Cymbidium Orchid with Brown Tips

[Julflower]

Hi,

I was in the grocery store and saw what was tagged "Double Bloom Cymbidium Orchid" with beautiful peach flowers. I returned 3 days later to buy it and the flowers were dead and the manager offered it to me 1/2 off. I re-potted with Cymbidium potting material since it was potted poorly with no drainage. It's very large, in the pot about 4 feet tall. I read what I could about them, but I really have no idea. I sat it outside with morning sun, afternoon shade. Daytime around 80 degrees, nights around 50 degrees. Watering once a week, have not fertilized yet. Purchased Miracle Grow Orchid Food 30-10-10 for it. Tips of leaves are turning brown with black stripes (see pics). Please help!!