ID this diode?

[chuho]

Are the square components here diodes? Dimensions are 50mm x 50mm. They have no markings I can find.



I'm trying to get an old benchtop power supply working. There are four of these and based on their location in the circuit they should be rectifying the AC.

If they really are diodes, the two shown here are shot. The two on the other side have a fwd bias voltage between 0.25 and 0.35 V.

[wraper]

Look like selenium rectifiers.

[amyk]

Could be selenium or copper oxide.