Oncidium Leaf Tip Burn

[NKYorchid]

My Oncidium Tsiku Marguerite has been doing well currently with multiple spikes. Noticed dead leaf tips and an increased number of black spots. Wonder if this is classic in appearance for a diagnosis or within normal limits.

Location: Midwest
Medium: RepotMe bark topped with sphagnum
Light and climate: North-facing windowsill all spring and summer into fall. Moved under lights a week ago - RH average 57%. 24Watt LED receiving ~100 PPFD.
Water: 1-2/week. Tap water (PH 7.3, Ca 30mg/L, ?Mag)
Fertilizer: Miracle Grow Orchid 30-10-10 about every other watering. 1/8 teaspoon into a 1/2 gallon watering can

Considerations:
1. Salt build-up resulting in fertilizer toxicity.
2. Dry climate, possible dehydration (I shriveled the pseudobulbs in late summer a few times)
3. Nutrient deficiency
4. Sunburn (probably unlikely)

Would appreciate any thoughts!

Oncidium Orchid - Album on Imgur