After a long, long sleep: ML-335

25+ years without power? Potential big trouble.

If it were mine, and I was short of cash, I’d pull the big caps and power them up one by one, very carefully with a variac, as @gs5556 suggested. And I’d put them inside a metal box beforehand - big caps can explode and throw caustic debris around. Then I’d let the cap under test sit, fully powered up, for a day. Then I’d turn off the power and see how long they kept their charge. Then re-install and power up carefully.

It doesn’t matter how good the quality is - an electrolytic cap is an electrolytic cap, and they do age. A sensible technician will power up his unused test equipment every month or two just for this reason. And some of that stuff, like my vintage Tektronix scopes, cost as much as a new Mercedes back in the day. That’s quality parts.

Or, you could just install new electrolytics. If it were mine and I had the cash, that’s what I would do.

@terry9, @bigkidz...you guys are techs, right? I was always told that electrolytic caps age with ’"use", not with just "age". Is that true?

The ML-335 was made in 1998.  That makes it just 25 years old.  Maybe new in box or demo unit.  That's why I asked about age.

My suggestion is to look inside first to make sure that it not too dusty or any dead fauna, insects etc inside. From personal experience I can tell that it’s very common to find some dead insects inside old equipment. Clean it if needed and live it ON for a couple of hours, not connected to anything except power, observe it, smell it for any burn. If everything looks fine then clean all the contacts with contact cleaner, make sure if the specifications are corresponding with your system and connect it to the system. Start to listen from lowest volume. 

Good point, @surfmuz , dust and wildlife can be a problem. But if it's been sitting in its box, perhaps less of an issue. I would not risk an exploding electrolytic in a high value classic, but YMMV.