Dendrobium with black spots on leaves

[Jling09]

I bought a couple of Dendrobiums from my favorite mail-order orchid place, and liked them so much I bought another few. The plants in the first shipment seem healthy.

In the second shipment, though, a couple of the plants had black spots on the leaves and yellowing (although, curiously, not around the spots themselves).

The first two pictures are of a Dendrobium aurantiflammeum; the third picture is of a Dendrobium tobaense var. giganteum.

What's going on with these plants? I repotted them in fresh soil (a seedling mix with small orchiata, Turface MVP, and some small cork chunks) but have not otherwise done anything to them but water with a half-strength solution of Jack's 30-10-10. Have otherwise not used any Physan or antifungals.

Should I throw them out, or can they be saved? Am I watering them too much? How do I know when they should be watered?

Detailed instructions would be super helpful - I seem to have a hard time keeping orchids alive. Thanks!