Selenium Rectifiers get weak unlike silicon, and sometimes they explode and give off a horrible smell. They were used for many years because they were much easier and cheaper than using a transformer and a rectifier tube .
This. Not just in something like a battery charger, but in many consumer electronics made into the early 1970s. I encountered it all the time in vintage tube equipment work.
And there is nothing you can do to stop them from degrading and failing. The selenium junctions, merely exposed to atmosphere, chemically degrade over time. And as the junctions degrade, their efficiency drops, their operating temperature rises, and the pass through voltage begins to drop. And that increased temperature just accelerates the process. Eventually, the junction overheats and explodes. And it's not unlike a pole cat thoroughly doing its business all over the chassis.
In some equipment, the declining voltage can cause a lot of collateral circuit damage before they blow. Most NOS seleniums you stumble on are not hermetically sealed, and are just as degraded and unstable now as the ones in service. So the seleniums are the first things to come out on any equipment I work on.
On a museum-grade piece where you want to retain original appearances, you can sometimes fit discrete silicon replacements in the gutted-out flat packs, but that's as far as you can go with them. On the open stacked ones, there is not much you can do except remove them.
Otherwise, a modern silicon replacement bridge is inexpensive, and will usually outlive the equipment.
I inherited a late 60s Sears 'Best' charger over 30 years ago that was stuffed with seleniums, and it surprisingly was still working within spec. It got a silicon transplant anyway, and soldiered along for another 15 years, until I replaced it with a much bigger Rizk (which are as old-school durable as it gets).
