I've never owned a R-390A, but I have grabbed a few IF decks to salvage the filters for other projects. I've found they can have greater than expected pass band ripple or are very lossy... some of the materials used in the filters outgasses and contaminates the resonant discs. Make sure the four filters are within spec before plunking down serious cash.
Having taken apart bad filters to fix them one of the issues is the foam rubber that they used as mechanical isolation on either end of the stack of dimes (my term for the little metal disks that are welded together with wires in strategic ways to determine the filter bandwidth). The foam breaks down after 50-60 years and turns in to a "goo"; The stuff gets everywhere inside of the filter and is a PITA to clean up (solvents and lots of cotton swabs).
It is a daunting task to take apart to restore the Collins tubular mechanical filters. The most common way a filter dies is to have the "killer cap" short out and it puts B+ through the input coil on each filter until it burns out a winding (also, not a fun thing to rewind this tiny little coil the size of a pencil eraser). Another way is from excessive mechanical movement (maybe amplified by the foam turning in to goo, or bad practices on disassembly). This tiny little steel needle that sticks out of each coil (facing to each other) can break off, and there is no way to fix that; you can't solder it, you might be able to spot weld it but it won't be the right length or temper or be in the exact perfect spot.
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In my parts scavenging I ran across an IF deck with a complete set of Clevite ceramic filters (rarer than rare); that deck is for a special radio I am building (the penultimate radio). I have added a roofing filter between the RF and IF stages, made changes to component values and capacitor types in the AF deck (to reduce THD), Upgraded power supply caps, installed a solid-state, german-made ballast tube replacement and hand picked tubes for every spot.
Even with all of that, it is probably one of the best, gold-plated versions of the R-390A that is out there; But still, it is an R-390A and lacks a decent SSB facility. I have Cubic's, Racal's and WJ's that perform better and even a SDRplay that is more fun to use.