Patching up a Mega Watt S700-12.

[MKwrench]

My "new to me" power supply suddenly and violently poped it's fuse, I'm assumung from some sort of surge, because the same setup operates just fine on another power supply. When the fuse popped it created a bright flash like a quick welding arc, so its a good short. I just began testing across "Line" and "Neutral" until I came to the 2 switching fets in the back, they both tested as resistors.
When I replace these is there anything else I should look for, or just replace for peace of mind?
These are obsolete and Google has recomended something different each time I asked... LOL
Here are some pics of suspect parts removed for testing.

 

[nomadradio]

When I replace these is there anything else I should look for,

It's all about the specs. If a replacement's data looks similar the failed one, you're good to go. Odds are you'll find specs at alldatasheet.com.

73

 

[brandon7861]

mouser and digikey both have the datasheet, both show it as obsolete, and both offer the ability to search for similar parts. Digikey will suggest parts directly and mouser wants you to check the parameters you want to match before suggesting parts. The part number of these mosfets tells some too (if you ever can't find the datasheet). 40N50. 40 is the current rating, N is the mosfet type and 50 (*100) is the voltage rating. So 40N50 is a 40A 500V N-channel mosfet.

https://www.onsemi.com/pdf/datasheet/fql40n50-d.pdf

IXFK80N50P may be an option.

 

[Lost Ram]

The owner of mega watt is a pretty nice guy. I have had some fail and asked about sending them back but he just offers me a great deal on a new one. I am not sure if he even repairs them or not. If he does he might have some suggestions for you.

 

[Eldorado828]

Hopefully i don't jinx myself, but I've been running one on the work bench for at least 10 years now and it's still just going. It's on a lot and I mean a whole lot unless it's at night or I'm not in the garage. Good supplies no doubt and especially for the price range.

 

[MKwrench]

Thanks for the input guys. I wish I knew what happened to cause it to suddenly Poof out. It was working fine, powering an NPCd PC122 and a DX250 on a dummy load to teast the pairing. I continued testing on my 36a Alinco P/S.
I might try a set of IXFK44N50s, they're real close in spec. I also read that I should replace the bias components, and that the 44N will switch faster because of a difference in gate capacitance and threshold voltage to possibly cause noise in the system. I guess the fix is to add resistance at the gate to slow the switching frequency if needed.
They're $16 a pop though, so I want to be careful with the repair.

 

[MKwrench]

I contacted Mega Watt by email today (Sunday) and John got back to me within a few minutes.
<<<<On A Sunday>>>>
. I wasn't expecting a response until Wednesday...LOL.
He recomended IRFP460s for replacement. I looked them up, they're a smaller package, have less drain current capability, less power dissapation and have a higher gate resistance. They switch 10X faster too. When choosing a replacement, I usualy go in the other direction, but I'm going to listen to the man and give these a whirl. We'll let you know how it goes.....
Otherwise, Im considering paralleling 2 of the S-400s for my next trick. He says the 400 has better filtering.

 

[Lost Ram]

Good deal!!! He is a good guy. I also run the S400's on his recommendation as well.

 

[MKwrench]

I found a pack of five for the price of just one at the electronics distributors. I know that bootleg and subpar parts are sometimes labeled and sold to unsuspecting victims.
The Amazon reviews of these were *mostly* favorable, and I'm not dumping a bunch of money into this thing when I'm probably just going to replace it with those two s400s anyhow.
So this is either going to be a learning experience, or a success story. Will be back with the results....

 

[MKwrench]

Well, guess what.....
It works, it works great, it even goes above 14v now...
All 5 of the Fets tested almost exactly the same with only Uf (forward V drop) being off by 1mv on 1, It might have just been the tester too....
Nothing else on the board looked cooked, so I tested what I believed to be the gate paths and bias components. Then *kind of* Hail Mary'd it.
I didnt have a 15A glass fuse handy, but I have a crap ton of 10A bladed fuses..... ......so...you know.
I 1st had a 60 desk lamp bulb where the fuse is (thats why the wires are so long). The bulb glowed brightly for a split second as the fan started to whirrr and the power LED lit up. After verifying a few unloaded startups, I swapped the fuse in. I didn't have high hopes, but I was wrong, a few more unloaded startups followed until I felt we were good for a load.
I had an old President Grant DX within reach so It was 1st, It ran fine on the dummy load, so I moved it to the antenna... fine there too. My DX250 and DX350 are both in the middle of power wire replacements, so I grabbed a little RM Giggity KL203 and adjusted the P/S output to exactly 13.8V. This combo doesn't offer much of a load, But it's what I have handy right now.
I'm happy so far, after testing the radio and baby amp were warmer than the P/S.

Here's what I "THINK" happened: We're all familiar with the voltage switch on the back of electrical appliances, and one of the 1st things I did when unpacking my used P/S was to (I have no clue why) flip that little switch back and forth one time... I'll bet a million Pesos that I didn't have it fully back into the 110 slot, causing the supply to overwork those Fets trying to keep output up with only half of the expected input.... Just a theory, but I really dont know.
So, have at it. the Amazon sourced BOJACK fets will work, and MegaWatt Power supplies is one heck of a great company with customer service that went above and beyond for one of their future customers....

 

[Eldorado828]

Well, guess what.....
It works, it works great, it even goes above 14v now...
All 5 of the Fets tested almost exactly the same with only Uf (farward V drop) being off by 1mv on 1, It might have just been the tester too....
Nothing else on the board looked cooked, so I tested what I believed to be the gate paths and bias components. Then *kind of* Hail Mary'd it.
I didnt have a 15A glass fuse handy, but I have a crap ton of 10A bladed fuses..... ......so...you know.
I 1st had a 60 desk lamp bulb where the fuse is (thats why the wires are so long). The bulb glowed brightly for a split second as the fan started to whirrr and the power LED lit up. After verifying a few unloaded startups, I swapped the fuse in. I didn't have high hopes, but I was wrong, a few more unloaded startups followed until I felt we were good for a load.
I had an old President Grant DX within reach so It was 1st, It ran fine on the dummy load, so I moved it to the antenna... fine there too. My DX250 and DX350 are both in the middle of power wire replacements, so I grabbed a little RM Giggity KL203 and adjusted the P/S output to exactly 13.8V. This combo doesn't offer much of a load, But it's what I have handy right now.
I'm happy so far, after testing the radio and baby amp were warmer than the P/S.

Here's what I "THINK" happened: We're all familiar with the voltage switch on the back of electrical appliances, and one of the 1st things I did when unpacking my used P/S was to (I have no clue why) flip that little switch back and forth one time... I'll bet a million Pesos that I didn't have it fully back into the 110 slot, causing the supply overwork those Fets trying to keep output up with only half of the expected input.... Just a theory, but I realy dont know.
So, have at it. the Amazon sourced BOJACK fets will work, and MegaWatt Power supplies is one heck of a great company with customer service that went above and beyond for one of their future customers....

Good work and thanks for your report.
As for the bojack labeled stuff on Amazon, I've bought several different parts under the bojack name and have found many of the parts to be either known manufacturer parts or known Chinese manufacturer parts but either way I've had nothing but success buying from them. Just my 2 cents for whatever it's worth.

You got her going so it's a win!