because of installed surge suppressors but apparently not. The unit did not fail during use, it was dead when I turned it on to use it. So, from now on I will unplug it when not using it. Hope this helps someone else. I'm leaving 4 stars because it seems like in this day and age designers could have learned how to protect their A/C inputs from routine voltage spikes by wiring in MOV's or whatever they use for this issue.
AD0AR Rating: 2015-07-10
Inexpensive and works with minor flaw Time Owned: more than 12 months.
First of all, everyone wants to save a buck on a basic thing such as a high current power supply which we all know can be very expensive. Our radios are expensive enough, why add insult to injury?? Well a linear supply is surely the least RF noise generating, but if that series pass transistor(s) electrically fails and shorts on a linear supply, our radios are instantly TOAST and more $$$ lost.
With switching supplies, the conveyed power is rectified AC from a high frequency electronically regulated oscillator , so it is highly unlikely to produce a over voltage condition from the electrical failure and shorting of the switching transistor. Most likely the AC mains fuse in the supply will blow, and the supply will need to be serviced and your precious radio will be safe being it is on the other side of a transformer which cannot convey a dc short. It is rare, but a over voltage condition can occur when the feedback loop on a switching supply opens, and then high voltage can be passed to the radio equipment.
Anyway these switcher type of supplies are much more energy efficient that the linear dinosaur counterparts that had huge heat sinks for the series pass transistors.
The major caveat of these power supplies is that the switching element can generate insurmountable broadband RF noise/hash.
After using three(3) of these MEGA-WATT S-400-12 for over two years, I have encountered only one supply that had an electrical anomaly- the DC+ was tied to the chassis case/AC ground.
After disassembling the faulty supply (lots of rf hash) and removing a tiny solder splash that shorted the ac/chassis ground to the + DC output, all was good.
Filtering is pretty rock solid and I could not notice any noise/hash when properly wired and using a separate dedicated ground for the radio.
Do not make the newbie mistake of tying the DC output ground (-) to the chassis of the power supply! No damage may result, but the conducted RFI can wipe out all HF and possibly some VHF ham bands.
To be RFI safe, I still used a large 2" 31 mix toroid on the DC output leads wrapped for common mode suppression but this is probably overkill with 14 turns on each side. I noticed no 80/160M changes to ambient RX conditions with or without this addition.
The power supplies have all performed admirably with my only nitpick is the cheap Chinese cooling fan. They all have failed with the wailing agony we are all too familiar with when a computer fan fails. I ended up not replacing the OEM fans as they were loud and annoying, but redrilled a few holes, added a 10 ohm resistor in series with a new 8cm fan and mounted the fans on the of the power supply with appropriately mounted finger saving fan guards covering all pertinent moving parts. The 10 ohm resistor slowed the fan down to unnoticeable audio levels while still keeping the supply cool to the touch, even running 100 watts of RTTY into my dummy load for a sustained 10 minute test.
These supplies are a best bargain in my book, even if they may need a fan mod which I am sure all of you amateurs could do with one hand behind your back. After all you guys did pass a basic electronics portion of the ham exam (I hope) ;-) Perfect for any project needing high current in a very small form factor.
Good luck, God Bless, In got we trust, all others we monitor de AD0AR
KA7NIQ Rating: 2013-07-06
Full Of Birdies! Time Owned: 6 to 12 months.
This little POS works great, where it works!
The 17 meter band is nearly unusable on my Kenwood TS 850, and I can find birdies on nearly every band.
The birdies on 17 meters are nearly S5 to S7, making weak signals unreadable.
LOL, I have to work between the birdies!
The Birdies are much much lower in amplitude, on the other bands, but they are still there.

I can not recommend this power supply, if you work 17 meters.
Other then that, I can live with this thing.
It runs the TS 850 to full power easily, and has never been turned off in the 6 + months I have owned it.