Are they selenium or copper oxide rectifiers?

[ratio]

Attached is a pic of a power supply I came across recently. The four big rectifiers screwed to the enclosure (top & either side), are they selenium, or copper oxide? At first, I though selenium, but after a little googling they seem to look more like copper oxide rectifiers. Is there an easy way to tell by looking?

[Gyro]

My guess would be Selenium. I can only see one junction per rectifier and I think Selenium ones have a higher PIV (20-25V?) than Copper oxide (4-6V?).

[MagicSmoker]

Unless this is a high current battery charger then those are almost certainly (99% probable) selenium rectifiers. If you want to make extra sure the voltage drop should be about 1V per plate, and the reverse breakdown rating about 25V per plate (limit current to a few mA at most if you elect to perform the latter test).

[ratio]

Thanks. This is a neat little power supply. The variable outputs seem to be controlled via a wiper on the front of the xfrmr, which can (apparently) be added or subtracted from the output voltage. I'd like to get it running, although the switch marked "line/isol" is a little worrisome.

[tautech]

I'd like to get it running, although the switch marked "line/isol" is a little worrisome.

I'd guess it just connecting mains earth/PSU chassis to the Neg output....easy enough to check with a continuity test. 

[TERRA Operative]

Depending on how much importance you put on originality, I'd lean towards installing silicon rectifiers to replace those selenium ones.. They are pretty nasty when (not if) they fail..
You wouldn't even need to remove the selenium rectifiers, just disconnect the wires and solder some beefy silicon diodes in their place, maybe with a decent wattage low ohm resistor in series to obtain the same voltage drop if needed.

[texaspyro]

Easiest way to tell is to do something that lets the magic smoke out of the rectifier.   If it stinks bad enough to make Satan puke his guts out it was selenium.   

[MagicSmoker]

They are pretty nasty when (not if) they fail..

Yes, this: Se rectifiers have a limited operational lifespan, and IIRC, they also gradually lose their ability to block reverse voltage if left unused for a long period (but can be reformed in much the same manner as the dielectric on elkos, by slowing increasing the voltage across them).

[ratio]

Great, thanks, that was just the kind of info I was hoping to hear. I'll plan on replacing them with silicon devices, and a resistor if necessary. I'd like to get this working as intended, but it's not going to be a period-accurate restoration. I might do something else with the line/isol switch too--I don't see a need to unisolate it any more.