Set of 3 Meanwell RSP-750-48 Power Supplies

[DAVEISGREAT]

Took a chance on a set of 3 thrift store power supplies wired in parallel only to find the huge caps on one were toast and all three have corrosion damage.  Cracked open a 2nd one, and the big caps test good.  Possibly just one went out.  I have not yet powered on any cause they were also pretty grimy so removed fans and cleaned up two a bit.  Looking for some advice on where to start troubleshooting.  Some questions:
1. Not sure what the "staggered" traces are on back of board but the ones next to the very bottom of each board have the most corrosion.  Wondering if building up solder there, or even bridging the worst ones together would be a good idea or will this only screw things up more? 
2. Should I power up the one with good caps or make some more tests beforehand?   Would be great to get at least one of them working since Meanwell is decent quality, also looks like voltage and amperage are adjustable from approximately 20V to 55V and amperage limits of 6A to 16A if I understand Meanwell's manual correctly.  These would make a good high power adjustable ps for my induction heating project, etc.
3. The two blown caps are 330uf @ 400V.  Do these require low ESR type & cost?
4. Not sure if this blue component blew up, was weakened by corrosion, or came off while brushing/cleaning or some combination. In the picture it's placed where it was approximately.  It tests open at room temperature. 
3. Any advice on tests, repair would be greatly appreciated. 

[DAVEISGREAT]

Forget these 4 pics.

[coromonadalix]

there is rust .......  sorry i would dump them,  you may had infiltrations in the xformers,   they are crusty dusty  etc ...  poor state ...

even doing some repairs on them, reliability and safety is not garanteed

[Haenk]

Awful. Probably running almost 10 years straight in some not-very-clean-and-moisty-and-not-suited-for-electronics environment.
I upvote the "dump them" suggestion.

[DAVEISGREAT]

Any advice very much appreciated.  Agree if transformers are burnt then repair may be not worth doing, but it should be easy to compare Ts by ohming em all out.  Transformer pins on back all look very clean. Rust is only an issue on bottom most part of circuit board.  Regarding safety Meanwell has incorporated  quite a bit of safety into the design and transformers in general are pretty rugged (I have reliably used Xs with 10x rust).  Of course, having 3 PSs makes repair of one much easier - once one is going...

[DAVEISGREAT]

UPDATE: As predicted one of the transformers had some copper corrosion found under the bottom of T's wire out connecting to threw hole. Lucky was only the outer pin next to bottom edge of circuit board.  Will spray some brake cleaner under there, polish the pin and touch it up with some solder.  Also removed two large 13 ohm open resistors on the "better" board #2.  The 3rd T also has two blown big caps and one blown fuse on the AC input side(first defective part found that is not located on lower bottom corroded area.
Any more specific advice on repair techniques, what/where to test, etc. would be greatly appreciated!!

Update answer to my own question:  A Bigclive video mentions purpose of exposed tin solder "staggered raised traces" are for greater heat dissipation.  Will scape, polish, add solder/reflow all corroded raised tracks.  (After cleaning back of circuit board it looks very shinny, new-like!

Update -possible answer to another repair advice question I had:  Seams to be common repair practice to insert a 100W lamp in series with mains when considering powering up damaged power supply.  PS #2 with the two good big caps will be first power on test after replacing the two large SMD 13ohm resistors(not sure of resistor's rated wattage so will use some big honkers.  Any other repair advice before powering up would be greatly appreciated...🤔

[DAVEISGREAT]

Update: Link to mobo corrosion vid worth a look. 

[DAVEISGREAT]

Link to SMPS reference repair guide
https://www.repairfaq.org/sam/smpsfaq.htm#smpsspsr

[fenugrec]

Will echo other poster's comments about the corrosion.... ooof.

There are RSP-1000 schematics floating around, they are very similar to the -750 which I have also have a YT repair video about:

[DAVEISGREAT]

I watched your video a few times last week.  Those were 2 sweet repairs! Very cool to see you in this awesome forum. Wish I had your mind. Bean counter here who recently started another hobby - will be a project for me but since I have no deadlines hopefully a fun one.    Any repair advice from a master like yourself would be greatly appreciated.
The boards were definitely cruddy but I have cleaned two amazingly well, and did not note any significant physical damage other than the corrosion on bottom most part of the boards. No pro tips yet from anyone, crickets?

[DAVEISGREAT]

Uncle Doug recommends a 250W bulb when working on high power vacuum tube amplifiers, and significantly lower watts on digital amps.  Wondering if someone can recommend best bulb size for testing when using 115V AC with one of these 750W power supplies?

[Vovk_Z]

Uncle Doug recommends a 250W bulb when working on high power vacuum tube amplifiers, and significantly lower watts on digital amps.  Wondering if someone can recommend best bulb size using 115V AC with this 750W power supply?

250 W seems like can be ok. You still can't power it 100% with any bulb.
But there is APFC I guess (since it is 750W but not 75W) which makes it more difficult to power it through a bulb.
I repaired 20W, 100W, 150W MDR and HDR series power supplies with a single 40W bulb (or it was even 25W?), 230VAC mains. Actually, I use this 25/40W? bulb to repair everything I get onto my table.

[DAVEISGREAT]

Do you think lamp can still be used to help find shorts, prevent further component damage, etc. despite APFC?

[Vovk_Z]

Of cause it can't prevent damage but it definitely prevents larger damage. It minimizes damage to a minimum possible. And very convenient is that it signals immediately about something wrong (APFC messes a bit this feature).
I think there is some bulb wattage which can live fine with APFC. Like, e.g. 100W may be not enough to start APFC module but 250W bulb may be just fine. You may find it with several experiments I guess. (I would buy several different bulbs - e.g. 100W, 250W etc.).