I have had issues with some failing for no apparent reason. It’s why I don’t like offering them for sale anymore. They are also no longer cheap. As the bitcoin miners have driven a the prices up to a point where it no longer pays to make them. My favorite is the dps600 pb, they rarely fail. I have been experimenting with a dps750 which is the same size but puts out 62.5 amps, so far so good but the RC guys give them mixed reviews . One plus is that they are cheap so if I smoke one while experimenting it won’t hurt too bad.

The DPS1200 and the 750 are the exact same footprint. Both easily get to 24 volts by floating the grounds on one of them,(Preferably the DC ground), then wiring them in series as you already know. The 1200 can push 75 amps on 120 volts and 100 on 220 which makes it extremely attractive to those of us that like to use power hungry chargers. The downside is that they sometimes fail for no apparent reason. When they were super cheap I did not care and would just make another. . By the way I do have one at 3+ years old and works perfect and a second at about 2 years old. I have also read that they are not all the same and certain revisions are more prone to fail than others. In the computer world that may not matter but in our world there seems to be something to that.  The DPS750 can reach 62.5 amps on 110 volts and is dirt cheap. Seems that certain revisions fail less than others but they don't have a great overall track record on the RC forums. The DPS600pb can reach 47 amps on 110 volts is also cheap and is super dependable. I have pushed mine to 53 amps just to see how much it will actually do and I did not smoke it.  Just for clarity sake I Do Not use the balance extensions anymore I have found them to sometimes be unreliable.

William, thanks for the good information and photos.  I have 2 sets of the dps1200's still working thankfully.  I found a post on an RC forum about the used PSUs in general.  Paraphrased, "they're all used, and were replaced because they don't meet ripple current specifications. They work well as long as not really loaded.  The problem is the output capacitors are going bad. if you stay under 40% load current, you will be OK. As you start to go up in load, the ripple increases, and your charger isn't going to like it.  If you need to run at a high current, add more caps on the output. Just make sure they have a low ESR."  Of course that quote doesn't speak directly to PSU death, but harm to the charger.  But they are all well used. 

If the above is true, it make some sense to use 24v supplies, two in series, instead of 12V where extra amperage load is used in the step up.  And when convenient, to limit the load.  Six 6s packs on a parallel board at 5 amps each is still in a reasonable range.

The fans on my DPS 1200 runs when plugged in.  I haven't tried to speed it up for a  while, but originally, I could get it to run louder/faster with heavy load.  Increase fan speed presumable in response to heat generated under load. Fans run even without the green lights on.

I have found that you have to push both the DPS 1200 and the 750 pretty hard to get the fans to kick in, but when they do they sound like a mini jet engine. With the DPS600 the fan can be controlled with an extra pin jump which lets it come on and off depending on load or skip the pin jump and let it run at high speed at all times. this is a link to the dps600pb build. https://www.rcgroups.com/forums/showthread.php?1581061-DPS-600PB-Builds-hints-tips-and-build-plans

I have never got the fan to come on full even the casing was quite warm. as it just spins at idle speed. I was going to rewire the fan so it'll work manually.

Hi Fred, Just wondering what value caps to use to filter out the ripple more?  Is 470uf/50V O.k.?

Joe, sorry but the author didn't give aby help on that.  I would assume somewhat like the OEM caps and I haven't looked at those since my PSU's are assembled.

MM
I recently tried t charge some batteries and got an error, "low supply voltage."  On the PL8 there is a setting for minimum supply voltage and I set that at 23V.  Turns out that one PSU 110v incoming had gotten loose so only one was working at about 12v.  I only knew because of the message.  I had another charger running without that setting and it was happily running.  My guess is that the heat would be proportioned to the load.  If your charger was outputting 40 amps at 6s voltage, then a 12 volt supply would be fairly loaded, one reason for using the 24v supply.  Of course if you are only outputting 20 amps, then it may all go un-noticed.  I beileve that there is a temperature cutoff if the supplies get too hot.

Joe: Maybe you aren't pushing them hard enough.They really only spool up hard when you get close to maximum capacity. They can get quite warm before they actually kick in. On most PS there seems to be a sensor on the fan that detects weather they are running or not. be careful if you try to modify anything to do with the fan, sometimes they wont run at all. you can probably find more specific info on the RC forums as I'm guessing that someone has"been there,done that" already.
  I also question, if its even worth it to fool around trying to fix these given the price and availability of so many different types of server supply's available to us for ridiculously cheap prices. I do like to tinker with these and probably have tried more models than most people and I hate to get "beat" by one so I understand your desire to try to get it working again and if you do I'd love to know what it was that failed. Also just for the record I have on a few occasions been able to get these brand new and the track record of failure has been no better that the used pulls we normally buy, so I really think its just random. In the end replacing them may still end up being the "Best Fix".

yeah,  you're right.. it is ridiculously cheap.     I think I'll  just replaced them instead of spending more time digging into it.   

Have a Merry Christmas! 

Joe