Roots dying - am I not watering enough?

[flyfishdoc]

I have an Oncidium Sweet Sugar that I repotted last month. I don't recall any root problems when I repotted. I changed to CHC, perlite and charcoal from whatever it came in from the dealer (bought last year from good seller). Today when I watered, I saw rotten roots. I am watering once a week, with rain water. Feeding 3 out of 4 weeks (weak). There are 2 new growths (seen on the right). The humidity even on the humidity tray gets to 35-40% a lot of the time now. Most of my other orchids have drying air roots, so I doubt over watering or staying moist. I switched my phals to a mix with some sphagnum moss to keep them from drying out.

My inclination is to water more often until it gets warmer and the heat goes off (next month I should see 70s and humidity and can open the windows)

Jerry