After a long, long sleep: ML-335

Just an amateur, but I've been doing this for 20 years. Actually, electrolytics usually go bad with heat, not use. They dry out. So, the power supply for my main amps is located in another room, a cool one, with all electrolytic caps well below all sources of heat.

Given the value of the ML piece, 25 years is pushing it too far. IMO. 

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.

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.

True, nothing like ESL's.

Very wise. Good luck!