Testing old caps from the junk bin: what I actually do

I went through this exact thing last winter with a jar of caps my uncle left me, mostly old military surplus with the se wax seals. Tested every one with a cheap component tester off Amazon and they all read close to rated value, which felt like a win until I put a few into a radio restoration and the thing hummed like a beehive. Swapped in fresh caps and the hum vanished. That was my introduction to why the bench reading alone lies to you. Now I keep a handheld ESR tester in the drawer next to the good meter and it has paid for itself many times over. The funny part is that old electrolytics in that jar had wildly varying internal resistance even when the farad reading looked fine. Some were five, ten times higher than a modern equivalent. One thing I'll add, and this goes against the buy-new advice a bit: collectors of old tube gear sometimes hunt for the paper-and-oil Russian caps on purpose because of how they sound in audio circuits. So before tossing them, it might be worth listing them on a forum classifieds section or asking around. Somebody out there probably wants what looks like junk to the rest of us. I gave a whole box away to a guy rebuilding a boat anchor transmitter and he was thrilled.

It is interesting how the internal molecular structure degrades over time, even when the external measurements suggest perfect functionality. Specifically, the thinning of the electrolyte within electrolytic caps creates high impedance pathways that will only show catastrophic failure later under load. I once tested a series of Soviet-era power caps that looked pristine, but thier internal leakage paths were alredy significantly expanded, meaning even a small current surge would likely cause premature catastrophic failure. The way the active compounds settle over several decades is frankly astonishing.

I agree with the ESR point completley; that’s where the real trouble starts. I've found that even if a cap passes the initial resistance check, the internal ripple current can actually accelerate the electrolyte dry-out process far faster than you think. On a recent project, I was running a power supply that pushed about 300 milliamps per cap. Even though the voltage held, the heat buildup alone was causing a measurable pressure change inside the casing. It’s not just about the measured resistance. I’ve seen capacitors fail prematurely after only a couple of months of service because they never got a chance to fully stabilize under load. You really need to look at the sustained temperture during operation, not just the bench test reading.